Rural Road Design
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AUSTROADS ROAD DESIGN SERIES Rural Road Design A Guide to the Geometric Design of Rural Roads
Transcript of Rural Road Design
AUSTROADS
ROAD DESIGN SERIES
ISBN: 0 85588 655 2AP-G1/03
Rural Road DesignA Guide to the GeometricDesign of Rural Roads
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Rural Road Design: A Guide to the Geometric Design of Rural Roads
© Austroads Inc 2003
NAASRA Guides: Austroads GuidesFirst published 1955 Seventh Edition 1989Second Edition 1961 Reprinted 1991Third Edition 1967 Reprinted 1993Reprinted 1967 Reprinted 1997Reprinted 1968 Reprinted 1999Fourth Edition 1970 Eighth Edition 2003Fifth Edition 1973Sixth Edition 1980
This work is copyright. Apart from any use permitted under the Copyright Act 1968,no part may be reproduced by any process without the written permission of Austroads.
National Library of Australia Cataloguing-in-publication data:
Rural Road Design: A Guide to the Geometric Design of Rural RoadsISBN 0 85588 606 4Austroads Project No. T&E.D.C.019Austroads Publication No. AP-G1/03Standards Australia and Standards New Zealand Handbook No. HB152:2002Project ManagerJohn Cunningham, VicRoads
Prepared byArup Group
Published by Austroads IncorporatedLevel 9, Robell House287 Elizabeth StreetSydney NSW 2000 AustraliaPhone: +61 2 9264 7088Fax: +61 2 9264 1657E-Mail: [email protected] www.austroads.com.au
Austroads believes this publication to be correct at the time of printing and does not accept responsibilityfor any consequences arising from the use of the information herein. Readers should rely on their ownskill and judgement to apply information to particular issues.
Design Kirk Palmer Design, Sydney
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RURAL ROAD DESIGN iii
SYDNEY 2002
Rural Road DesignA Guide to the Geometric Design of Rural Roads
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RURAL ROAD DESIGNiv
Austroads is the association of Australian and New Zealandroad transport and traffic authorities whose purpose is tocontribute to the achievement of improved Australian andNew Zealand transport related outcomes by:
● developing and promoting best practice for the safe andeffective management and use of the road system
● providing professional support and advice to memberorganisations and national and international bodies
● acting as a common vehicle for national and internationalaction
● fulfilling the role of the Australian Transport Council’s RoadModal Group
● undertaking performance assessment and development ofAustralian and New Zealand standards
● developing and managing the National Strategic ResearchProgram for roads and their use.
Within this ambit, Austroads aims to provide strategicdirection for the integrated development, management andoperation of the Australian and New Zealand road system —through the promotion of national uniformity and harmony,elimination of unnecessary duplication, and the identificationand application of world best practice.
Austroads membership comprises the six State and two Territoryroad transport and traffic authorities and the CommonwealthDepartment of Transport and Regional Services in Australia, the Australian Local Government Association and Transit NewZealand. It is governed by a council consisting of the chiefexecutive officer (or an alternative senior executive officer) ofeach of its eleven member organisations:
● Roads and Traffic Authority New South Wales● Roads Corporation Victoria● Department of Main Roads Queensland● Main Roads Western Australia● Transport South Australia
● Department of Infrastructure, Energy and ResourcesTasmania
● Department of Infrastructure, Planning and EnvironmentNorthern Territory
● Department of Urban Services Australian Capital Territory● Commonwealth Department of Transport and
Regional Services● Australian Local Government Association● Transit New Zealand
The success of Austroads is derived from the synergies ofinterest and participation of member organisations and othersin the road industry.
In December 1993 Austroads and Standards Australia signed aMemorandum of Understanding regarding the developmentof Standards and related documents primarily for thedevelopment and management of the Australian road system.Standards Australia's support for this handbook reflects thecooperative arrangement between the two organisations toensure there is a coordinated approach in this area.
In August 1995 Austroads, Transit New Zealand and StandardsNew Zealand signed an agreement regarding the developmentof Standards and related documents for endorsement of theappropriate Austroads publications as SNZ handbooks. StandardsNew Zealand and Transit New Zealand's support for thishandbook reflects the cooperative arrangement with Austroadsto ensure that there is a coordinated approach in this area.
AUSTROADS INCORPORATED
AUSTROADS MEMBERSHIP
H A N D B O O K E N D O R S E M E N T
HB 152:2002
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The Austroads Reference group for the guide:
Members
Project Manager John Cunningham, Manager VicRoads Design, VictoriaTechnical Editor Dennis Maxwell, VicRoads, Victoria
Michael Brauer/Peter Ellis Roads and Traffic Authority, New South WalesJohn Byrden VicRoads, VictoriaDennis Davis Transit New ZealandGeoff Clarke Commonwealth Department of Transport and Regional ServicesTony Gill Department of Urban Services, Australian Capital TerritoryGeoff Glynn Municipal Association of VictoriaRob Grove Main Roads, Western AustraliaArthur Hall Department of Main Roads, QueenslandFritz Nabholtz Department of Infrastructure, Planning and Environment, Northern TerritoryGraeme Nichols Department of Infrastructure, Energy and Resources, TasmaniaRichard Saunders Department of Transport South Australia
Project Research and Writer ARUP Group
A U S T R OA D S R E F E R E N C E G R O U P
This guide represents the combined experience and international best practices of Austroads memberagencies and industry experts in the area of geometric design of rural roads. The Guide has been preparedas the common design tool for Australia and New Zealand. For a more detailed explanation of specificmatters, which may vary from place to place, designers should check with the relevant road authority.
It has been the aim of the Consultant and the Reference group to validate all tables, figures and graphsincluded in the Guide. The validation took the form of developed formulae, laboratory test results, fieldobservations or references.
In some cases the designer has been provided with a range of desirable and absolute values. A design canbe produced which may take into account the design topography, the safety of the occupants and thedesign parameters. Care should be taken to ensure the combined use of absolute values does not create aninappropriate design. Each circumstance should be individually evaluated based on local conditions byexperienced personnel.
This document does not cover the geometric design of unsealed roads. The designer is directed to the ARRBdocument “Unsealed Roads Manual – Guidelines to Good Practice, 1993”. The document referred to willprovide the practical and basic aspects for the maintenance design and construction of unsealed roads.
FOREWORD
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RURAL ROAD DESIGN vii
P R E FAC E
This is the eighth edition of the Geometric Design of Rural Roads. The guide was last revised in 1989.
This revision of Rural Road Design: Guide to the Geometric Design of Rural Roads follows the 2002 release of UrbanRoad Design: Guide to the Geometric Design of Major Urban Roads.
(Text to be added/revised)
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TOPIC PAGE NO.
FOREWORD v
PREFACE vii
GLOSSARY OF TERMS xii
PART 1 INTRODUCTION 1
1. A BALANCED APPROACH 11.1 General 1
1.2 Design Standards 1
1.3 Speed Concept 1
1.3.1 General 1
1.3.2 High Speed Roads 2
1.3.3 Intermediate Speed Roads 2
1.3.4 Low Speed Roads 3
1.3.5 85th Percentile Speed 3
2. ROAD FUNCTIONAL CLASSES 33. DESIGN APPROACH 43.1 General 4
3.2 The Driver’s View 4
3.3 Co-ordination of Horizontal and Vertical Alignment 4
3.3.1 General 4
3.3.2 Curvilinear Design 5
3.3.3 Combined Horizontal and Vertical Alignment 5
PART 2 FUNDAMENTAL DESIGN CONSIDERATIONS 8
4. TRAFFIC VOLUME & TRAFFIC COMPOSITION 85. DESIGN VEHICLE 86. ENVIRONMENTAL CONSIDERATIONS 96.1 Traffic Related Intrusion 9
6.1.1 Visuals 9
6.1.2 Noise 9
6.1.3 Vibration 11
6.1.4 Air Pollution 11
6.1.5 Erosion 11
6.1.6 Environmentally Sensitive Areas 11
6.1.7 Clearing 12
6.2 Environmental Related Intrusion 12
6.2.1 Snow and Ice 12
6.2.2 Floods 12
6.2.3 High Winds 12
6.2.4 Animals and Birds 12
6.3 References 12
PART 3 DESIGN INPUTS 13
7. SPEED, USED FOR GEOMETRIC DESIGN 137.1 Introduction 13
7.2 Explanation of Terminology 13
7.2.1 Vehicle Speed on Roads 13
7.2.2 Operating Speed 14
7.2.3 Operating Speed of Trucks 14
7.2.4 Section Operating Speed 14
7.2.5 Design Value 14
7.3 Estimating Operating Speeds on Rural Roads 14
7.3.1 General 14
7.3.1.1 Driver Behaviour 14
7.3.1.2 Road Characteristics 14
7.3.1.3 Vehicle Characteristics 14
7.3.2 Operating Speed Estimation Model 14
7.3.3 Acceleration On Straights Graph 17
7.3.4 Deceleration On Curves Graph 17
7.3.5 Section Operating Speeds 17
7.3.5.1 Length Of Road to be included in The Study 17
7.3.5.2 Identification of Sections 19
7.3.6 Estimating Speed on a Section of Road 21
7.3.6.2 Step 2 – Estimate Speed at Point C 21
7.3.6.3 Step 3 – Estimate Speed at Point D 21
7.3.6.4 Step 4 – Estimate Speed at Point E 21
7.3.6.5 Step 5 – Estimate of Speed at Point F 21
7.3.6.6 Step 6 – Estimate of Speed at Point G 21
7.3.6.7 Step 7 – Estimate of Speed at Point H and I 21
7.3.7 Effects Of Grades 21
7.3.8 Effect of Cross-Section 23
7.3.9 Effect of Pavement Condition 23
7.3.10 Use of Operating Speed in the Design of Rural Roads 23
7.4 Operating Speed of Trucks 24
7.5 Use Of Truck Operating Speeds 24
8. SIGHT DISTANCES 248.1 General 24
8.2 Sight Distance Parameters 25
8.2.1 Object Height 25
8.2.2 Driver Eye Height 25
8.2.3 Driver Reaction Time 26
8.2.4 Ageing of Drivers 26
8.3 Stopping Sight Distance (SSD) 26
8.3.1 Derivation 26
8.3.2 Longitudinal Friction Factor 27
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8.3.3 Car to Road Object Stopping Sight Distance 27
8.3.4 Truck to Road Object Stopping Sight Distance 27
8.4 Overtaking Sight Distance 30
8.4.1 General 30
8.4.2 Overtaking Model 30
8.4.3 Determination of Overtaking Provision 30
8.4.4 Determination of Percentage of Road Providing Overtaking 31
8.5 Manoeuvre Sight Distance 33
8.5.1 Derivation 33
8.6 Headlight Sight Distance 33
8.7 Horizontal Curve Perception Distance 34
PART 4 GEOMETRIC DESIGN GUIDELINES 35
9. HORIZONTAL ALIGNMENT 359.1 General 35
9.2 Movement on a Circular Path 35
9.3 Horizontal Curves 35
9.3.1 Types of Horizontal Curves 35
9.3.1.1 Reverse Curves 35
9.3.1.2 Compound Curves 35
9.3.1.3 Broken Back Curves 35
9.3.1.4 Transition Curves 35
9.4 Side Friction Factor 36
9.5 Minimum Radii Values For Horizontal Curves 37
9.5.1 Minimum Radius Values 37
9.5.2 On Steep Down Grades 38
9.6 Horizontal Alignment Design Procedure 38
9.7 Superelevation 39
9.7.1 Maximum Values of Superelevation 42
9.7.2 Minimum Values of Superelevation 42
9.7.3 Application of Superelevation 42
9.7.4 Length of Superelevation Development 42
9.7.4.1 Rate of Rotation 43
9.7.4.2 Relative Grade 43
9.7.4.3 Design Superelevation Development Lengths 44
9.7.5 Positioning Of Superelevation Runoff 44
9.7.5.1 Without Transitions 44
9.7.5.2 With Transitions 46
9.7.6 Superelevation on Bridges 48
9.8 Curves With Adverse Crossfall 48
9.9 Minimum Horizontal Curve Length 48
9.10 Pavement Widening on Horizontal Curves 48
9.11 Sight Distance on Horizontal Curves 51
9.11.1 Benching for Visibility on Horizontal Curves 51
9.11.2 Other Restrictions to Visibility 51
9.12 Curvilinear Alignment Design in Flat Terrain 52
9.12.1 Introduction 52
9.12.2 Theoretical Considerations 52
9.12.3 Advantages of Curvilinear Alignment 52
9.13 Bridge Considerations 53
10. VERTICAL ALIGNMENT 5410.1 Introduction 54
10.2 Grades 54
10.2.1 General 54
10.2.2 Vehicle Operation on Grades 54
10.2.3 Maximum Grades 55
10.2.4 Length of Steep Grades 55
10.2.5 Steep Grade Considerations 55
10.2.6 Minimum Grades 56
10.3 Vertical Curves 56
10.3.1 General 56
10.3.2 Forms and Types of Curve 56
10.3.3 Crest Vertical Curves 56
10.3.3.1 Appearance 56
10.3.3.2 Sight Distance Criteria (Crest) 57
10.3.4 Sag Vertical Curves 57
10.3.4.1 Appearance and Comfort 57
10.3.4.2 Sight Distance Criteria (Sag) 58
10.3.5 Reverse/Compound/Broken Back Vertical Curves 58
11. CROSS SECTION 6011.1 General 60
11.2 Traffic Lane Width 60
11.3 Traveled Way 61
11.3.1 Single Carriageways 61
11.3.2 Divided Carriageways 62
11.3.2.1 Independent Design of Carriageways 63
11.3.2.2 Superelevation Issues 63
11.3.2.3 Transitions Between Divided and Undivided Carriageways 63
11.4 Pavement Crossfall and its Considerations 63
11.5 Shoulder 65
11.5.1 Function 65
11.5.2 Width 65
11.5.3 Shoulder Sealing 66
11.5.4 Crossfalls 67
11.6 Verge 67
11.7 Batters 67
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11.7.1 Benches 69
11.7.2 Batter Rounding 69
11.8 Medians 69
11.9 Roadside Drains 72
11.9.1 Table Drains 72
11.9.2 Catch Drains 72
11.9.3 Median Drains 72
11.10 Noise Barriers 72
11.11 Right of Way 72
11.12 Widths of Bridges 72
PART 5 OTHER DESIGN CONSIDERATIONS 75
12. PRINCIPAL FACTORS 7512.1 Financial Level 75
12.2 Safety 75
12.3 Energy 75
12.4 Stage Construction 75
13. AUXILIARY LANES 7513.1 General 75
13.2 Types of Auxiliary Lanes 75
13.3 Speed Change Lanes 76
13.3.1 Acceleration Lanes 76
13.3.2 Deceleration Lanes 76
13.4 Overtaking Lanes/Climbing Lanes 76
13.4.1 Overtaking Lanes 76
13.4.1.1 Overtaking Demand 76
13.4.1.2 Overtaking Opportunities 76
13.4.1.3 Warrants 79
13.4.1.4 Length 79
13.4.1.5 Location 80
13.4.1.6 Spacing 80
13.4.1.7 Improvement Strategy For Overtaking Lanes 81
13.4.2 Climbing Lanes 81
13.4.2.1 General 81
13.4.2.2 Warrants 81
13.4.2.3 Length 83
13.5 Slow Vehicle Turnouts 83
13.5.1 Partial Climbing Lanes 83
13.5.2 Passing Bays 83
13.6 Descending Lanes 85
13.7 Runaway Vehicle Facilities 85
13.7.1 General 85
13.7.2 Types of Escape Ramps 86
13.7.2.1 Sand Pile 86
13.7.2.2 Descending Grade 86
13.7.2.3 Horizontal Grade 86
13.7.2.4 Ascending Grade 86
13.7.3 Location of Runaway Vehicle Facilities 86
13.7.4 Arrester Beds and Escape Exits 86
13.7.4.1 Arrester Beds 87
13.7.4.2 Escape Exits 89
13.7.4.3 Spacing 89
13.7.4.4 Summary of Design Considerations 90
13.7.5 Brake Check and Brake Rest Areas 90
13.8 Geometry of Auxiliary Lanes 90
13.8.1 Starting and Termination Points 90
13.8.2 Tapers 91
13.8.3 Cross Section 91
13.8.3.1 Pavement Width 91
13.8.3.2 Shoulder Width 91
13.8.3.3 Crossfall 91
13.8.3.4 Lane Configurations 91
13.8.4 Line marking and Signing 92
13.8.4.1 Signs 92
13.8.4.2 Linemarking 92
14. VEHICLE STOPPING AREAS 9214.1 General 92
14.2 Service Facilities 92
14.2.1 Rest Areas 92
14.2.1.1 Major Rest Areas 93
14.2.1.2 Basic Rest Areas 93
14.2.1.3 Other Areas 94
14.2.2 Location of Vehicle Stopping Areas 95
14.2.3 Heavy Vehicle Considerations 95
15. COMMUNITY CONSULTATION 9616. DRAINAGE 9616.1 General 96
16.2 Flood Estimation 96
16.3 Rational Method 97
16.4 Design Considerations 98
16.5 Water Quality 99
17. ROADSIDE SAFETY 10017.1 Safety Objectives 100
17.2 On-Road Safety 100
17.2.1 Intersections 100
17.2.2 Mid Block 101
17.3 Recovery Area 101
17.3.1 Clear Zone 101
17.3.2 Existing Hazards Within a Clear Zone 102
17.4 Safety Barriers 105
17.5 Landscaping 108
17.6 Lighting 108
17.7 Pedestrians and Cyclists 108
17.8 Temporary Works During Construction 108
17.9 Road Safety Auditing 108
18. RAILWAY LEVEL CROSSINGS 10918.1 Horizontal Alignment 109
18.2 Vertical Alignment 109
18.2.1 Road Grading 109
18.2.2 Cross Section 112
19. COMPUTER SOFTWARE FOR ROAD DESIGN 112
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REFERENCES 113
APPENDICES 117Appendix A – Characteristics of the Euler Spiral
(Clothoid) 117
Appendix B – Vertical Curve Formulae 119
Appendix C – Derivation of Sight Distance Requirements at Railway Level Crossings 121
1. General 121
2. Case 1: Sight Distance Required for Give Way Control 121
3. Case 1(i): Decelerate and Safely Stop at the Stop or Holding Line 122
4. Case 1(ii): Proceed and Clear the Crossingwith an Adequate Safety Margin 122
5. Case 2: Sight Distance Required for Stop Sign Control 123
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AADT Annual Average Daily Traffic is calculated by counting the number of vehiclespassing a roadside observation point in a year and dividing this number by 365.
Abutment An end support of a bridge or similar structure.
Acceleration Lane An auxiliary lane used to allow vehicles to increase speed without interferingwith the main traffic stream. They are often used on the departure side ofintersections.
Access The driveway by which vehicles and/or pedestrians enter and/or leave propertyadjacent to a road.
Adverse Crossfall A slope on a curved pavement that generates forces detracting from the abilityof a vehicle to maintain a circular path.
Alignment The geometric form of the centreline (or other reference line) of a carriagewayin both the horizontal and vertical directions.
Alignment Co-ordination A road design technique in which various rules are applied to ensure that (coordinated alignment) the combination of horizontal and vertical alignment is both safe and
aesthetically pleasing.
Aquaplaning Full dynamic aquaplaning occurs when a tyre is completely separated from theroad surface by a film of water.
Arrester Bed An arrester bed is a safe and efficient facility used to deliberately decelerate andstop vehicles by transferring their kinetic energy through the displacement ofaggregate in a gravel bed.
Arterial Road A road that predominantly carries through traffic from one region to another,forming the principal avenue of communication for traffic movements.
Auxiliary Lane The portion of the carriageway adjoining the through traffic lanes for speedchange, or for other purposes supplementary to the through traffic movement.
Average Recurrence Interval (ARI) The Average Recurrence interval (ARI) is the average interval of time duringwhich an event will be equalled or exceeded once. It should be based on alengthy period of records of the event. Statistically it is the inverse of theAverage Exceedence Probability. The term replaces recurrence interval.
Batter The uniform side slope of walls, banks, cuttings or embankments, expressed asa ratio of 1 vertical on x horizontal as distinct from grade.
Batter rounding Curvature that is applied to improve the stability and appearance of the roadat the intersection of the extension of the road crossfall and/or existing surface(hinge point), with the batter slope of an embankment or cutting.
Barrier An obstruction placed to prevent vehicle access to a particular area.
Barrier Kerb A kerb with a profile and height sufficient to prevent or discourage vehiclesmoving off the carriageway.
Bench A ledge constructed in a batter or natural slope for the purpose of providingadequate horizontal sight distance, greater security against batter slippage orto assist with batter drainage.
Border The area between the carriageway and the property line. It allows provision forservices, footpaths, cycle path, shared paths, street trees and street furniture.Additional width will be required for bus bays or where major transmissionservices are to be provided in the verge. It includes the shoulder if provided.
Braking Distance The distance required for the braking system of a vehicle to bring the vehicleto a stop from the operating speed.
Broken Back Curve Two horizontal curves in the same direction separated by a short straight (aspecial case of the compound curve).
GLOSSARY OF TERMS
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Bunching Grouping of vehicles travelling in the same direction with restricted speed causedby the slow moving head of the bunch and limited overtaking opportunities.
Bus Bay An auxiliary lane of limited length at a bus stop or terminus usually indented intothe shoulder or verge.
Carriageway That portion of a road or bridge devoted particularly to the use of vehicles,inclusive of the shoulders and auxiliary lanes.
Catch drain A surface channel constructed along the high side of a road or embankment,outside the batter to intercept surface water.
Catchment Area The area that will contribute to the discharge of a stream after rainfall at the pointunder consideration.
Channelised Intersection An intersection provided with channelised islands.
Centreline The basic line that defines the axis or alignment of the centre of a road or otherworks.
Clear Zone An area adjacent to the traffic lane that should be kept free from featurespotentially hazardous to errant vehicles.
Clearance The space between a stationary and/or moving object.
Climbing Lane A special case of an overtaking lane located on a rising grade.
Coefficient of Run-off The ratio of the amount of water that runs off a catchment area to the amountthat falls on the catchment.
Compound Curve A curve consisting of two of more arcs of different radii curving in the samedirection and having a common tangent point or being joined by a transitioncurve.
Crossfall The slope, measured at right angles to the alignment, of the surface of any partof a carriageway.
Cross Section The transverse elements of the longitudinal elements.
Crown The highest point on the cross section of a carriageway with two-way crossfall.
Curvilinear Alignment The alignment is a continuous curve with constant, gradual and smooth changesof direction.
Cycle Lane A paved area adjacent to and flush with the traffic lane pavement, for themovement of cyclists. A lane designated for the exclusive use of cyclists.
Deceleration Lane An auxiliary lane provided to allow vehicles to decrease speed.
Deck The bridge floor directly carrying traffic loads.
Design Life The period during which the quality of a structure (eg riding quality of apavement) is expected to remain acceptable.
Design Speed A speed fixed for the design and correlation of those geometric features of acarriageway that influence vehicle operation. Design speed should not be lessthan the operating speed.
Design Traffic The predicted cumulative traffic at the design year, expressed in terms of vehicles.
Design Vehicle A hypothetical road vehicle whose mass, dimensions and operatingcharacteristics are used to determine geometric requirements.
Design Year The predicted year in which the design traffic would be reached.
Discharge The volumetric rate of water flow.
Divided Road (divided carriageway) A road with a separate carriageway for each direction of travel created by placingsome physical obstruction, such as a median or barrier, between the opposingtraffic directions.
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Drainage The natural or artificial means for the interception and removal of surface orsubsurface water.
Ease Section of rounding.
Footpath A public way reserved for the movement of pedestrians and manually propelledvehicles. A separate facility for pedestrians remote from the road carriageway.It may also be the paved part of the “footpath” used by pedestrians.
Footway Pedestrian facility on a bridge.
Formation The surface of the finished earthworks, excluding cut or fill batters.
Frangible Term is used to describe roadside furniture designed to collapse on impact. Theseverity of potential injuries to the occupants of an impacting vehicle isreduced, compared to those that could occur if the furniture was unyielding.
Freeway A divided highway for through traffic with no access for traffic betweeninterchanges and with grade separation at some interchanges.
Grade The rate of longitudinal rise (or fall) of a carriageway with respect to thehorizontal, expressed as a percentage.
Grade Separation The separation of road, rail or other traffic so that crossing movements, whichwould otherwise conflict, are at different elevations.
Hinge Point The point in the cross-section of a road at which the extended batter line wouldintersect the extended verge line.
Horizontal Alignment The bringing together of the straights and curves in the plan view of acarriageway.
Horizontal Curve A curve in the plan view of a carriageway.
Intensity of Rainfall The rainfall in a unit of time.
Interchange A grade separation of two or more roads with one or more interconnectingcarriageways.
Intermediate Sight Distance The ISD is equal to 2 x stopping distance for the operating speed.
Intersection A place at which two or more roads meet.
Intersection Angle 1. The angle between two intersecting roads.
2. The angles between the centrelines of two intersecting carriageways.
Intersection (at-grade) An intersection where carriageways cross at a common level.
Intersection Leg Any one of the carriageways radiating from and forming part of anintersection.
K Value The length required for a 1% change of grade on a parabolic vertical curve.
Kerb A raised border of rigid material formed at the edge of a carriageway.
Kerb and Channel The kerb and channel combine to form an open drain to capture and dischargerun off.
Kerb Clearances A distance by which the kerb should be set back in order to maintain themaximum capacity of the traffic lane.
Lane (Traffic) A portion of the carriageway allocated for the use of a single line of vehicles.
Lane Separator A separator provided between lanes carrying traffic in the same direction todiscourage or prevent lane changing, or to separate a portion of a speedchange lane from through lanes.
Lateral Friction The force which, when generated between the tyre and the road surface,assists a vehicle to maintain a circular path.
Level of Service (LOS) A qualitative measure describing operational conditions within a traffic streamand their perception by motorists and passengers.
GLOSSARY OF TERMS (con t ’d)
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Limiting Curve Speed Standard The curve speed at which f just equals f max, Vs.
Line of Sight The direct line of uninterrupted view between a driver and an object of specifiedheight above the carriageway in the lane of travel.
Longitudinal Friction Factor The friction between vehicle tyres and the road pavement under locked wheelbraking conditions, measured in the longitudinal direction.
Longitudinal Section A vertical section, usually with an exaggerated vertical scale, showing the existingand design levels along a road design line, or another specified line.
Median A strip of road, not normally intended for use by traffic, which separatescarriageways for traffic in opposite directions.
Median Island A short length of median serving a localised purpose in an otherwise undividedroad.
Median Lane The traffic lane nearest the median.
Median Opening A gap in a median provided for crossing and turning traffic.
Minimum Turning Path The path of a designated point on a vehicle making its sharpest turn.
Minimum Turning Radius The radius of the minimum turning path of the outside of the outer front tyre ofa vehicle.
Motorway A divided highway for through traffic with no access for traffic betweeninterchanges and with grade separation at some interchanges.
Multiple Combination Vehicles The full range of truck, prime mover and semi trailers and road trains.
Normal Cross Section The cross section of the carriageway where it is not affected by superelevation orwidening.
Off-tracking The radial offset between the path traced by the centre of the front axle and thecentre of the effective rear axle.
One-way Road A road or street on which all vehicular traffic travels in the same direction.
Operating Speed The 85th percentile speed of cars at a time when traffic volumes are low and willallow a free choice of speed within the road alignment.
Overtaking The manoeuvre in which a vehicle moves from a position behind to a position infront of another vehicle travelling in the same direction.
Overtaking Distance The distance required for one vehicle to overtake another vehicle.
Overtaking Lane An auxiliary lane provided to allow for slower vehicles to be overtaken. It is line-marked so that all traffic is initially directed into the left-hand lane, with the innerlane being used to overtake.
Overtaking Zone A section of road on which at least 70 per cent of drivers will be prepared to carryout overtaking manoeuvres subject to availability of adequate gaps in theopposing direction.
Passing The manoeuvre by which a vehicle moves from a position behind to in front ofanother vehicle, which is stationary or travelling at crawl speeds.
Passing Bay A very short auxiliary lane (of the order of 100 m) that allows a slow vehicle topull aside to allow a following vehicle to pass.
Pavement That portion of a road designed for the support of, and to form the runningsurface for, vehicular traffic.
Perception Distance The sight distance required accessing the curvature of horizontal curves onapproach.
Property Line The boundary between a road reserve and the adjacent land.
Rainfall Intensity The rate of rainfall (mm/hr).
Rate of Rotation The rate of rotation required achieving a suitable distance to uniformly rotate the
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crossfall from normal to full superelevation. The usual value adopted is 0.025rad/sec; 0.035 rad/sec is the maximum value.
Reaction Distance The distance travelled during the reaction time.
Reaction Time The time between the driver’s reception of stimulus and taking appropriateaction.
Re-alignment An alteration to the control line of a road that may affect only its verticalalignment but, more usually, alters its horizontal alignment.
A method of widening a road reservation.
Reverse Curve A section of road alignment consisting of two curves turning in oppositedirections and having a common tangent point or being joined by a shortlength of tangent.
Residual Median The remnant area of the median adjacent to right turn lanes.
Road Furniture A general term covering all signs, streetlights and protective devices for thecontrol, guidance and safety of traffic, and the convenience of road users.
Roadside Safety Barrier A device erected parallel to the road to retain vehicles that are out of control.
Road (way) A route trafficable by motor vehicles; in law, the public right-of-way betweenboundaries of adjoining property.
Roundabout An intersection where all traffic travels in one direction around a central island.
Run-off That part of the rainfall on a catchment which flows as surface discharge pasta specified point.
Sag Curve A concave vertical curve in the longitudinal profile of a road.
Section Operating Speed The 85th percentile speed of cars traversing a section of road alignment.
Semi-Mountable Kerb A kerb designed so that it can be driven across in emergency or on specialoccasions without damage to the vehicle.
Shared Path A paved area particularly designed (with appropriate dimensions, alignmentand signing) for the movement of cyclists and pedestrians.
Shoulder The portion of formed carriageway that is adjacent to the traffic lane and flushwith the surface of the pavement.
Sideways Friction Coefficient The ratio of the resistance to side ways motion of the tyre of a vehicle (on aspecified pavement) and the normal force on that wheel due to the vehiclemass.
Sight Distance Approach Sight Distance (ASD)The distance required for a driver to perceive marking or hazards on the roadsurface approaching an intersection and to stop.Car Stopping Distance (SSD)The distance required for a car driver to perceive a hazard, react and brake toa stop. For design purposes, wet weather conditions and locked wheel brakingare assumed.Entering Sight Distance (ESD)The sight distance required for minor road drivers to enter a major road via aleft or right turn, such that traffic on the road is unimpededManoeuvre Sight DistanceThe distance required for an alert car driver to perceive an object on the roadand to take evasive action.Minimum Gap Sight Distance (MGSD)“The minimum sight distance based on the gap necessary to perform aparticular movement.”Overtaking Sight Distance The sight distance required for a driver to initiate and safely complete anovertaking manoeuvre.Railway Crossing Sight Triangle
GLOSSARY OF TERMS (con t ’d)
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The clear area required for a truck driver to perceive a train approaching anuncontrolled railway crossing and to stop the truckSafe Intersection Sight Distance (SISD)The distance required for a driver in a major road to observe a vehicle enteringfrom a side road, and to stop before colliding with it.Sight Distance Through Underpass The distance required for a truck driver to see beneath a bridge located across themain road, to perceive any hazard on the road ahead, and to stop.Stopping Sight Distance The sight distance required by an average driver (car or truck depending ondesign requirements), travelling at a given speed, to react and stop before strikingan object on the road.Truck Stopping Sight Distance The distance required for a truck driver to perceive a hazard, react and brake toa stop.For design purposes, the braking of an unladen vehicle in wet weather conditionswithout locking the wheels is assumed.
Sight Triangle The area of land between two intersecting roadways over which vehicles on bothroadways are visible to each driver.
Skid Resistance The frictional relationship between a pavement surface and vehicle tyres duringbraking or cornering manoeuvres. Normally measured on wet surfaces, it varieswith the speed and the value of ‘slip’ adopted.
Slope 1. The inclination of a surface with respect to the horizontal, expressed as rise orfall in a certain longitudinal distance.
2. An inclined surface.
Speed 85th Percentile Speed The speed at which 85 percent of car drivers will travel slower and 15 percent willtravel faster.Operating Speed of TrucksThe 85th percentile speed of trucks measured at a time when traffic volumesare low.Section Operating Speed The value at which vehicle speeds on a series of curves tend to stabilise, arerelated to the range of radii on the curves.
Speed-change Lane A subdivision of auxiliary lanes, which cover those lanes used primarily for theacceleration or deceleration of vehicles. It is usual to refer to the lane by its actualpurpose (eg. deceleration lane).
Sub-arterial Road Road connecting arterial roads to areas of development, and carrying trafficdirectly from one part of a region to another.
Superelevation A slope on a curved pavement selected so as to enhance forces assisting a vehicleto maintain a circular path.
Superelevation Development The length over which the crossfalls on a carriageway are gradually changed fromnormal crossfall to full superelevation crossfall.
Superelevation Runoff That part of superelevation development that goes from flat crossfall to fullsuperelevation crossfall (on the outside of the curve, when there are segmentsrotating either side of the axis of rotation).
Swept Path The area bounded by lines traced by the extremities of the bodywork of a vehiclewhile turning.
Swept Width The radial distance between the innermost and outermost turning paths of avehicle.
Table drain The side drain of a road adjacent to the shoulder, having its invert lower than thepavement base and being part of the formation.
Tangent Runout The length of roadway required to accomplish the change in crossfall from a
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normal crown section to a flat crossfall at the same rate as the superelevationrunoff.
Terrain Topography of the land.Level Terrain Is that condition where road sight distance, as governed by both horizontal andvertical restrictions, are generally long or could be made to be so withoutconstruction difficulty or major expense.Undulating TerrainIs that condition where road sight distance is occasionally governed by bothhorizontal and vertical restrictions with some construction difficulty and majorexpense but with only minor speed reduction.Rolling TerrainIs that condition where the natural slopes consistently rise above and fall belowthe road grade and where occasional steep slopes offer some restriction tonormal horizontal and vertical roadway alignment. The steeper grades cause trucks to reduce speed below those of passengercars.Mountainous TerrainIs that condition where longitudinal and transverse changes in the elevation ofthe ground with respect to the road are abrupt and where benching and sidehill excavation are frequently required to obtain acceptable horizontal andvertical alignment. Mountainous terrain causes some trucks to operate at crawlspeeds.
Time of Concentration The shortest time necessary for all points on a catchment area to contributesimultaneously to run-off at a specified point.
Traffic A generic term covering all vehicles, people, and animals using a road.
Traffic Control Signal A device that, by means of changing coloured lights, regulates the movementof traffic.
Traffic Island A defined area, usually at an intersection, from which vehicular traffic isexcluded. It is used to control vehicular movements and as a pedestrian refuge.
Transition Transition length for increasing or decreasing the number of lanes.
Traffic Lane A portion of the carriageway allocated for the use of a single line of vehicles.
Traffic Sign A sign to regulate traffic and warn or guide drivers.
Transition Curve A curve of varying radius to model the path of a vehicle entering or leaving ahorizontal circular curve.
Transition Length for alignment The distance within which the alignment is changed in approach from straightto a horizontal curve of constant radius.
Transition Length for crossfall The distance required rotating the pavement crossfall from normal to thatappropriate to the curve. Also called superelevation development length.
Transition Length for widening The distance over which the pavement width is changed from normal to thatappropriate to the curve.
Travelled way That portion of a carriageway ordinarily assigned to moving traffic, andexclusive of shoulders and parking lanes.
Turning Lane An auxiliary lane reserved for turning traffic.
Typical Cross Section A cross section of a carriageway showing typical dimensional details, furniturelocations and features of the pavement construction.
Verge That portion of the formation not covered by the carriageway or footpath.
Vertical Alignment The longitudinal profile along the design line of a road.
Vertical Curve A curve (generally parabolic) in the longitudinal profile of a carriageway toprovide for a change of grade at a specified vertical acceleration.
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RURAL ROAD DESIGN 1
1 . A B A L A N C E D A P P R OACH
1.1 General
Roads will continue to be an important part of our transportsystem for the foreseeable future by providing for the safe andoperationally efficient movement of people and goods. Abalanced approach towards road planning and design canimprove road safety and public amenity, and reduce the effectof noise, vibration, pollution and visual intrusion on the areasthrough which a road passes. The objectives of new andexisting road networks should be carefully considered toachieve the desired balance and must take into account theavailable resources to achieve them.
In every situation designers will be faced with competingdemands from different sections of the community as theyendeavour to design safe, operationally efficient roads.
The various chapters in this publication provide a guide topractitioners on the standards that can be achieved withinsocial, environmental, economic and other constraints usingbest local and overseas practice.
1.2 Design Standards
Geometric road design standards are used as an aid toachieving consistent and operationally effective road designs.Rapid expansion and improvement to road networksprecipitated the need for standards to:
● maintain a degree of uniformity, particularly acrossadministrative boundaries;
● enable satisfactory designs to be produced, even wherethere was not a high degree of expertise; and
● ensure that road funds were not miss-spent, throughinappropriate designs, or through inadequate provision forfuture traffic growth or for current operations.
Prior to the 6th edition of this guide, many of the standardsadopted in Australia were based heavily on those used in theUSA and other developed countries. However, with the 6th
edition, standards that were more appropriate for Australiawere promoted. There were two aspects to these newstandards:
● Technical – relating to safety and efficiency of trafficoperations and particularly to alignment design.Experience has shown that rigid adherence to the earlierstandards did not always ensure a safe, operationallyefficient road; and
● Costs of desirable road construction projects almost alwaysexceed the total of funds that can be made available. Inthis situation, each upward increment in design standards
to which a road project is built, results (due to the slightlyincreased cost) in the deferment of other projects to enablethe higher cost project to be funded. Improved provisionfor future traffic results in greater deficiencies on thebalance of the road system with respect to present traffic.The more constrained the financial situation, the morethese tradeoffs become evident.
There are three distinct stages in the development of acountry’s road system. The importance of geometric standardsdepends very much on the stage reached.
● Stage 1 – Basic Network. The establishment of a basicnetwork so that transport links exist where they arerequired. The roads must be trafficable. Geometricstandards are relatively unimportant except as they affectmatters like drainage and gradient;
● Stage 2 – Increasing Capacity. Improving the road’s abilityto carry increasing volumes of traffic. This includesstructural strength, but geometric standards assumegreater importance; and
● Stage 3 – Quality of Service. Building operational safety,efficiency and convenience into the network, as embodiedin a concept of ‘quality of service’. Alignment standardsbecome important, and cross section standards need to bemore generous to accommodate significant volumes ofhigh-speed traffic.
The development of the Australian and New Zealand roadnetwork is a mixture of increasing the network capacity andproviding for an improved quality of service. Parts of the moreremote areas still have road development problems associatedwith the establishment of a basic network. Many of theimported geometric standards that were used prior to the 6th
edition related to the quality of service that a road provides.Problems arose through their inappropriate application inareas where a basic network was still being developed.
The main problem now for geometric standards is that thereare many areas where the road system exhibits all three stagesof development. In these areas motorists are more likely to beinfluenced by the geometry of the ‘Stage 2’ roads. Hence,they are likely to be more demanding of the standard ofgeometry on ‘Stage 1’ roads.
1.3 Speed Concept
1.3.1 General
When assessing the major roles that a road should fulfil andthe standard of this provision, engineering judgement will berequired. Identified problems or concerns need to be carefullyconsidered and a range of alternative solutions examinedbefore deciding upon a particular course of action. Judgementof what is considered “acceptable” for the road in questionwill involve a balance between such issues as traffic capacity,the environment, speed, safety and road user comfort. It is
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important to determine which of the various demands shouldbe given priority, taking into account function and operatingconditions of the road and its relationship with other roads inthe adjacent network.
Use of the traditional design speed concept as a criterion foralignment consistency on rural roads was introduced in theUSA in the 1930s in response to increasing numbers ofaccidents at horizontal curves. This concept was developed asa mechanism for designing rural road alignments permittingthe majority of drivers to operate uniformly at their desiredspeed. However, as identified by researchers in variouscountries, the concept has not always produced safe andconsistent alignments.
Various speed studies in Australia, New Zealand and overseashave shown that on roads designed for speeds less than 100km/h the 85th percentile driver exceeds the design speed by upto 20 km/h. The revised design procedure in Guide to theGeometric Design of Rural Roads (NAASRA, 1989) incorporatedconsiderations of operating speeds to improve alignmentconsistency. The guide had four basic speed parameters:
● desired speed;● speed environment;● design speed; and● limiting curve speed standard.
There was some uncertainty in the application of the NAASRA(1989) design parameters because:
● different interpretations were given to the term speedenvironment;
● designers were reluctant accept the predicted speeds onsome low radii;
● no clear instructions were available on the use of thedesign curves;
● results obtained by different designers were not consistent;and
● very long lengths of relatively straight road were requiredfor vehicles to reach the speed environment.
In spite of these problems, the basic procedure providedappropriate outcomes. However, in order to make theprocedures more transparent, there is a need for a morespecific method for determining speeds on straight andhorizontal curves.
From observations of driver behavior in hilly terrain, it wasnoted that drivers initially reduce speed over the first fewcurves until they reach a speed that is the highest at which thedriver feels comfortable. The driver then tends to maintain thisspeed unless confronted with a curve with a radiussignificantly below the general range of radii on the section ofroad. Conversely, the driver will not increase speed unless astraight (or near straight) is available and is >200 meters. Onshorter straights drivers tend to maintain the speed attainedon the preceding section of curves. This speed is called“section operating speed”.
The research findings and accumulated design experiencesuggest that there are effectively three ranges of speed standardfor roads, and that different design philosophies should beemployed for each range. All have the fundamental objective ofproviding a road which accords with driver expectations.
1.3.2 High Speed Roads
These are roads with design speeds in excess of 100 km/h. Onthese high-speed roads operating speeds are not constrainedby the geometry of the road but by a number of other factors,which include:
● The degree of risk the drivers are prepared to accept;● Speed limits and the level of policing of these limits; and● Vehicle performance.
Roads with design speeds of 110 km/h and 130 km/h are likelyto have similar operating speeds.
McLean (Ref. 71) noted that drivers generally wish to travel ataround 100 km/h to 110 km/h. On roads designed for lowerspeeds, drivers tend to “overdrive” the road. Conversely onroads designed for higher speeds, drivers adopt an operatingspeed of 100 km/h to 110 km/h.
1.3.3 Intermediate Speed Roads
These are roads designed with minimum operating speeds of80 km/h to 100 km/h. Operating speeds on these roads aregenerally constrained by the geometry. Drivers will, however,accelerate whenever the opportunity arises, such as on anystraight or large radius curve. Curve radii on these roads aregenerally in excess of 160 m.
1.3.2 High Speed Roads
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1.3.4 Low Speed Roads
These are roads having many curves with radii less than 150m. Operating speeds on the curves vary from 50 km/h to 70km/h. These roads are only used when difficult terrain andcosts preclude the adoption of higher speeds. Thealignments provided in these circumstances could beexpected to produce a high degree of driver alertness, sothose lower standards are both expected and acceptable. Themost pragmatic approach to the design of individualelements in such constrained situations is to provide the bestthat appears practicable, and to check that it is within theabsolute minimum standards for the predicted 85thpercentile speed. Innovative, non-standard treatments willoften be required when these standards cannot be met.
On roads with speed limits less than 100 km/h, the operatingspeed of vehicles will be determined by the geometricconstraints of the road on the imposed speed limits and the corresponding operating speeds refer Section 7.2 andFigure 7.1.
1.3.5 85th Percentile Speed
The term “eighty fifth percentile speed” indicates that 85percent of car drivers will travel at or below this speed and 15percent will travel faster. In effect, this means that designsbased on the 85th percentile speed will cater for the majorityof drivers. For design purposes, the 15% of drivers whoexceed this speed are considered to be aware of the increasedrisk they are taking and are expected to maintain a higher levelof alertness, effectively reducing their reaction times.
2. ROAD FUNCTIONAL CLASS ES
Roads fall into a hierarchy of functional classes ranging frommajor arterial to local access. Austroads has defined a systemof functional classification for rural roads (see Table 2.1).
Functional classes are not always clear-cut since almost allroads have some degree of local importance.
Rural roads of higher functional class generally cater for ahigher (though normally still modest) proportion of longerlength journeys, and it may be appropriate to select higherdesign standards for such roads so that the quality of serviceis more appropriate to the longer trip’s duration. Howeverdesigners must be aware of placing too much importanceon functional class alone where traffic volumes are low.Further discussion on functional classification of roads isgiven in Ref. 22.
1.3.3 Intermediate Speed Roads 1.3.4 Low Speed Road
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3 . D E S I G N A P P R OACH
3.1 General
The subsequent sections are concentrated primarily on thephysical attributes of good road design to satisfy therequirements of safety and performance. Whilst these needsare of prime importance, some compromise may be necessaryin the need for convenience of access, amenity and economy.
Considerations of amenity are those which concern the effectthat a road and its traffic has upon the environmental andaesthetic senses of users and of those others who are affectedby its construction and operation. The pleasing coordination ofalignment and grading, the fitting of the road to the naturalcontours of the land surface, and the preservation orenhancement of the natural vegetation is all involved.
The road, therefore, must be considered at all stages of designas a three-dimensional structure that should be safe,functional and economical but also aesthetically pleasing.
3.2 The Driver’s View
The driver sees a foreshortened and, thus, distorted view ofthe road, and unfavourable combinations of horizontal andvertical curves can result in apparent discontinuities in thealignment, even though the horizontal and vertical designseach comply separately with the provisions of theirindividual design requirements. Such combinations canmask from the driver a change in horizontal alignment oreven a sag curve deep enough to conceal a significanthazard (the hidden dip problem). Only the consideration ofthe road as a three dimensional entity can reveal suchdeficiencies, and good design practice requires theelimination of all avoidable hazards even though someadditional expense may be incurred. The removal of hazardsis not, however, the only benefit, as the improved safety andperformance potential is invariably accompanied bysignificantly enhanced amenity.
Not only is the driver’s view constantly changing, but theduration of his view of successive elements of the road is alsovarying. Features situated in long, low sag curves remain inview for a considerable length of time whereas other featuresat or near an abrupt crest or on a tight curve are in view onlyfleetingly. It follows then that important features such asintersections are most favourably located on long sag curves.
Visual cues to the driver from peripheral areas must be givenadequate attention. While the designer views the whole roadlayout at once, and is aware of all changes in alignment, thedriver sees much less at any one time. The driver’s inherentlyrestricted view can be further limited at night, or in other timesof poor visibility. The designer must, therefore, provide thedriver with as many clues as possible as to what lies ahead, butmust make sure that the roadside conditions do not conveymessages which are ambiguous or misleading.
3.3 Co-ordination of Horizontal and Vertical Alignment
3.3.1 General
It has been shown that the operation of a road is influencedpartly by the nature of the terrain and partly by thehorizontal alignment. It follows, therefore, that if theindications of these two factors are similar, the road willprovide the best level of consistency in driver expectancy andthus safety. Further, a road having both horizontal andvertical curvature carefully designed to conform to the terrainwill result in the desirable aesthetic quality of being inharmony with the landform.
Perfect harmony of course is not always possible, and thedesigner must consider what matters are beyond his controland make full allowance for their influence on driverbehaviour. From Section 7 it will be clear that, while it ispossible to build a road with a high operating speed inadverse terrain, it is unlikely that there will ever be sufficientcurvature in flat or gently rolling terrain to produce a low-speed environment. Operating speeds will be high in thelatter cases because of the terrain. The grading needs to
RURAL ROAD DESIGN4
Table 2.1 Austroads Functional Rural Road Classification
ARTERIAL ROADS
Class 1
Those roads, which form the principal avenue forcommunications between major regions, including directconnections between capital cities.
Class 2
Those roads, not being Class 1, whose main function is toform the principal avenue of communication formovements between:● A capital city and adjoining states and their capital
cities; or ● A capital city and key towns; or ● Key towns.
Class 3
Those roads, not being Class 1 or 2, whose main functionis to form an avenue of communication for movements:● Between important centres and the Class 1 and Class
2 roads and/or key towns; or● Between important centres; or● Of an arterial nature within a town in a rural area.
LOCAL ROADS
Class 4
Those roads, not being Class 1, 2 or 3, whose mainfunction is to provide access to abutting property(including property within a town in a rural area).
Class 5
Those roads, which provide almost exclusively for one activityor function, which cannot be assigned to Classes 1 to 4.
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ensure adequate sight distances to potential hazards on theroad and, where such sections merge into more constrainedalignment sections, such transition must be accomplishedgradually rather than suddenly.
In flat open terrain, long straight road sections are common,but generally there is advantage in avoiding excessive lengthsof straight road. A gentle curvilinear design, as discussed inSection 9, always helps to keep the operating conditions‘under control’ and at the same time, affords scope for farmore sympathetic fitting of the road to terrain. The increasedflexibility of this approach enables more pleasing designs tobe produced at no extra cost; economies in earthworks canoften be achieved by fitting the road more closely to theterrain. In addition, safety is enhanced by making the drivermore aware of his speed, by allowing him to make betterassessments of the distances and speeds of other vehicles, byreducing headlight or sun glare in appropriate circumstancesand by reducing boredom and fatigue. Even in flat countrycurvilinear designs can be used. Radii must be very large, sothat all of the benefits of a curving alignment are achieved.Estimation of speed of oncoming vehicles is not significantlyimproved over a straight alignment when radius exceedsabout 5,000m to 10,000m. It is the opinion of experienceddesigners, however, that sufficient benefits do still remain tomake the exercise worthwhile.
3.3.2 Curvilinear Design
Curvilinear design is most readily applicable to dividedroads with their less stringent sight distancerequirements but the principles are just as relevant tosingle carriageway roads provided care is taken to ensureadequate overtaking opportunities are available.
Very large radius curves can provide overtaking opportunitiesand, as mentioned above, retain at least some of the benefitsof curvilinear alignment. If the topography is such that‘natural’ curvature precludes the provision of overtaking sightdistance, then the provision of overtaking zones may producean economical as well as an aesthetic solution.
Figure 3.1 illustrates basic examples of the method andbenefits of proper fitting of the road to the terrain and ofproper coordination of horizontal and vertical elements. Inaddition, there are some examples of poor design form, withindications of appropriate remedial measures. These latterexamples are typical of the results likely if the designer doesnot consider the vertical and horizontal views simultaneously;particularly if a ‘minimum’ vertical standard is superimposedon a relatively unrestricted horizontal regime.
The diagrams are not intended to be comprehensive, but servemerely to demonstrate the general concepts that should (orshould not) be followed. In all cases, recognition of thedeficiency is sufficient to indicate the appropriate remedy, andthe recognition of the deficiency is dependent only on thedesigner taking a three-dimensional, rather than a two-dimensional view of the problem.
Specific rules are not appropriate to good design, as eachparticular project has its own peculiar problems andconstraints. However, some benefit can be obtained from aconsideration of what combinations of horizontal and verticalelements are most likely to produce satisfactory results, and
visualising the schemes in these dimensions using whateveraids are available.
3.3.3 Combined Horizontal and Vertical Alignment
The most pleasing three-dimensional result is achieved ifthe horizontal and vertical curvature is kept in phase, asthis relates most closely to naturally occurring forms. Wherepossible, the vertical curves should be contained within thehorizontal curves. This enhances the appearance in sag curvesby reducing the three-dimensional rate of change of direction,and improves the safety of crest curves by indicating thedirection of curvature before the road disappears over thecrest. Thus, the best appearance requires the scale of thevertical and horizontal movements to be comparable: a smallmovement in one direction should not be combined witha large movement in the other.
Drainage structures in sag curves that are combined withhorizontal curves require careful design if a disjointed orkinked appearance is to be avoided. Culverts should introducelittle aesthetic difficulty if they are contained withinembankments and are made sufficiently long to accommodatefull road formation widths.
Bridges built on combined horizontal and vertical curvaturecan present considerable aesthetic problems, especially ifreduced formation widths are used. Particular care should bedevoted to the design of the bridge kerbs and railings, as wellas to the location and transitioning of approach guard fences.In general, the more generous the curvature, the morepleasing and safer will be the result.
Horizontal curves combined with crests have less influence onthe appearance of a road than those combined with sags.Nevertheless, the effect on safety can be much greater, as thecrest can obscure the direction and severity of the horizontalcurve. Minimum radius horizontal curves, therefore, shouldnot be combined with crest vertical curves.
3.3.3 Combined Horizontal and Vertical Alignment
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Figure 3.1(a): Coordination of Vertical and Horizontal Alignments
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Figure 3.1(b): Coordination of Vertical and Horizontal Alignments
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4 . T R A F F I C VO L U M E & T R A F F I C COMPOSITION
Guide to Traffic Engineering Practice Part 2 (ref 15) providesdetails of highway capacity analysis. The Highway CapacityManual Transportation Research Board, HCM 2000, provides acollection of state-of-the-art techniques for estimating thecapacity and determining the level of service for transportationfacilities, including intersections and roadways as well asfacilities for transit, bicycles and pedestrians (Ref 93). Whilst asummary of key principles and issues is provided here, thesereferences should be consulted for more detailedconsideration of capacity issues.
Level of Service (LOS) is defined as a qualitative measuredescribing operational conditions within a traffic stream asperceived by drivers and/or passengers. A level of servicedefinition generally describes these conditions in terms offactors such as speed and travel time, freedom to manoeuvre,traffic interruptions, comfort and convenience and safety.
Level of Service A provides the best traffic conditions with norestrictions on desired travel speed or overtaking. Level ofService B to D describes progressively worse traffic conditions.Level of Service E occurs when traffic conditions are at or closeto capacity, and there is virtually no freedom to select desiredspeeds or to manoeuvre within the traffic stream. Flow isunstable and minor disturbances within the traffic stream willcause breakdown of flow.
The service flow rate is defined as the maximum hourly rate atwhich vehicles can reasonably be expected to traverse auniform section of a lane or roadway during a given timeperiod under the prevailing traffic and control conditions whilemaintaining a designated level of service. The service flow ratefor LOS E therefore is taken as the capacity of a lane orroadway.
Capacity of rural road sections is influenced by the followingkey characteristics:
● Traffic volume;● Road configuration – such as two lane two way, multi-lane
divided or undivided;● Operating speed;● Terrain;● Lane and shoulder width;● Heavy vehicle (trucks and buses) proportions; and● Grades.
In the case of two lane two way roads the following additionalfactors are important:
● Directional distribution of traffic flow; and● Overtaking opportunities - sight distance, overtaking lanes,
climbing lanes or slow vehicle turnout lanes.
Designers need to consider future traffic demands for a roadsection to determine the required cross sectional configuration.A design period of 20 years is to be considered in determiningcapacity requirements. Consideration should be given to thestaged construction or widening of roads over this period.
Design requirements for rural roads are typically assessed byreference to forecasts of AADT. Design hour volumes may bederived by consideration of the flow pattern across hours ofthe year. A 30th highest hourly volume is often adopted as adesign volume. In areas of high peak demands, such asrecreational routes, special consideration may be required.Research (Ref. 57) has suggested an alternative specification ofthe design volume according to the percentage of traffic forwhich a selected level of service is to be exceeded (eg. provideLOS D or better for 85% of all traffic).
In addition to capacity considerations traffic volume andcomposition is a key input to the structural design of pavement,culverts and bridges. Truck volumes are a critical input.
5 . D E S I G N V E H I C L E
The physical and operating characteristics of vehicles usingmajor rural roads are controls in geometric design. The designvehicle is a hypothetical vehicle whose dimensions andoperating characteristics are used to establish lane width,intersection layout and road geometry. This chapter discussesthe design vehicle for mid-block sections.
Historically, three general classes of vehicles have beenselected for design purposes, namely:
● Design prime mover and semi-trailer (19.0 m);● Design single unit truck/bus (12.5 m); and● Design car (5.0 m).
These three vehicle types are the basic design vehicles for mostroad and traffic design situations.
The 19.0m prime mover and semi-trailer is to be used asthe design vehicle for cross section elements and the caras the design vehicle for horizontal and verticalgeometry.
The geometric design should be checked for the largest designvehicle expected to use the road, as outlined below. Thedimensions of the design vehicles are provided in DesignVehicles and Turning Path Templates (Ref. 36).
Additional considerations for motorcycles are outlined inGuide to Traffic Engineering Practice, Part 15–MotorcycleSafety (Ref. 28).
A functional layout based on the characteristics of a designvehicle should represent an economical level of design thatcaters safely and comfortably for at least 85% of vehicles
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operating in accordance with normal traffic regulations. Largervehicles (33 metre B-triple and 30 metre super B-double) andthose operating under restricted access conditions may also becatered for, but this will usually involve encroachment intoother traffic lanes. This may cause some inconvenience toother road users, but may be acceptable where there is a lowfrequency of occurrence together with the effect of specialconditions associated with the permit.
Where the route is designated for the use of special vehiclesthat fall outside the three general classes (other freightefficient vehicles, over-length buses, type 1 or 2 road trains), orwhere regular use of the route by these vehicles couldreasonably be expected (access to industrial areas, bus routes),the design should satisfy the needs of such vehicles. Theoperation of these vehicles should not be compromised byhaving to encroach into other traffic lanes.
The geometric design should be checked for B-doubles andspecial vehicles where the need is demonstrated and at the areaswhere problems are most likely to occur. Most arterial rural roadsare likely to have some B-double operation even if they are notspecific B-double routes. Table 5.1 describes the provisions thatneed to be made for trucks. These can also be used for specialvehicles. Design guidelines for the various geometric issues in thetable are discussed in subsequent sections.
Guide to Traffic Engineering Practice, Part 5 – Intersections atGrade (Ref. 18) provides detailed guidance on intersection design.
6. ENVIRONMENTAL CONSIDERATIONS
6.1 Traffic Related Intrusion
The various impacts of roads in the rural environment are ofgrowing concern to individuals and communities. It isimportant to fully consider the impact of these issues in anyroad design. Reduction of adverse environmental impactshould be one of the main objectives of any road project.
New rural roads should not only be constructed to link majorrural centres, but also to bypass areas sensitive to trafficimpacts. Good design should aim to ensure that sensitiveenvironments are not disturbed.
The careful design of rural roads can incorporate the means toameliorate the environmental intrusion of road infrastructureand associated traffic. In particular, consideration should begiven to visual amenity through the use of landscaping andcreativity with structures and noise barriers. At the designstage, measures to address safety and access issues for all roadusers will reduce the impact of road projects. Traffic relatedintrusions perceived by people include:
● Visual;● Noise;● Vibration;● Air pollution;● Erosion;● Risk of accidents and intimidation (Chapter 17);● Deterioration of water quality (Chapter 16);● Adverse effect on environmentally sensitive areas; and● Clearing.
6.1.1 Visual
The visual intrusion of a road project can have a dramaticeffect on abutting individual residents and communities. Thevisual amenity of a project can be greatly enhanced by thedesign of creative and functional landscaping. The expense ofvisual landscaping can be shared by the other functions thatthe landscaping will aid, such as soil erosion control,replacement vegetation and amenity.
6.1.2 Noise
The potential for noise disturbance to individuals andcommunities resulting from traffic use of road networks ishigh. Concern regarding the adverse effects of noise in theenvironment has resulted in strict noise regulations beingdeveloped and enforced by relevant authorities.
Factors affecting noise levels that should be considered bydesigners include:
● Number, speed, type and condition of vehicles;
● Road surface type, condition and gradient;
● Distance of the noise sensitive land use from the road(particularly intersections);
● Shielding (natural/built) between the road and noisesensitive area;
● Type of terrain (reflective/absorptive) between the road andnoise sensitive area; and
● Meteorological conditions (prevailing winds).
Methods available to the road designer to reduce the impactof noise from traffic include:
● Where possible, locating the route away from noisesensitive areas;
● Using pavement surfaces that have been developed forreduced tyre/surface noise (eg. open graded friction courseasphalt);
● Using geometric design features that encourage thesmoother flow of traffic, such as flatter grades and theelimination of at-grade intersections;
● Locating the road in a cutting or a tunnel where the effectsof noise are constrained except at the ends. Cuttings,tunnels and retaining walls could be fitted with noiseabsorptive cladding; and
● Providing shielding with landscape features such as earthmounds with appropriate plantings, or with noiseattenuation barriers. These barriers may be an architecturalfeature or designed to blend into the surroundings.Transparent barriers can be used to maintain views.
The required height, location and material type of barriersshould be based on acoustic modelling. Cross-sectional detailto provide for noise barriers is shown on Figure 11.7.
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RURAL ROAD DESIGN10
LOCATION
Intersections
Table 5.1: Provision for Trucks
PROVISION FOR TRUCKS
Provide for the swept paths of trucks. Refer to DesignVehicles and Turning Path Templates. (Ref. 36). Roadsideobstructions shall be located 600 mm clear of the swept paththat is travelled when the vehicle’s wheels are in the tray ofthe kerb and channel.
Provide truck stopping sight distance shown on Table 8.3(b)(lateral sight distance restrictions are often critical, particularlyat intersections in hilly terrain or near bridge piers).
Provide truck stopping sight distance (refer to Table 8.3(b))for intersections on or near crest vertical curves.
Provide truck stopping sight distance (refer to Table 8.3(b)) toallow large/special vehicles to turn safely into each road.
Vehicle stability should be considered for turning movementsby providing radii appropriate for the turning speeds andproviding a uniform rate of change for crossfall.
Provide stopping sight distance to railway crossings, speedchange areas and merge areas such as lane drops.
Horizontal curves As far as possible, avoid locating features that are likely torequire large/special vehicles to break on curves, such asintersections where the major road is on a low radius curve.Note that the extra braking distances required on horizontalcurves are not compensated by higher driver eye height.
Reverse curves Provide a straight 0.6V metre long or transition curvesbetween reverse curves to allow for the spiral tracking oftrucks. Where deceleration is required on the approaches toa lower radius curve, sufficient distance must be provided toenable drivers to react and decelerate.
Compound curves If deceleration is likely to be required, allow sufficientdistance for drivers to react and decelerate. However, the useof compound curves is not desirable.
Transition curves Provide transition curves wherever possible. However, anytransition should involve a shift of >0.25m.
Grades Provide sufficient signs to warn drivers of steep downhillgrades.
Provide adequate sight distance on approaches to curves onsteep downhill grades.
Sag vertical curves
Superelevation
Provide stopping sight distance and adequate clearancebeneath overpasses.
Avoid adverse superelevation where practicable.
The length of superelevation development should beadequate to ensure safe vehicle rotation. Check thatsuperelevation has been increased on downgrades.
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6.1.3 Vibration
Vibration from traffic on rural roads is very unlikely to besignificant and action to ameliorate the intrusion will notusually be necessary.
However, where vibration is an issue, the airborne soundpressure issue can be mitigated through noise attenuation orwindow design.
6.1.4 Air Pollution
Motor vehicles have an adverse effect on air quality. Thisresults from the discharge into the air of reactive and non-reactive pollutants. The amount of vehicle emissions isdependent on traffic volume, composition of traffic, trafficflow characteristics and road geometry. The impact of theadverse effect of the emissions is dependent on topography,meteorological and atmospheric conditions and the distanceof the receptor from the road. On rural roads this intrusion hasminimal effect and need not be considered further.
6.1.5 Erosion
The construction of rural roads can rapidly disturb theenvironment, leaving extensive scars on the landscape. Thecooperation of road engineers, soil conservationists and allpersonnel involved is essential to reduce the impact of roadconstruction on the environment. The large areas cleared byearthmoving equipment during road construction are apotential soil erosion hazard. Areas, that do not have a coverof grass to slow and reduce water runoff, are subject toexcessive water flows and can result in severe loss of soil.Erosion on construction sites can affect adjacent propertiesand cause the sedimentation of private and public lands,streams, water storage dams, rivers, harbours and lakes.Sediment can destroy vegetation and the natural habitat ofnative fauna. Soil erosion and sediment can pose a seriousthreat to the safety, stability and durability of the road itself.
These problems can be greatly reduced if adequate planning isundertaken during design and control measures areimplemented for each stage of construction. An erosionmanagement plan, which has been developed by all responsibleagencies and authorities, shall be the corner stone of rural roadprojects. The best results will be achieved when an erosionmanagement plan, developed by agreement between theresponsible agencies, is in place and a suitably qualified personis engaged to manage and control its implementation.
Specific control measures may include:
● Training construction personnel to understand andimplement the control measures;
● Developing culvert and drainage works prior to majorconstruction;
● Minimising disturbance of natural vegetation cover,particularly adjacent to drainage lines;
● Stockpiling topsoil for later respreading to assist therevegetation of areas disturbed during construction;
● Building sedimentation traps;
● Using earth banks to divert water from disturbed areas;
● Lining drains to prevent scouring and gollying;
● Establishing vegetation using suitable plant species; and
● Implementing an appropriate post-constructionmaintenance program.
The advantages of a properly managed erosion managementplan are:
● Greatly reduced erosion repair costs;
● Marked decrease in down-time following wet weather,resulting in substantial financial benefits;
● Significant improvements in catchment protection and amore acceptable environment adjacent to the site;
● Increased safety.
The cost of erosion and sediment control is likely to be afraction of the total project costs, but the aesthetic and generalbenefits of implementing control measures are far greater.
6.1.6 Environmentally Sensitive Areas
A proposed rural road may highlight other environmentalissues either within or close to the road reserve, such as:
● Native flora and fauna;● Cultural heritage (indigenous and non-indigenous); and● Water quality.
6.1.5 Controlling Erosion
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The construction, use and maintenance of the road must besensitive to these issues. For example, it is important to retainsignificant areas of remnant native vegetation, includinggrasses, in and adjacent to the road reserve. Road design,construction works and maintenance activity should all aim toreduce impact on native flora and fauna habitat.
Identifying and managing any potential impact on sites ofhistorical or archaeological interest should involve a qualifiedarchaeologist and representatives of relevant local AboriginalLand Councils and heritage bodies. If required, a program forarchaeological monitoring should be developed inconsultation with the road authority to determine the mostappropriate construction methods to avoid or reducedisturbance to the site.
Designers should also consider the influence of social issueswhen planning and designing rural roads (see Section 15Community Consultation).
Runoff from the road surface contains pollutants, which canbe detrimental to the receiving waters. When AADT is greaterthan 30,000, the amount of resultant pollutants is very highand the runoff from rural roads should be considered fortreatment over the full length of the project. When AADT isless than 30,000, lengths of a project traversing sensitivereceiving environments should be considered for treatment toimprove the runoff water quality (Ref Section 16.5).
6.1.7 Clearing
The clearing of all forms of vegetation should be kept to aminimum within the works area. Cleared areas rob soil of thenatural protection from erosion, which vegetation provides.Close attention is to be given to determining the extent ofclearing when preparing the erosion protection strategy planfor a project.
6.2 Environmental Related Intrusion
6.2.1 Snow and Ice
Snow and ice can pose a traffic hazard that may requiremaintenance action and signage to accommodate the safepassage of vehicles.
6.2.2 Floods
Many areas are inundated with flood waters that over-top therural road formation. Special signage and possible routerelocations may result from these incidents.
6.2.3 High Winds
High winds that blow adjacent to the road alignment inexposed locations need to be considered in the design stage.The winds can cause concern to all vehicles and specialsignage and wind socks are used to bring the attention of thedriver to the intrusion.
6.2.4 Animals and Birds
Animal and bird intrusion can be in the form of farm (fenced)or station (unfenced) animals or natural animals and birds. Allthese require signage and in some cases road cattle grids or
special crossings or fences to limit intrusion. Cattleunderpasses or overpasses can be installed to allow for thesafe movement of stock. In the case of natural animals thespecial crossings and fences may be installed to provide a safecrossing for migratory reasons.
6.3 References
Guidelines prepared by Austroads (Ref. 31) establish a range ofprocedures to evaluate environmental impacts and summarisethe legislation and operation of Australian Federal and Stateprocedures for use when assessing major road projects.
The impacts that need to be addressed to meet the objectivesof ecologically sustainable development strategy are describedin another Austroads publication (Ref. 35). This strategy is akey document to assist road planners and designers. Furtherconsideration of these issues is set out in Ref. 32 and 39 andvarious environmental protection policies or guidelinesprepared by local environmental authorities.
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6.2.3 High Wind
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7 . S P E E D , U S E D FO R G E O M E T R I CDESIGN
7.1 Introduction
Among the principal parameters used in road design are“stopping distance”, “sight distance”, “curve radius”, “lanewidth” and “superelevation”. As these parameters are relateddirectly to the speed of traffic on the road, one of the firstrequirements in design is to establish the appropriate speed orspeeds to use for design.
Historically, a single “design speed” was used as the basicparameter for each road. Although roads designed in this wayhad consistent minimum design standards, problems arosebecause vehicle operating speeds differed from the designspeed and, in some cases, and the speed difference wassufficient to create a hazard. The most common locationwhere problems occur is at the end of straights where vehicleoperating speeds often exceed the design speed of the curve.
To overcome these problems, designers are now required toobtain more rigorous estimates of 85th percentile vehicleoperating speeds on each element of the road and then toensure that the design speed of every element is either equalto, or greater than, the 85th percentile operating speed onthat element.
The decision to use the 85th percentile operating speed wasbased on:
● The need to overcome the problems associated with theuse of a single design speed as mentioned above;
● Recent design practice in Europe and the USA;
● The premise that drivers of the fastest vehicles, generallytravel in a more alert state than the average driver andtherefore a reduction in reaction time can be assumedwhich will compensate, to some extent, for the differencebetween the operating speed of these vehicles and thedesign speed of the road; and
● Practical constraints. It is not possible for practical reasons(mainly economic) to design for the 100th percentilevehicle.
Operating speeds can either be measured or estimated.Wherever possible, operating speeds should be measured bothfor cars and for trucks. As this is not possible with new roadproposals, operating speeds have to be obtained by othermeans including measurements of speed on similar roads, andestimates using the method described in Sections 7.3 and 7.4.
The estimation procedure in Sections 7.3 to 7.4 was developedto simulate or model the actual behaviour of vehicles on theroad. This correlation between the mathematical model and
actual vehicle operation in the field enables the designers tovisualise themselves in the position of a driver negotiating theroad. Use of this procedure can help designers to identifyfeatures, which could influence the operating speed; it is alsoa useful technique for identifying other problems associatedwith the design.
In addition to simulating vehicle behaviour on curves, theestimation model has the following built-in safety factors:
● The model identifies the use of lateral friction factorswhich exceed specified values; and
● The model identifies the development of excessive speedinconsistencies along the alignment. The model restrictsspeed differences between design elements to less than10km/h and in most cases the difference is significantly lessthan this.
7.2 Explanation of Terminology
7.2.1 Vehicle Speed on Roads
Vehicle speed range is as follows:
High speed: 100km/h or greater
Intermediate: 80km/h to 99km/h
Low speed: 79km/h or less
Driver operating speeds are not constrained by the geometryof the road but by a number of other factors, which include:
● The degree of risk the drivers are prepared to accept;
● Speed limits and the level of policing of these limits; and
● Vehicle performance.
Figure 7.1: Comparison between Observed 85thPercentile Speeds and pre 1980 Curve Speed Standard
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Figure 7.1 indicates that on roads designed for lower speeds,drivers tend to overdrive the road. Conversely on roadsdesigned for higher speeds, drivers adopt an operating speedof 100 km/h to 110 km/h. In some cases, where a speed limitis 110km/h, operating speeds may be higher such as on longdownhill grades.
7.2.2 Operating Speed
The term “Operating Speed” in this guide is the 85thpercentile speed of cars at a time when traffic volumes are low,that is when drivers are free to choose the speed at which theytravel. In effect, this means that designs based on the 85thpercentile speed will cater for the majority of drivers. Fordesign purposes, the 15% of drivers who exceed this speedare considered to be aware of the increased risk they aretaking and are expected to maintain a higher level of alertness,effectively reducing their reaction times.
On straight flat rural roads with low traffic volumes, the 85thpercentile Operating Speed of cars is generally close to110km/h. On higher standard roads with a posted speed of110km/h, the Operating Speed may be marginally higher.
A procedure for estimating vehicle speeds of cars in rural areasis provided in Section 7.3.
7.2.3 Operating Speed of Trucks
The term “Operating Speed of Trucks” is the 85th percentilespeed of trucks at a time when traffic volumes are low.
Operating speeds of trucks are required for checking designdetails such as stopping distances for trucks at intersections.
7.2.4 Section Operating Speed
Vehicle speeds on a series of curves and short straights tend tostabilise at a value related to the range of curve radii. Thisspeed is called the “Section Operating Speed”.
7.2.5 Design Value
Operating Speed is the value adopted for the design of eachelement of the road.
On roads designed for high-speed travel, speeds remainrelatively constant permitting the use of a single design valuefor the road. Note that although operating speeds arerelatively constant, they can differ significantly from the designvalue as indicated on Figure 7.1.
On roads with operating speeds less than 100 km/h, operatingspeeds vary along the length of the road depending on theroad geometry and, to some extent on other factors such asspeed limits and policing. For the design of rural roads, mostweight is given to the effects of the geometry of the road asspeed limits and the level of policing can change. On theseroads operating speed needs to be determined for eachelement of the road. For design purposes on two-waycarriageways, operating speeds are either measured orestimated for each element of the road and for each directionof travel. In many cases the higher of the two values will beadopted as the design value of the curve. There will be somecircumstances where each direction has to be considered
separately, such as on the approaches to intersections. Atintersections, the stopping distance on each approach shouldbe based on the operating speed for that approach. Operatingspeeds can be affected by the frequency of intersections.
7.3 Estimating Operating Speeds onRural Roads
7.3.1 General
The following procedure will enable designers to consider thebehaviour of a typical 85th percentile driver. There are threebasic elements: the driver, the road and the vehicle.
7.3.1.1 Driver Behaviour
Consider first a typical driver approaching a straight section ofroad, which is followed by a series of curves at the end of thestraight.
The driver's initial response will depend on the speed at this timeand the length of straight. If the straight is too short, the driveris likely to continue at the same speed. On longer straights, thedriver will accelerate until terminal speed is reached, which isrelated to the length of straight and the initial speed. They willthen continue at this speed to within approximately 75m of thecurve. The driver then decelerates to a speed, which isconsidered safe for the curve ahead. Truck drivers will generallydecelerate to the appropriate speed for the curve because of thedangers associated with braking trucks on curves. Car drivers arelikely to enter at a speed that is high for the curve as indicatedby some further deceleration, which commonly occurs withinthe first 80m of the curve. Speeds remain at this level until thedriver has a clear view of the curve or straight ahead. If it is astraight, the driver will accelerate out of the curve; if anotherrelatively low radius curve follows, the driver is likely to reducespeed further. This loss of speed continues until the vehiclereaches a speed at which he feels comfortable. This is thesection operating speed for the series of curves. This speed isthen maintained until the end of the section.
7.3.1.2 Road Characteristics
The effect of grading, cross section and surface conditions allimpact on the operating speed. There is insufficientinvestigation to accurately understand their impact but it isimportant to be aware of their characteristics. This is explainedfurther in Sections 7.3.7 to 7.3.9.
7.3.1.3 Vehicle Characteristics
Two design vehicles are considered: cars and the truck (designsemi trailer 19.0m). Speeds are determined first for cars. Truckspeeds are then obtained using Table 7.2.
7.3.2 Operating Speed Estimation Model
The model used to estimate Operating Speeds is based on a largenumber of observations of the behaviour of traffic. TheOperating Speed of vehicles is estimated by establishing theapproach speed of the vehicle for the direction of traffic flowbeing considered. The approach speed is then applied to the firstcurve and an operating speed is read. This speed then becomesthe approach speed for the subsequent curves and separatingstraights. The Operating Speed estimating graphs are:
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Figure 7.2: Acceleration on Straights (Hilly to Mountainous Terrain)
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Figure 7.3: Deceleration on Curves
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● Acceleration on Straights (Figure 7-2),● Deceleration on Curves (Figure 7-3).
7.3.3 Car Acceleration On Straights Graph
The car Acceleration on Straights graph Figure 7.2 allows thedesigner to estimate the speed at which a vehicle canaccelerate over a given length. Large radius curves may beconsidered as straights, as depicted on Figure 7.3, where theOperating Speed from 50 to 120km/h is no longer influencedby a further increase in the radius. The change in speed, readfrom Figure 7.2 assumes that the terrain is constant and themaximum visible length of straight is 1000 metres.
7.3.4 Car Deceleration On Curves Graph
The car Deceleration on Curves graph Figure 7.3, allows thedesigner to estimate the speed to which a vehicle deceleratesto, when entering a given curve radius line, or matches theSection Operating Speed. The intersect with the higher speedvalue is the element Departure Speed. Figure 7.3 allows thedesigner to then consider the given curve radius against theDesirable Minimum Radius and check that it does notapproach the Absolute Minimum Radius for the ApproachSpeed. The example on Figure 7.3 shows an Approach Speedof 100km/h intersecting with a given radius of 320m, resultingin a Departure Speed of 93km/h. The curve radius intersectionis about the Desirable Minimum Radius limit. The DepartureSpeed is more than the Section Operating Speed in this case. Itis necessary to redesign the alignment in those circumstanceswhere the intersection of the Approach Speed and the SectionOperating Speed (or radius) encroaches more than half waytoward the Absolute Minimum Radius line. An acceptablesolution would be for the intersect to be midway or betterbetween the Desirable and Absolute Minimum radius lines. Inthe case of existing roads, adequate signage needs to beprovided to inform drivers of the restricted alignment.
Information required to use this graph includes:
● The approach speed to the curve. This is likely to be either:- The speed on the preceding curve; or- The speed at the end of the preceding straight;
● The length of the curve or straight;
● The section operating speed being considered;
● Radii.
7.3.5 Section Operating Speeds
As previously stated, when drivers travel along a series ofcurves of similar radii, their speed will stabilise at a level atwhich the driver feels comfortable. This is the SectionOperating Speed. The effects of grade, cross-section andpavement conditions, as explained further in Sections 7.3.7,7.3.8 and 7.3.9 may influence Section Operating Speed.
7.3.5.1 Length Of Road to be included in The Study
Section Operating Speeds can be obtained directly from Table7.1. However, as a first step, it is necessary to segment thealignment into sections commencing approximately 1km to
Operating Speed (in sequence)
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Figure 7.5: Single Curve Disparity
Figure 7.6: Road Length Sections
Figure 7.4: Road Study Length
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1.5km before the start of the section for which speedestimates are required.
If, for example, speed estimates were required for the curvesbetween C and I in Figure 7.4, the speed study would extendfrom A to I (Assuming a one way road in the direction from Ato I). If the diagram represented a two-way road, the studywould include the section from A to J.
The extensions are necessary because the first speed estimateat the start of the extensions, at points A and J, are notparticularly accurate. Accuracy then increases with distancedepending on the alignment. The choice of 1.5km isconsidered conservative.
7.3.5.2 Identification of Sections
In some circumstances, the radius of a single curve cannot begrouped with curves to create a section because of thedisparity between the radii. In this instance, the single curvehas to be treated as a section as shown on Figure 7.5.
A series of similarly sized curves, separated by small straights,or spirals that can be grouped together function as a singleelement and drivers will travel along this portion of road at theSection Operating Speed.
Spiral lengths should be divided in two, with the length of thetwo halves being included in the adjoining elements. Table 7.1
only includes radii up to 600m, radii beyond that range should beconsidered as a straight. Also refer Section 7.3.3 and Figure 7.3.
Further research is required to establish a minimum length ofstraight that may be considered as a section. In the meantime,it is suggested that 200m should be adopted as the minimumlength of straight that may be considered as a section. Straights,shorter than 200m have no effect on vehicle operating speed.
It is also considered that:
● Individual curves separated by straights longer than 200mare treated as individual elements.
● Curves inconsistent in radius to the preceding curves whereacceleration is likely are treated as individual elements.
Acceleration occurs whenever speed has been reduced belowthe Section Operating Speed or the section speed. Forexample, the stable speeds on sections 1 and 2 of Figure 7.6could be 70km/h and 80km/h respectively. Speed can thus beexpected to increase on the first few curves of section 2 untilstability is reached at 80km/h. The rate of increase can be 1km/h for every 30m with limited sight distance (Figure 7.2) to1km/h for every 5m with unlimited sight distance (flat toundulating terrain).
Section Operating Speeds for single curve sections and curvegroups are listed in Table 7.1.
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45-65 55 50 180-285 235 84
50-70 60 52 200-310 260 86
55-75 65 54 225-335 280 89
60-85 70 56 245-360 305 91
70-90 80 58 270-390 330 93
75-100 85 60 295-415 355 96
80-105 95 62 320-445 385 98
85-115 100 64 350-475 410 100
90-125 110 66 370-500 440 103
100-140 120 68 400-530 465 105
105-150 130 71 425-560 490 106
110-170 140 73 450-585 520 107
120-190 160 75 480-610 545 108
130-215 175 77 500-640 570 109
145-240 190 79 530+ 600 110
160-260 210 82
Table 7.1: Section Operating Speeds
Range of Radii InSection
(m)
Single CurveSection Radius
(m)
SectionOperating Speed
(km/h)
Range of Radii InSection
(m)
Single CurveSection Radius
(m)
SectionOperating Speed
(km/h)
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In the following example the series of curves are joined byshort straights and transition curves unless otherwise stated.
Example Calculations
Identify individual sections for the alignment shown in Figure 7.7.
Between A and B the curve radii range is from 230m to 320m.
This range fits within the “Range of Radii in Section” columnin Table 7.1, suggesting that 89km/h should be adopted as theSection Operating Speed.
The next section is the straight between points B and C.
Consideration must then be given to the curves betweenpoints C and I where radii range between 165m and 320m.As this range will not fit within any listed in Table 7.1, thecurves must be grouped into two or more sections. Theproblem curve is clearly the one with a radius of 165m. Asthis curve cannot be grouped with any of the adjacent curves
to form a section, it must be treated as a Single CurveSection Radius in Table 7.1.
The curves between C and F range in radii between 270m and320m. This range fits within the section in Table 7.1, which hasa Section Operating Speed of 93km/h.
Section FG is an isolated 165m radius curve section. Interpolatedfrom column 2 in Table 7.1, this curve has a Section OperatingSpeed of 76km/h.
The two curves between G and I both have radii of 300m.From Table 7.1, the section operating speed of this section is91km/h. In this case the Section Operating Speed can beobtained from the single curve column or alternatively bypicking a range of radii which is spread evenly on each side ofthe 300m radius. Both methods give the same result.
The sections identified above are shown diagrammatically onFig. 7.8.
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Figure 7.7: Road Length Detail
Figure 7.8: Section Identification
Figure 7.9: Section Operating Speed
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Section operating speeds within these sections are shown inFig. 7.9.
7.3.6 Estimating Speed on a Section of Road
An estimate of Section Operating Speed is required between Cand I in the direction from C to I on Figure 7.7. For the purposeof this exercise the pavement condition and cross sectionremain constant. The undulating terrain is also constant. Therehas been no allowance for steep grades. See Section 7.3.7 to7.3.9 for further clarification.
As vehicle speed at every site depends on the road geometryon the approaches, it is necessary in all investigations toconsider the alignment for 1km and 1.5km on each approach.As this is a one-way road it is only necessary to consider theapproach between points A and C.
7.3.6.1 Step 1 – Estimate Speed onSection A – B.
From Table 7.1 for radii 230m to 320m the section operatingspeed is 89km/h and the speed at point B may be taken as89km/h.
7.3.6.2 Step 2 – Estimate Speed at Point Cthat is near the end of the straight.
From Figure 7.2, the speed at the end of the straight is100km/h (assuming an initial speed of 89km/h on straight600m long).
7.3.6.3 Step 3 – Estimate Speed at Point D(departure speed on first curve)
On Figure 7.10, follow the 100km/h curve approach speed linedown until in intercepts either with the radius of 320m or withthe section operating speed – determined earlier as 93km/h(whichever comes first). In this case the departure speed forthis curve is 93km/h.
Note the location of the intercept with the radius line. The factthat this is close to the Desirable Minimum indicates that thecurve radius is using desirable lateral friction. The curve isacceptable.
7.3.6.4 Step 4 – Estimate Speed at Point E(departure speed on second curve)
On Figure 7.11, follow the approach speed line (which is now93km/h) to the intercept with the radius or the SectionOperating Speed (whichever comes first). In this case theSection Operating Speed is 93km/h. The departure speed atPoint E is equal to the Section Operating Speed (93km/h).Note also the location of the intercept between the radius(270 m) and the curve speed. In this case it is close to thedesirable minimum radius line. This indicates that the radiusused is desirable.
7.3.6.5 Step 5 – Estimate of Speed atPoint F
As for Step 4, Figure 7.11 can be used again to demonstratethat the Section Operating Speed (93km/h) again prevails.
7.3.6.6 Step 6 – Estimate of Speed at Point G
On Figure 7.12, follow the approach speed line (93km/h)down to the intercept with the radius (165m) or the SectionOperating Speed (76km/h) whichever comes first. The radiusintersect is first and the departure speed is 81 km/h.
The intersection of the approach speed 93km/h and theradius 165 m is at the Absolute Minimum Radius line. Thisis an unacceptable solution. The radius needs to beincreased to relocate the intercept of the Approach Speed(93km/h) and the radius to at least midway between theDesirable and Absolute Minimum Radii for the ApproachSpeed (93km/h). If this cannot be achieved, warning curvesigns need to be provided to inform drivers of the restrictedalignment.
A radius of 220m intersects midway and results in a DepartureSpeed of 85km/h.
In absolute situations where it is unavoidable to increase sucha radius careful attention needs to be given to clearing runoffareas, sight distance lines, lighting and sufficient advancedwarning signs, in an attempt to minimise the potential foraccidents.
It is desirable to redesign the alignment in those circumstanceswhere the intersection of the approach speed and the SectionOperating Speed (or radius) is to the left of the absoluteminimum radius line.
7.3.6.7 Step 7 – Estimate of Speed at Point H and I
The 85th percentile vehicle, having significantly reduced speedon curve F G Figure 7.9 will accelerate on subsequentelements. Most drivers will attempt to achieve the sectionoperating speed again, provided the driver can see somebenefit. The driver will not accelerate over a short length onlyto decelerate around another tight curve. Acceleration willapply on both straights and curves provided the driver doesn’texceed the element operating speed. The acceleration onstraights graph can be used to estimate the increase in speed.If the distance between points G and I was 310m withapproach speed 81km/h, from Figure 7.2, the approximatespeed of a car at point I would be 88km/h.
7.3.7 Effects Of Grades
Insufficient information is available to provide firm guidelineson the effect of grades. However, designers are expected toconsider the grading and make adjustments to speedestimates, refer Table 10.1. The following assumptions canbe made.
These corrections for grade must be made for each element ofthe road as the speed estimate is made.
● The operating speed of cars may be reduced on up hillgrades longer than 200m.
● The operating speed of laden trucks will be significantlyreduced on long up hill grades.
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Figure 7.11: Speed at Point ‘E’ & Point ‘F’ (see Figure 7.3)
Figure 7.10: Speed at Point ‘D’ (see Figure 7.3)
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Figure 7.12: Speed at Point ‘G’ (see Figure 7.3)
● Cars will generally travel at the operating speed on steepdown hill grades, however, some increase could beexpected toward the end of the down hill grade.
● Trucks may be required to significantly reduce their speedprior to steep down hill grades.
Corrections for grade should be considered for each elementof the road. This is particularly necessary when there is asignificant change in topography.
7.3.8 Effect of Cross-Section
Speed estimates in preceding sections are appropriate fortypical road cross-sections, such as those with traffic laneswider than 3m. On roads with lanes narrower than 3m, thespeed estimates can be reduced by up to 3km/h.
7.3.9 Effect of Pavement Condition
Average pavement conditions were assumed for the speedestimates in the preceding sections. On roads with poor orbroken surfaces, speeds can be reduced by 5km/h to 10km/h.
7.3.10 Use of Operating Speed in theDesign of Rural Roads
The normal design procedure is to prepare a preliminaryalignment and grading with standards that are as high aspossible within realistic constraints. The minimum standardsused must be appropriate for the terrain, consistent with the
hierarchy of the road and either equal to or greater than thepredicted 85th percentile operating speed for the road withconsideration given to both cars and trucks.
If the road being designed is a high-speed road with operatingspeeds of 110km/h, then a single operating speed can beadopted and the road designed using this speed to select thedesign standards used.
On other roads the operating speeds will vary along the lengthof the road. The basic steps to be followed in the design of thistype of road are listed below:
● Prepare a draft alignment and grading in the normalmanner taking into account desirable minimum curve radii,road hierarchy and terrain. A design feature is the use ofrelatively large radii at the end of straights where highspeeds can be expected;
● Using the draft alignment, estimate the operating speedsin each direction of travel using the procedure outlined inSections 7.3 to 7.5. The location of intersection points onthe deceleration on curves graph will indicate whether thedesign is appropriate or not. If any of the intersectionpoints between the curve radius and operating speed lieon the left of the desirable minimum radius line, then someadjustments will be required, either to the design to reducethe approach speed to the curve, or to increase the radiusof the curve – usually the latter;
● Modify the alignment;
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● Check the operating speeds on the modified alignment.(Repeat if necessary until all intercept points on the speedon curve graph are either on, or to the right of thedesirable minimum radius line).
● If a very short length smaller radius curve exists, the driverusually transitions the vehicle path to a larger radius thanthe curve centre line. A short length curve can therefore bedefined as a curve where the radius of the transitioneddriver path is considerably greater than the radius of thecentre line of the roadway. Any short length curves canusually be found by visual inspection of the alignment. Theradius of the transitioned driver path can be obtained byassuming a 2m wide vehicle approaching and departingthe curve in the centre of the lane and transitioning to justtouch the centre line of the roadway or the edge linemidway around the curve. By using this method largerradius curves can be used in the analysis of short absoluteminimum radius curves.
● Check that the maximum difference in speeds betweendesign elements does not exceed 10km/h.
● Compare the operating speeds for each direction of trafficon each element of the roadway (other than those atintersections) and adopt the higher of the two speeds as thedesign speed for each element. Where intersections areinvolved, both operating speeds have to be used as speedson each approach can differ and the appropriate speed hasto be used for sight distance checks on each approach;
● Check sight distances on all curves noting where benchingis likely to be required. It is often impractical in steepcountry to meet the sight distance requirements. In thesecircumstances consideration should be given to alternativetreatments such as the use of sealed shoulders of sufficientwidth to enable one vehicle to manoeuvre around astationary vehicle in the lane ahead.
● Using the checklist in Table 5.1, check the alignment forpotential problem sites for trucks. If any problem areas areidentified, then it is necessary to estimate the 85thpercentile truck operating speeds for each site. Truck sightdistances can then be checked. If the site proves to be aproblem for trucks, the design should be reviewed and, ifnecessary, amended;
● Prepare superelevation diagrams based on the criticalspeeds obtained for each element; and
● Prepare detail design plans for the project.
7.4 Operating Speed of Trucks
As with cars, truck speeds should be measured whereverpossible. Where it is not practical to measure the speed oftrucks, speed has to be estimated. The following rules shouldbe used as a guide:
● On high-speed roads, truck speeds can be taken to be thesame as that of cars.
● Provided sufficient length of acceleration is available, truckspeeds will closely match car speeds on flat terrain.
● Otherwise, truck speeds in Table 7.2 may be used.
Additional factors that support the truck speeds in table 7.2are:
● The lower operating speed for trucks is an averagecondition with truck speeds varying more than car speedsdue to grades, poorer acceleration etc.
● When checking braking and stopping sight distanceprovision for trucks, it is acceptable to use the lower truckoperating speed for a corresponding car operating speed.This is because an acceptable level of safety is providedthrough the assumptions of:- Wet conditions- Unladen state- No antilock braking system
No further reduction in operating speed due to wet conditions.
Table 7.2 Truck/Car Speed Relationships
Car Speed110 100 90 80 70 60 50(km/h)
Truck Speed110* 100* 80 70 60 52 43(km/h)
Note: *On high-speed rural roads truck speeds equal caroperating speeds.
7.5 Use Of Truck Operating Speeds
Although the basic design vehicle for road alignments is stillthe car, designers are now required to check all designs toensure that they are safe for trucks. Specific locations whereproviding for trucks is likely to be required are listed in Table5.1.
Further research is required to determine the speed of truckson individual geometric elements and the maximum allowabledecrease in speeds between successive geometric elements.
8. SIGHT DISTANCES
8.1 General
The principal aim in road design is to ensure that the driver isable to see any possible road hazards in sufficient time to takeaction to avoid mishap. To provide a calculable parameter thatcan be related to the geometry of the road, the concept ofSight Distance is used. This concept is based on a number ofsomewhat stylised assumptions of particular hazards andcorresponding driver behaviour. The hazard is assumed to bean object of sufficient size to cause a driver to take evasiveaction, intruding into the driver’s field of view. Specific valuesare assumed for the driver’s reaction time (though in practicethere would be a distribution of values) and the dimensionsdetermining the geometry of the sight line.
Normally, selection of extreme values for every parameter isnot appropriate, as the probability of all factors occurring
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together is extremely low, and the resultant designs wouldbecome impractical. The assumed parameter values lead tosight distances that produce a satisfactory design. Greaterdistances allow for less probable hazard situations and thusproduce greater margins of safety. Subject to their effect onovertaking and economics they may be an advantage. Wherepossible the sight distance provided should be greater than thevalues used in these guidelines.
Adequate sight distance is essential for safe and efficienttraffic operation. The designer should consider the length ofvertical curves, the radius of horizontal curves and the terrainon the inside of horizontal curves in providing adequate sightdistance.
This section does not consider sight distance at intersections.For required sight distances at intersections, includingroundabouts, refer to Guide to Traffic Engineering Practice,Part 5 and 6 (Ref. 18 and 19).
8.2 Sight Distance Parameters
When determining sight distance, assumptions must be madeabout the following elements:
● Object height;● Driver eye height; and● Driver reaction time.
Sight distance is measured between the driver eyes and anobject or pavement marking on the road ahead, as shown onFigure 8.1.
An object in view may not always be perceived. There isevidence that when a driver is travelling on sharp curves orwhen the vehicle is rapidly accelerating or decelerating and thedriver is subject to unusual forces, his ability to perceive anobject is reduced. Fatigue and drugs add to the time ofperception and may increase an individual’s reaction time.
8.2.1 Object Height
The object height to be used in the calculation of stoppingsight distance is a compromise between the length of sightdistance and the cost of construction. Stopping is generally inresponse to another vehicle or large hazard in the roadway. Torecognise a vehicle as a hazard at night, a line of sight to its
head lights or taillights would be necessary. Larger objectswould be visible sooner and provide longer stopping distances.
To perceive a very small hazard, such as a surface defect, a zeroobject height would be necessary. However, at the requiredstopping sight distances for high speeds, small pavementvariations and small objects (especially at night) may not bevisible to most drivers. Thus, most drivers travelling at highspeeds would have difficulty in stopping before such a smallobstruction.
The length of vertical curve required at crests increasessignificantly as the object height approaches zero. The generalfigure adopted which produces satisfactory design is 200mm.
Lower object heights, even zero, can be used at intersections,where it is necessary to see road markings, and at locationssuch as causeways, floodways and cuttings, kerb and channelnoses, where there is a high probability of water, rocks or otherdebris being on the road (Ref. 51).
For geometric design of rural roads the object heights shownin Table 8.1 are to be used.
Table 8.1: Object Heights
Object height Situation
0.0m • Intersection design(Pavement) • Sight to line-marking configuration
0.2m • Mid-block crest curve design(Object) • Horizontal curve line of sight
0.6m • Impact on vertical clearance(Car taillights) • Sight to vehicles at end of (Car traffic indicator) intersection queues
• Sight over roadside safety barrierinstallations
8.2.2 Driver Eye Height
Driver eye height is a combination of the height of driverstature and driver seat height. A number of studies (eg. Ref. 4,41, 50 and 64) have investigated car driver eye height trendsand found that they have progressively reduced over time,
Figure 8.1: Sight Distance
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consistent with the changing vehicle fleet. Historically andinternationally, car driver eye heights used range between 1.15m and 1.00 m. Based upon recent research and considerationof the characteristics of the vehicle fleet and the ageing ofdrivers, a car driver eye height of 1.05 m is to be used forthe geometric design of rural roads.
For general geometric design a truck driver eye height of2.4m is to be used. The 2.4m value for sag curves isparticularly important for checking the effect of overheadstructures on sight distance.
The reduction of car driver eye height will have implications ongeometric design elements (such as length of vertical curves)used in other road design publications, which should beconsidered by the designer when this guide is used inconjunction with previously published guides.
8.2.3 Driver Reaction Time
Reaction time is the time for a driver to perceive and react toa particular stimulus and take appropriate action. This timedepends on the complexity of the decision or task involved.
Research studies have shown that an average reaction time of2.5 seconds is typical although the variance of the distributionof reaction times is very high (Ref. 6, 54, 68 and 94). Values ofup to 7 seconds have been recorded at one extreme, and atthe other extreme, 1.0 second has been measured with forcedstops (Ref. 6). One reason for the large variability is thatreaction time depends on a driver’s level of alertness at thetime. Similarly, anticipation or pre-signalling of an event, theabsence of uncertainty on multiple choices, and the familiaritywith the task can each lower reaction time.
Given the above, it has been reported that most drivers canreact simply to a clear stimulus in less than 2.5 seconds in anurgent situation. This represents an upper (possibly the 85thpercentile) value for normal drivers and is close to the mean fordegraded drivers (Ref. 94). Consequently, the reaction time of2.5 seconds is a commonly adopted value, although a numberof European countries specify a value of 2.0 seconds.
A recent study investigating road safety and design for olderdrivers (Ref. 53) recommended a minimum reaction time of2.5 seconds at intersections. For mid-block sections a desirableminimum reaction time of 2.5 seconds and an absoluteminimum of 2.0 seconds is to be used. The aging of drivers(refer to Section 8.2.4) emphasizes the importance of thesevalues.
A driver reaction time of 2.5 seconds is to be used in thisGuide for the geometric design of rural roads. However,in mid-block situations where there is an expectation forincreased driver alertness, such as locations with additionalsigns or line marking, or where it may not be practicable todesign for a 2.5 second reaction time, such as low speedalignments in difficult terrain, a minimum reaction time of 2.0seconds may be considered.
It is noted that the driver reaction time will have implicationson geometric design elements (such as sight distance) used inother road design publications, which should be considered bythe designer when this Guide is used in conjunction withpreviously published guides.
For truck drivers, the 2.5 second time actually consists of a 2.0second initial reaction time (which is a reflection of the factthat truck drivers are professional drivers and in traffic, areusually able to see over vehicles in front) plus a 0.5 secondinherent delay in the operation of the air brake system that isused on heavy vehicles (see Ref 55). Braking tests by MackTrucks Australia support this time delay, being in the range of0.47 seconds to 0.6 seconds.
8.2.4 Ageing of Drivers
As people age, they experience decreasing physical and mentalcapabilities and become more susceptible to injury and shock.Human functions subject to deterioration due to ageing include:
● Visual ability;● Attention capacity;● Reaction time; and● Contrast sensitivity.
As a group, older drivers do not currently represent a majorroad safety problem in most Western societies when comparedwith other age groups. However, older drivers are involved insignificantly more serious injury and casualty crashes perkilometre travelled. Furthermore, as the proportion of olderpeople in Australia and New Zealand is expected to roughlydouble over the next 40 years, older drivers are likely to becomea more significant problem in the years ahead (Ref. 53).
Recent research (Ref. 53) indicates that a number of roaddesign elements may be associated with older driver crashes inAustralasia. In particular, it was concluded that improvementsto intersection sight distances, provision for separate turnphases at traffic signals, more conspicuous traffic signallanterns and more clearly defined vehicle paths have thepotential to reduce crash and injury risk for older drivers. Theresearch includes a detailed description of measures thatshould be implemented immediately in Australia to increasethe safety of older road users.
8.3 Stopping Sight Distance (SSD)
Stopping sight distance is the distance to enable a normallyalert driver, travelling at the design speed on wet pavement, toperceive, react and brake to a stop before reaching a hazardon the road ahead. This distance is considered to be theminimum sight distance that should be available to a driver.
8.3.1 Derivation
Stopping sight distance has two components, namely thedistance travelled during the driver’s perception-reaction timeand distance travelled during braking.
SSD = d1 + d2
whered1 = reaction distance = (m)
d2 = braking distance = (m)
RT = reaction time (2.5 secs)
V = operating speed (km/h)
F = longitudinal friction factor
g1 = longitudinal grade (%, + for upgrades and – for downgrades).
RURAL ROAD DESIGN26
(RTV)3.6
(V2)254(F+0.01g1)
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Values of RT (from Section 8.2.3) and F must be assumed inorder to compute the SSD appropriate to the operating speed,Table 8.2.
8.3.2 Longitudinal Friction Factor
The longitudinal friction factor is a measure of the longitudinalfriction between the vehicle tyres and the road surface. Itdepends on factors such as the speed of the vehicle, the tyrecondition and pressure, the type of road surface and itscondition, including whether it is wet or dry. Currently designvalues of the longitudinal friction factor for bituminous andconcrete surfaces are shown in Table 8.2.
The review of available literature indicates that the longitudinalfriction factors for cars that are currently in use appear toohigh relative to the actual friction that can be confidentlyexpected on wet surfaces. The friction factors appear to havebeen increased relative to those given in NAASRA, 1973“Policy for geometric design of rural roads (Metric Units)”without direct vindication.
McLean (Ref. 71) notes that the limiting values for longitudinalfriction factor were based on producing stopping sightdistance requirements leading to what was considered to bean appropriate balance between horizontal and crest verticalcurve standards. The balance achieved appears to be generallyconsistent with international practice, although, relative toNorth America and earlier Australian (1973) practice,minimum sight distance requirements are a little low.
Concerns have been raised in relation to the high values oflongitudinal friction factor for trucks. However, little mentionof truck longitudinal friction factors is given in current or pastresearch literature. The adopted figures in Table 8.2 were
derived from US research (Ref. 55) and were based on thebehavior of an empty prime mover-trailer combination on awet pavement.
8.3.3 Car to Road Object Stopping Sight Distance
The concept of car stopping sight distance is illustrated inFigure 8.2. It is measured between the driver’s eye and a smallobject on the road.
SSD values for cars are calculated using the adoptedlongitudinal friction factor values, are shown in Table 8.3(a).
8.3.4 Truck to Road Object Stopping Sight Distance
A comparison of international sight distance design practices(Ref. 56) noted that SSD only refers to cars. Truck stoppingsight distance is not considered by most of the countriesreviewed. A typical reason for this can be found in AASHTO(Ref. 1):
“The derived minimum stopping sight distances directly reflectpassenger car operation and might be questioned for use indesign for truck operations. Trucks as a whole, especially thelarger and heavier units, require longer stopping distances fora given speed than passenger vehicles do. However, there isone factor that tends to balance the additional braking lengthsfor trucks for given speeds with those for passenger cars. Thetruck operator is able to see the vertical features of theobstruction substantially farther because of the higher positionof the seat in the vehicle. Separate stopping sight distances fortrucks and passenger cars, therefore, are not used in highwaydesign standards.”
RURAL ROAD DESIGN 27
Operating Speed (km/h)
50 60 70 80 90 100 110 120 130
Cars 0.52 0.48 0.45 0.43 0.41 0.39 0.37 0.35 0.35
Trucks 0.29 0.29 0.29 0.29 0.29 0.28 0.26 0.25 0.24*
*Extrapolated
Table 8.2: Longitudinal Friction Factors
Figure 8.2: Stopping Sight Distance
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Operating Speed (km/h)
50 60 70 80 90 100 110 120 130
Longitudinal Friction Factor 0.52 0.48 0.45 0.43 0.41 0.39 0.37 0.35 0.35
SSD 2.5 Des. min. 54 71 91 114 140 170 205 245 280
(m, level grade) 2.0 Abs. min. 47 63 82 103 128 157 190 229 262
Correction for Grade (m)
Upgrade 2 % - -1 -2 -3 -4 -5 -7 -9 -11
4 % -1 -2 -4 -5 -7 -9 -13 -17 -21
6 % -2 -3 -5 -7 -10 -14 -18 -24 -31
8 % -3 -4 -7 -9 -13 -17 -23 -30 -38
Downgrade - 2 % - 1 2 3 4 6 7 10 14
- 4 % 2 3 4 6 8 12 16 21 27
- 6 % 3 4 7 10 13 18 25 34 44
- 8 % 4 6 9 13 19 26 36 48 62
However, this is quite contrary to the findings of a review ofreferences on truck performance characteristics (Ref. 48, 52and 87), which suggest that the sight distance advantagesprovided by the higher driver eye level in trucks do notcompensate for the inferior braking of trucks. Particularly atlocations with lateral sight distance restrictions, the benefits ofthe higher eye level could be lost and provision of longer SSDor other remedial measures such as signing and higher frictionsurfaces would be needed.
The reasons for the longer truck braking distances include:
● Poor braking characteristics of empty trucks.Empty trucks have poor braking characteristics and this isreflected in comparatively high crash rates. The problemrelates to the suspension and tyres, which are designed formaximum efficiency under load.
● Uneven load between axles.When the load is not evenly distributed between axles, oneaxle can slip sideways and create instability in others (up to15% of braking efficiencies can be lost).
● Inefficient brakes of articulated trucks.Fifty percent of trucks tested on the roads in the US couldnot meet the required braking standards. Many driversimmobilise their front brakes to reduce the possibility ofjack-knifing.
● Effect of road curvature.Trucks require longer SSD on curves than on straightsbecause some of the friction available at the road/tyreinterface is used to hold the vehicle in a circular path.
● The braking of articulated vehicles must be in the form ofcontrolled braking without wheel locking in order to avoidjackknifing if wheels lock at different times. Without theaid of antilock braking systems, the friction coefficientused in controlled braking is usually less than that forlocked wheel braking. The friction coefficient for cars inTable 8.2 involve locked wheel braking.
● Truck tyres are designed primarily for wear resistance.Consequently, they tend to have lower wet frictioncoefficients than cars.
In situations where driver eye height provides no advantage,the only parameter that offsets the poorer brakingperformance of trucks is the assumed lower operating speedas per Table 7.2. Therefore, some further justification or basisof the truck operating speeds should be given. For example:
● The lower operating speed for trucks is an averagecondition with truck speeds varying more than car speedsdue to grades, poorer acceleration, etc.
● When checking braking and stopping sight distanceprovision for trucks, it is acceptable to use the lower truckoperating speed for a corresponding car operating speed.This is because an acceptable level of safety is providedthrough the assumptions of:
● Wet conditions;● Unlade state;● No antilock braking system; and● There is no additional assumption of a reduction in
operating speed due to wet conditions.
RURAL ROAD DESIGN28
Table 8.3(a): Minimum Car Stopping Sight Distances (1.05m to 0.2m)
Note:• Desirable minimum stopping sight distances are calculated for a reaction time of 2.5 seconds and absolute minimum stopping
sight distances are calculated for a reaction time of 2.0 seconds.• Corrected stopping sight distances should be rounded conservatively to the nearest 5 metres.
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Figure 8.3: Truck Stopping Sight Distance
Table 8.3(b): Minimum Truck Stopping Sight Distances (2.4m to 0.2m)
Operating Speed (km/h)
50 60 70 80 90 100 110 120 130
Longitudinal Friction Factor 0.29 0.29 0.29 0.29 0.29 0.28 0.26 0.25 0.24*
SSD 2.5 Des. min. 69 91 116 143 173 210 259 310 367
(m, level grade) 2.0 Abs. min. 62 82 106 131 160 197 244 294 349
Correction for Grade (m)
Upgrade 2 % -6 -9 -12 -16 -20 -24 -30 -35 -42
4 % -11 -16 -22 -28 -36 -44 -53 -64 -78
6 % -15 -22 -30 -39 -49 -60 -73 -87 -110
8 % -19 -27 -36 -47 -60 -74 -90 -107 -125
Downgrade - 2 % 8 11 15 20 25 31 37 45 55
- 4 % 18 26 35 46 58 71 86 103 122
- 6 % 32 46 62 81 102 126 153 182 212
- 8 % 52 74 101 132 167 206 249 296 345
Note:• Desirable minimum stopping sight distances are calculated for a reaction time of 2.5 seconds and absolute minimum stopping
sight distances are calculated for a reaction time of 2.0 seconds.• Corrected stopping sight distances should be rounded conservatively to the nearest 5 metres.* Extrapolated
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To balance between the costs and benefits for makingprovision for trucks, rural roads are to be designed to cater forcars. Truck stopping sight distances should be used forchecking purposes at locations that could be potentiallyhazardous for trucks (as summarised in Table 5.1).
At crest and sag points truck stopping sight distance ismeasured as shown on Figure 8.3.
The designer should consider measures such as additionalsigns and line marking to improve safety if stopping sightdistance is found to be inadequate for trucks and it is notpossible to improve the geometric design. However, it isemphasised that signage and line marking are not substitutesfor achieving standard design practices.
SSD values for trucks have been calculated using the adoptedlongitudinal friction factor values are shown in Table 8.3(b).
8.4 Overtaking Sight Distance
8.4.1 General
Overtaking sight distance is the distance required for the driverof a vehicle to safely overtake a slower moving vehicle withoutinterfering with the speed of an oncoming vehicle. It ismeasured between the driver’s eyes of the overtaking andoncoming vehicles.
Overtaking sight distance is considered only on two-lane two-way roads. On these roads, the overtaking of slower movingvehicles is only possible when there is a suitable gap in theoncoming traffic accompanied by sufficient sight distance andappropriate line marking. Sections with adequate overtakingsight distance should be provided as frequently as possible, asthey are an essential safety measure by reducing driverfrustration and risk taking. The desirable frequency is related tothe operating speed, traffic volume and composition, terrainand construction cost. Overtaking demand increases rapidly astraffic volume increases, while overtaking capacity in theopposing lane decreases as volume increases. As a general rule,if overtaking sight distance cannot be economically provided atleast once in each 5km of road or V/20 which is 3 to 5 minutesof driving time apart, (Ref. 95), consideration should be givento the construction of overtaking lanes (Refer Section 13.4.1).
In practice, overtaking zones will usually be the fortuitousresult of road alignment and cross section. Because of thelarge sight distances involved, it is often not practical toachieve overtaking zones through design alone (costly toprovide). However, good design practice will include a checkon the overtaking zones that are provided and may result incases where an overtaking zone can be achieved through apractical refinement of the design. More commonly though,the proportion of road that provides overtaking is used inconjunction with traffic volumes to assess the level of serviceprovided by a section of road and hence determine whetherovertaking lanes are warranted.
The Austroads parameters for determining the start and finishof overtaking zones dictate that there are few passingopportunities on New Zealand roads. The New Zealandpractice to provide a desirable minimum overtaking sightdistance for vertical curve design is to double safe stoppingsight distance.
8.4.2 Overtaking Model
The overtaking manoeuvre has a large number of variables:
● The judgement of the overtaking driver and the risks he isprepared to take;
● The speed and size of vehicles to be overtaken;
● The speed of the overtaking vehicle;
● The speed of a potential on-coming vehicle; and
● The evasive action or braking undertaken by the vehicle orthe overtaken vehicle.
Since the 6th edition of this guide, overtaking has beenassessed by means of a model that was derived from researchinto overtaking on Australian Rural Roads (Ref 95). There aretwo main considerations with the Overtaking Model: ReferFigure 8.4
● Establishment: A minimum sight distance that is adequateto encourage a given proportion of drivers to commencean overtaking manoeuvre. This is called the EstablishmentSight Distance (ED) as it establishes a length of road as apotential overtaking zone.
● Continuation: A critical sight distance, which if maintainedfor some length of road after the ED has become available,will enable an overtaking driver to either complete orabandon a manoeuvre already commenced with safety.This is called the Overtaking Continuation Sight Distance(OSD). After the establishment sight distance first becomesavailable, an overtaking zone is assumed to extend as longas this shorter distance remains available, subject to theconstraint in the next paragraph.
8.4.3 Determination of OvertakingProvision
ARRB has carried out a major research project on overtakingon Australian rural roads. (Ref. 95). The values in this Guide
RURAL ROAD DESIGN30
8.4.2 Overtaking Sight Distance
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(refer Tables 8.4 (a) & 8.4 (b)) are the distances for the 85thpercentile overtaking manoeuvres, adopted from the research.These distances indicate the overtaking sight distances to beused in determining the overtaking zones on MCV (MultipleCombination Vehicle) routes.
Briefly:
● Establishment Sight Distance is derived from the size of thetime gap accepted by a potential overtaking driver and isderived by the time taken to complete phases 1,2,3 and 4of the total manoeuvre (see Figure 8.4).
ED = GT85
where:
GT85 = 85th%ile critical time gap secs.u = V/1.17 (speed of slow vehicle)V = operating speed
● Continuation Sight Distance is derived from the timetaken to complete phases 2 and 3 of the manoeuvre (seeFigure 8.4).
● The oncoming vehicle is assumed to travel at the operatingspeed.
● The overtaken vehicle is assumed to travel at a lesserspeed, taken as the mean speed for its direction of travel.
● The sight distances with the 1.05m driver eye height to1.05m object height are used in this guide.
● The distance travelled by oncoming traffic is represented inFigure 8.4 by phase 4.
In checking a length of road, the OSD will be found to be thecritical parameter in allocating a ‘percent allowing overtaking’to the road section. The OSD ensures that the road distanceused by the overtaking vehicle would be visible at the ‘point ofno return’, and an approaching vehicle would be visible if it iswithin the zone where it could affect the manoeuvre.
8.4.4 Determination of Percentage ofRoad Providing Overtaking
Sections of road assumed to provide overtaking will:
● Commence at a point where ED is available; and
● Terminate where OSD ceases to be available, or alternativelyat a distance equal to Operating Speed divided by 20 (km)from the last location where ED was available if this is less thanthe length over which OSD has been maintained. As longas the OSD remains available, any overtaking manoeuvrecommenced can be successfully completed. However if theED does not occur again at intervals, insufficient drivers will beencouraged to commence overtaking, and capacity (at highvolumes) or quality of service (at low volumes) will suffer. Thedistance equal to Operating Speed divided by 20 should betreated as an approximate rather than a precise figure. Itcorresponds to about 3 to 5 minutes travel time.
The Operating Speeds to be used in selection of the overtakingdistances will be the Section Operating Speed over a length ofroad in both directions. A section of road must be used ratherthan an individual geometric element, as Operating Speed mayvary. Also, since one element in the overtaking provision is thespeed of the oncoming vehicle, and as Operating Speed may varyby direction of travel, the mean of both directions must be used.
The proportion of road offering overtaking provision is the sumof such sections, divided by the overall length of the roadsection being considered.
O.P. = x 100
where:
O.P. = Proportion of road offering overtaking provision (%)∑O.L’s = Sum of overtaking lengths in road section (m)T.S.L. = Total road section length (m)
The sight distances to be used in the analysis of overtaking arepresented in Table 8.4. The time gaps from which they werederived are also shown.
RURAL ROAD DESIGN 31
Figure 8.4: Overtaking Manoeuvre
∑O.L’sTSL
(V + u)3.6
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RURAL ROAD DESIGN32
70 50 50 490 510 540 580 260 280 310 350
80 59 59 610 630 670 730 320 340 380 430
90 67 67 740 770 820 890 370 400 460 530
100 76 76 890 930 990 1,080 450 490 550 650
110 84 84 1,070 1,120 1,200 1,310 540 580 660 770
Given a low eye height of 1.05m, most car drivers cannot adequately distinguish differences in sight distance for values greaterthan about 1000m. Therefore, listed sight distance values greater than 1000m can be assumed to be satisfied whenever theactual sight distance exceeds 1000m.
The listed sight distance values have been derived from the Troutbeck (1981) overtaking model. Sight distance values have beenrounded to the nearest 10m. Given the inherent level of precision in the overtaking model, it would be incorrect to determinethat an overtaking zone does not exist when the actual sight distance falls below a relevant listed value by about 10m.
Table 8.4 (a): Overtaking Sight Distances for Determining Overtaking Zones on MCV Routes when MCV speeds are10km/h less than the Operating Speed.
Road Section
OperatingSpeed(km/h)
OvertakenVehicle speed
(km/h)
EstablishmentSight Distance
(m)
ContinuationSight Distance
(m)
OvertakenVehicle
Semi-trailerB-Dble
RoadTrains
PrimemoverSemi-trailer
B-Double Type 1Road Train
Type 2Road Train
PrimemoverSemi-trailer
B-Double Type 1Road Train
Type 2
Road Train
70 60 60 570 600 640 690 300 320 360 420
80 69 69 710 740 790 860 370 400 450 510
90 77 77 850 890 950 1,040 440 470 530 620
100 86 84 1,020 1,070 1,130 1,240 530 560 630 740
110 94 84 1,230 1,290 1,200 1,310 620 680 660 770
Given a low eye height of 1.05m, most car drivers cannot adequately distinguish differences in sight distance for values greaterthan about 1000m. Therefore, listed sight distance values greater than 1000m can be assumed to be satisfied whenever theactual sight distance exceeds 1000m.
The listed sight distance values have been derived from the Troutbeck (1981) overtaking model. Sight distance values have beenrounded to the nearest 10m. Given the inherent level of precision in the overtaking model, it would be incorrect to determinethat an overtaking zone does not exist when the actual sight distance falls below a relevant listed value by 10m.
RoadSection
OperatingSpeed(km/h)
OvertakenVehicle speed
(km/h)
EstablishmentSight Distance
(m)
ContinuationSight Distance
(m)
OvertakenVehicle
Semi-trailerB-Dble
RoadTrains
PrimemoverSemi-trailer
B-Double Type 1RoadTrain
Type 2RoadTrain
PrimemoverSemi-trailer
B-Double Type 1RoadTrain
Type 2RoadTrain
Table 8.4 (b): Overtaking Sight Distances for Determining Overtaking Zones on MCV Routes when MCV speeds areequal to the Operating Speed.
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8.5 Manoeuvre Sight Distance
Designers shall make every effort to provide car stopping sightdistance along traffic lanes on all roads. However, in somecircumstances manoeuvre sight distance (MSD) may be used toavoid costly construction. MSD is generally only 6% less thanSSD. MSD, therefore, is the absolute sight distance that mustbe provided. For example, on a two-lane two-way road, it maybe much cheaper to provide full width paved shoulders on anexisting substandard crest curve than to reconstruct withimproved vertical geometry.
Manoeuvre sight distance may be used on isolated verticalcurves on a straight or sufficiently large radius horizontal curvewhere lowering of the grade line would mean expensiveexcavation into hard rock materials or major geologicalproblems. Manoeuvre sight distance should not be used on ahorizontal curve with a radius that requires close to theabsolute maximum side friction. The designer must ensure thatthe pavement width is sufficient to enable drivers tomanoeuvre around stationary or slow moving vehicles or anobject on the road. Sealed shoulders with a desirable minimumwidth of 2.5m (or absolute minimum width of 1.5m) canprovide a reasonable space for evasive action provided thecombined seal width of lane plus sealed shoulder exceeds 5m.However if the area adjacent to the shoulder is clear of hazardsand traffic volumes are low, an unsealed shoulder may beaccepted.
8.5.1 Derivation
Manoeuvre sight distance, for a single vehicle to manoeuvrearound on obstruction is the sum of two components: MSD = d1 + d3
where:
d1 = the distance travelled during the reaction time = (see section 8.3.1) (m)
d3 = the distance travelled during the evasive action (m)
Evasive action distance is the distance a driver requires toundertake an evasive manoeuvre. The evasive manoeuvreconsists of braking to comfortable speed followed by aswerving manoeuvre to avoid the object. The values given inTable 8.5 are based on empirical evidence gained in Australia.
The manoeuvre sight distance for a range of operating speedsis shown in Table 8.6.
8.6 Headlight Sight Distance
The most common obstruction on a normal rural road isanother vehicle that may or may not be stopped. Even if itslights are not operating, it will have retro-reflective material atstrategic locations, situated higher than the ‘object cut offheight’ used in the stopping sight distance calculations.
As far as small, unilluminated objects are concerned, researchhas shown that:
● Only larger, light-coloured objects can be perceived atspeeds above 80 km/h at the stopping sight distances setout herein;
● Significant improvement is unlikely, as a fivefold lightincrease is necessary for a 15 km/h increase in speed, anda tenfold increase for a 50% reduction in object size;
● In any case, the joint requirements of driving vision andminimising glare for oncoming traffic set limits to beamintensity.
A general limit of 120m to 150m sight distance is all that canbe safely assumed for visibility of an object on a bitumenroadway. This corresponds to a satisfactory stopping distancefor 80 km/h to 90 km/h, and a manoeuvre time of about 5seconds at 100 km/h. Beyond this, it is only large or light-coloured objects that will be perceived in time for reasonableevasive action to be taken on unlit roads. The relatively smallnumber of accidents involving objects on the roadway at nightis probably due to the factor of safety implicit in the variousassumptions in sight distance calculations.
RURAL ROAD DESIGN 33
Operating speed (km/h)
Evasive ActionDistance
(m)*
Speed RangeSlowed To
(km/h)
Table 8.5: Evasive Action Distance
50 15.0 30 - 35
60 25.0 35 - 40
70 35.0 40 - 50
80 50.0 40 - 50
90 70.0 40 - 60
100 95.0 35 - 60
110 125.0 35 - 60
120 155.0 25 - 60
130 190.0 25 - 60
Table 8.6: Manoeuvre Sight Distance
OperatingSpeed (km/h)
Reaction Time (sec)
ManoeuvreTime (sec)
ManoeuvreSight
Distance (m)
50 2.0 3.2 45
60 2.0 3.6 60
70 2.0 3.9 75
80 2.0 4.3 95
90 2.0 4.8 120
100 2.0 5.6 155
110 2.5 6.3 195
120 2.5 7.0 235
130 2.5 8.0 275
(RTV)3.6
Note: *Derived from Queensland “Road Planning and DesignGuide”.
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RURAL ROAD DESIGN34
In addition to the problem of beam illumination, the questionof the angle of the beam is relevant in sags. It is inappropriatefor the beam to be aimed above the horizontal positionbecause of glare to opposing drivers and a figure of 0.5odepression is an appropriate assumption. A headlight aimingangle of 0.5 degrees depressed will allow on effective 1 degreeelevation of the beam to be used in design due to verticalspread.
The length of sag curves to give stopping sight distancemeasured from a headlight height of 750 mm to zero isconsiderably more than that required to achieve reasonableriding comfort. In addition, increasing the length of sag curveto produce a theoretical sight distance may not give thedesired result. If there is a horizontal curve in addition to thesag, the headlights shine tangentially to the horizontal curveand off the pavement (refer Figure 10.1).
The only method of achieving full compatibility betweentheoretical sight distances by day and night is by roadwaylighting. However, two matters act to redress the imbalance,one outside the control of designers and one at least partly intheir domain. Firstly, the majority of hazards encounteredcomprise other vehicles, which are either illuminated or visiblebecause of the requirement for retro-reflective fittings.Secondly, because retro-reflective materials respond too muchlower light levels than the non-reflective objects, they areperceived well outside the direct headlight beam. Thus, theprovision of retro-reflective road furniture (including items likeflood gauge markers, which frequently occur in sags) is animportant offset to the difficulties of night time driving.
8.7 Horizontal Curve Perception Distance
A major characteristic of low speed roads and intermediatespeed roads is the way drivers will speed up on longer straightsand through larger radius horizontal curves then slow downwhere necessary for smaller radius curves. Since the 6th editionof this guide, such roads have been designed so that thegeometric elements matched the operating speeds along theroad. This means that when vehicles have to slow down for ahorizontal curve, drivers must see a sufficient amount of thecurve in order to perceive its curvature, react and slow downappropriately for the curve.
As a result of not perceiving the curvature, drivers may notslow down appropriately for them. Therefore, these curvesshould only be used when the perceived curve operating speedis no more than 5 km/h less than the operating speed on theapproach to the curve.
Normally, sufficient sight distance for a horizontal curve isprovided through the practice of not having a horizontalcurve start over a crest. However, there are times where thiscannot be avoided and the following criteria should beapplied in order to check that sufficient visibility is providedfor the curve.
● A driver eye height of 1.05 m.
● A zero object height because the driver needs to see theroad surface in order to perceive the curvature. Road edgeguide posts and cut batters can only be considered assupplementary aids.
● A driver needs to see sufficient length of the curve in orderto judge its curvature. The driver must be able to see theminimum of :– 5 degrees of arc– About 80 metres of arc– The whole curve.
However, if the curve is transitioned, at least 80% of thetransition length needs to be seen and desirably all of thetransition.
● The length of arch that needs to be perceived must be seenfrom a point that allows the driver to react then decelerate.See section 8.2.3 for the reaction time. The decelerationshould only require comfortable braking. Therefore amaximum deceleration rate of 2.5m/s/s should be used.Typically, this means a distance of 25 m will accommodatea 10 km/h speed reduction from 90 km/h and 40 m willaccommodate a 15 km/h speed reduction. Decelerationdistances should be adjusted for the effect of grade.
● The sight distance is the sum of the reaction distance, arclength for perception and deceleration distance. If thecurve is transitioned, it is possible for the decelerationdistance to coincide with up to the first half of thetransition. If the curve is untransitioned, deceleration up tothe curve tangent point can be assumed.
Provision of horizontal curve perception distance may require alarger crest than is required for stopping sight distance.
8.7 Horizontal Curve Perception Distance (sequential)
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RURAL ROAD DESIGN 35
9 . H O R I Z O N TA L A L I G N M E N T
9.1 General
The horizontal alignment of a road is usually a series ofstraights (tangents) and circular curves that may or may not beconnected by transition curves. The following section outlinesvarious design criteria that are to be considered whenadopting a horizontal alignment.
9.2 Movement on a Circular Path
As a vehicle traverses a circular curve, it is subject to acentripetal force that must be sufficient to balance the inertialforces associated with the circular path. For a given radius andspeed a set force is required to maintain the vehicle in thispath. In road design, this is provided by side friction developedbetween tyre and pavement and by superelevation.
For the design vehicle types and side friction coefficient andnormal values of superelevation, side friction coefficient andcurve radius the following formula is accepted:
e + f = ...... (9.1)
where e = pavement superelevation (m/m or tangent of angle).
This is taken as positive if the pavement falls toward the centre of the curve
f = side frictional factor (see Section 9.4)V = speed of vehicle (km/h)R = curve radius (m).
9.3 Horizontal Curves
9.3.1 Types of Horizontal Curves
9.3.1.1 Reverse Curves
A reverse curve is a section of road alignment consisting of twocurves turning in opposite directions and having a commontangent point at the end and start of transition curves or beingjoined by a short length of tangent. This tangent length is desirably0.6V metres long. However, where deceleration is required on theapproaches to a lower radius curve, sufficient distance must beprovided to enable drivers to react and decelerate.
9.3.1.2 Compound Curves
Curves comprising two or more contiguous curves of differentradii in the same direction are known as compound curves.Generally the following guidelines apply to compound curves:
● Radii less than 1,000 m are undesirable;
● Where radii less than 1,000 m are unavoidable, thereshould be no more than 10 km/h difference in the design
speed of successive geometric elements; and
● Diminishing radii should be avoided on steep downgrades;and
● Motorcyclists may experience instability of the motorcycleas a result of the abrupt changes in centripetal forcerequired due to the change in radius.
Although inconclusive, some literature suggests that a smallradius curve immediately following a large radius curve (bothturning in the same direction) gives drivers inadequateperception of the small radius. This is reported to lead to ahigher single vehicle accident rate. Generally, this geometryshould be avoided.
9.3.1.3 Broken Back Curves
Broken back curves have a straight less than 0.60V long or alarge radius curve between two relatively low radiusunidirectional curves. Generally the following guidelines applyto broken back curves:
● These curves are unsightly and should be avoided wherepossible; and
● Where unavoidable, the length of straight should be noless than the design speed in metres.
9.3.1.4 Transition Curves
Transition curves are normally used to join straights andcircular curves, although they may be omitted when large-radius curves are used. Transition curves:
● Provide a natural path for vehicles moving from a straightto a circular curve and enable centripetal acceleration toincrease gradually from zero at the start of the transition totheir maximum value at the start of the circular curve. If atransition curve is not provided some drivers will occupyadjoining lanes when entering and leaving the curve;
G E O M E T R I C D E S I G N G U I D E L I N E S 4PA R T
9.3.1.1 Reverse Curves
V2
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RURAL ROAD DESIGN36
● Allow for superelevation development and pavementwidening; and
● Improve the appearance of the curve ahead.
The need for transition curves was learned from the early days ofrailway building when problems were encountered withpassenger comfort and track wear due to the sudden applicationof curvature with untransitioned curves. However, the fact thatroad vehicles are not rigidly confined to a specific path togetherwith the characteristics of road vehicle steering mean that shortertransition lengths are more appropriate than those used forrailways. This is why it is current road design practice to basetransition lengths on superelevation runoff length (see Section9.7.4) instead of a comfort criterion that was once used.
The use of longer transitions than those based onsuperelevation runoff length should be avoided when curveoperating speeds are such that drivers have to reduce speedfor the curve. Drivers regulate their speed from the apparentcurvature of the road ahead and in practice, there is somevariation in curve entry speeds. In these circumstances, longertransitions may cause drivers to perceive a higher standard ofcurvature than there is, with consequent increased speed andfriction demand on the circular section of the curve. Overseasstudies have found that there have been higher accident rateson some curves with a combination of long transition (typicallywith more than twice the length based on superelevationdevelopment) and small to medium radius.
For most curves the average driver can achieve a suitabletransition path within the limits of normal lane width. However,with particular combinations of high speed, heavy vehicles anda large difference in curvature between successive geometricelements, the resultant vehicle transition path can result in asideways movement within the lane and sometimes actualoccupation of adjoining lanes. Trucks have more problemsbecause of their wider wheelbase and heavier, less responsivesteering. Trucks also require more width on curves because:
● Rear axles of semi trailers track outwards when travellingaround curves at speed;
● At low speeds the trailers track inwards;
● Truck trailers swing from side to side at speed; and
● The effective width of trucks increases on curves (vehicleswept path considerations).
In the abovementioned circumstances, transition curves havebeen applied to obtain the following advantages:
● A properly designed transition curve allows the vehicle’scentripetal acceleration to increase or decrease gradually asthe vehicle enters or leaves a circular curve. This transitioncurve minimises encroachment on adjoining traffic lanes.
● The transition curve length provides a convenient desirablearrangement for superelevation runoff. The transitionbetween the flat cross slope and the fully superelevatedsection on the curve can be effected along the length ofthe transition curve in a manner closely fitting the speed-radius relation for the vehicle traversing it.
● Where superelevation runoff is affected without atransition curve, it has been common practice to match thesuperelevation runoff with the likely transition path thevehicles take when entering or leaving the circular curve.
● A transition facilitates the change in width where thepavement section is to be widened around a circular curve.Use of transitions provides flexibility in the widening onsharp curves.
● The appearance of rural roads is enhanced by theapplication of transitions.
Despite the advantages of using transition curves, there arealso possible adverse effects associated with transitions. Someresearch studies undertaken indicate the following:
● Transitions at the start of horizontal curves give theimpression of magnifying the radius of the curve ahead.This encourages drivers to approach the curve too quickly;
● Transitions hide the tangent-to-curve point making itdifficult to identify the start of the curve. This results indrivers reducing speed on the approach to curves so thatthey can judge when to commence braking;
● Transition curves at the start of circular curves are reportedto lead to a higher single vehicle accident rate than circularcurves without transitions, for the above reasons.However, other studies indicate that single vehicle accidentrates on circular curves without transitions are similar tothose for circular curves with transitions (Ref. 66); and
● When drivers brake on curves, a combination of forcesapplies on the tyres, effectively reducing the maximumforce that can be developed for braking or cornering.Articulated trucks also have problems with braking oncurves because of the tendency of these vehicles to jack-knife. On curves with transition approaches, brakingoccurs on the spiral. This could create a problem if thedriver does not commence braking sufficiently early.
Sections of road where the operating speed is less than 60km/h do not require transition curves.
The most frequently used form of transition is the clothoid (orEuler) spiral where the curvature changes at a uniform ratealong the curve. The clothoid is easier to set out in the fieldcompared with other forms of transition curves (theLemniscate and the cubic parabola). Basic properties of theclothoid transition are shown in Appendix A
A transition may be omitted when the associated shift (seeAppendix A) is less than 0.25m.
9.4 Side Friction Factor
A vehicle travelling round a circular horizontal curve requires aradial force that tends to effect the change in direction andconsequent centripetal acceleration. This force is provided byside friction between the tyres and the road surface. If thereis insufficient force provided by side friction, the vehicle willtend to slide tangentially to the road alignment.
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Side friction factor f is the friction force divided by the weightperpendicular to the pavement and is expressed as thefollowing formula:
f = – e ...... (9.2)
whereV = operating speed, km/hR = radius of horizontal curve, me = superelevation, m/m.
The upper limit of this factor is that at which the tyre isskidding or at the point of impending skid (Ref 1). The sidefriction factor at which side skidding is imminent depends on:
● Vehicle operating speed;● The type and condition of the roadway surface; and● The type and condition of the tyres.
If the vehicle speed were less than the permissible operatingspeed V, the side friction factor being called upon would beless than the design maximum side friction factor fmax , and asthe travel speed approaches V, then f will approach fmax . Thespeed at which f just equals fmax can be considered as alimiting (safe) speed Vs and if a vehicle is travelling in excess ofVs, then the side friction factor being called upon will exceedfmax . Vs, is called the Limiting Curve Speed Standard.
The amount by which Vs exceeds V can be considered toindicate a lower bound for the margin of safety against thefriction being demanded exceeding the friction that isavailable. That is, the quantity Vs – V can be considered adesign margin of safety.
The available friction can vary both spatially (from one curve toanother, at the same time) and temporally (from one time toanother time at the same curve). Temporal variations in theavailable side friction factor are often due to changes inweather and are inevitable, and the most practicable way tominimise the total variation is to minimise the spatial variationsby providing a spatially uniform road surface.
Variation in the margin of safety arises from both variations inthe available friction (friction supply) and the frictiondemanded (friction demand) by drivers. The geometric designwill have little (if any) effect on the available friction, but it caninfluence the behaviour of drivers (and particularly their choiceof speed) (Ref 82).
The values of side friction factor f for use in geometric designare shown in Table 9.1.
It is important to note that the absolute maximum values for fgiven in Table 9.1 assume construction and maintenancetechniques that will ensure an adequate factor of safetyagainst skidding. The susceptibility of the wearing surface topolishing, the macro-texture of the surface and the amount ofbitumen used, evident at wearing surface, are all importantmatters in the initial construction of a pavement contributingto skid resistance. Freedom from contamination by oil spillageor loose aggregate and resealing when surface texturebecomes too smooth are important aspects in maintenance ofskid resistance. Normally, a pavement, which is properlymaintained, will retain adequate resistance to skidding underall but extreme conditions of driver behaviour or weather.
The desirable maximum values should be used on intermediateand high-speed roads with uniform traffic flow, on whichdrivers are not tolerant of discomfort. These values should beadopted, if possible, to allow vehicles to maintain their lateralpositions within a traffic lane and be able to comfortablychange lanes if necessary.
On low speed roads with non-uniform traffic flow, drivers aremore tolerant of discomfort, thus permitting employment ofabsolute maximum amount of side friction for use in design ofhorizontal curves (Ref. 1)
The f values given in Table 9.1, which apply only to sealedpavements, have been derived from observations of driverspeed behaviour on rural road curves and revised by ARRB (Ref42). A reduction of 0.04 is applied to all values when appliedto unsealed pavements (Ref. 66).
9.5 Minimum Radii Values For Horizontal Curves
9.5.1 Minimum Radius Values
The minimum radius of a horizontal curve for a givenoperating speed can be determined from the formula (9.1). Itcan be rearranged as follows:
Rmin =
whereRmin = minimum radius (m)V = operating speed (km/h)emax = maximum superelevation (m/m)fmax = maximum coefficient of side frictional force developed
between vehicle tyres and road pavements.
Using the values for fmax from Table 9.1, the approximateminimum radii for various vehicle speeds for typical maximumsuperelevations are as shown in Table 9.2.
RURAL ROAD DESIGN 37
Table 9.1: Side Friction Factors
Operating Speed f(km/h) Des max. Abs max.
50 0.30 0.35
60 0.24 0.33
70 0.19 0.31
80 0.16 0.26
90 0.13 0.20
100 0.12 0.16
110 0.12 0.12
120 0.11 0.11
130 0.11 0.11
V2
127R
V2
127(emax + fmax )
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9.5.2 On Steep Down Grades
On steep down grades, the minimum curve radius fromSection 9.5.1 should be increased by 10% for each 1%increase in grade over 3%.
RMIN on Grade = RMIN from Table 9.2 [1 + (G – 3)/10]
whereG = grade (%)R = radius (m)
9.6 Horizontal Alignment Design Procedure
Step 1
Identify all major controls on the alignment and categorisethem as mandatory or discretionary.
Step 2
Decide upon an operating speed that is appropriate both forthe class of road and for the terrain. Minimum radii for theseoperating speeds are then obtained from Table 9.2. Radiiused are chosen to fit the terrain and desirably should exceedthe minimum.
Step 3
Prepare a trial alignment using a series of straights andcurves, using the radii determined in Step 2. On low andintermediate speed alignments, curves used should generallybe consistent. Special care must be taken with curves at the
end of straights because of the high speeds that can bedeveloped at these locations.
Step 4
Prepare a trial grade line, taking into account vertical controlsand drainage aspects. Co-ordinate horizontal and verticalalignments as in Section 11.
On down grades, minimum curve radii should be increasedby 10% for each 1% increase in grade over 3%. ReferSection 9.5.2.
Step 5
Check that all radii are compatible with estimated vehicleoperating speeds using the procedure described in Section 7.
Step 6
Adjust the alignment so that:
● All mandatory controls are met;
● Discretionary controls are met as far as possible;
● Curve radii are consistent with operating speeds at alllocations;
● Other controlling criteria are satisfied with specialconsideration given to the location of intersections andpoints of access to ensure that minimum sight distancesand critical crossfall controls are met; and
RURAL ROAD DESIGN38
Table 9.2 Minimum Radii of Horizontal CurvesBased on Superelevation and Side Friction at Maximum Values
Operating Km/h Minimum Radius m (rounded up)
Speed Flat Terrain Undulating Terrain Rolling Terrain Mountainous Terraine = 3% e = 6% e = 7% e = 10%
Des min Abs min Des min Abs min Des min Abs min Des min Abs min
50 60 52 56 40 53 47 49 44
60 105 79 95 73 91 71 83 66
70 175 113 154 104 148 102 133 94
80 265 173 229 157 219 153 194 140
90 315 219 335 245 319 236 277 213
100 525 415 437 358 414 342 - -
110 635 635 529 529 501 501 - -
120 810 810 667 667 - - - -
130 950 950 782 782 - - - -
(km/h)
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ee + f
● Earthworks are minimised.
Where minimum standards cannot be achieved andcompromises have to be made, the designer requires a broadunderstanding of basic theory and the assumptions made inthe development of the guidelines.
9.7 Superelevation
The superelevation to be adopted is chosen primarily on thebasis of safety, but other factors are comfort and appearance.The superelevation applied to a road should take into account:
● Operating Design Speed of the curve, which is taken as thespeed at which the 85th percentile driver is expected tonegotiate it;
● Tendency of very slow moving vehicles to track towards thecentre;
● Stability of high laden commercial vehicles;
● Difference between inner and outer formation levels,especially in flat country; and
● Length available to introduce the necessary superelevation.
However, it is noted “although the dynamics of vehiclemovement show that the selection of superelevation isimportant for traffic safety, research findings suggest that itdoes not make much of a difference for drivers, who areprimarily affected by the radius of curvature in choosing theirspeed” (Ref. 61).
The proportion of centripetal acceleration as a result of thecombination of superelevation and sideways friction needs tobe controlled to provide a constant driving experience.
There are a number of methods to determine thesuperelevation (and hence resultant side friction) for curveswith a radius larger than the minimum radius for a givendesign speed. It must also be reiterated that the length of suchcurves should be checked to ensure that the length does notcause the operating speed to increase beyond the curve designspeed when the design speed is less than 110 km/h.
The “linear method” distribution to be used in this Guide is forthe superelevation and side friction to be varied linearly from0 for R = infinity to emax for Rmin. This then results in theproportions of the required centripetal acceleration due tosuperelevation and side friction being the same for larger radiias they are at Rmin, considering the following practicalconsiderations:
● For construction expediency, superelevation values arenormally rounded (upwards) to a multiple of 1% so thatthere is a corresponding adjustment of side friction.
● The perceived benefits of uniformity are only possible onhigh-speed rural roads (where the design or operatingspeed exceeds 100 km/h), because section operatingspeeds vary on intermediate and low speed rural roads.
● Other methods have been used in the past so that thereare likely to be many cases where the reuse of existing
pavement will dictate a different superelevation. This isacceptable if the resultant side friction is suitable for thecurve design speed and consistent with that for anyadjacent curves.
With the “linear distribution method”, the superelevation (e1)for a curve of radius R, which is greater than Rmin is given by:
e1 =
Note that fmax may be either the absolute maximum value orthe desirable maximum value for the design speed V.
The value of e1 is usually rounded upwards (eg. 4.0% but4.1% becomes 5%) and the corresponding coefficient of sidefriction is calculated from:
f1 = – e1 rounded
However, if specific controls cannot be met then actual evalues may be used. With different possibilities for emax andfmax (absolute maximum vs. desirable maximum) differentvalues of superelevation may be attributed to a givencombination of radius and design speed. However, thesubjective basis of the “linear distribution method” (andindeed most other methods) and the practice of rounding thesuperelevation value, allows a practical rationalisation to bemade, refer Figures 9.1(a) & (b) and Figures 9.2(a) & (b).
For rural roads, rationalisation of the parameters hasbeen achieved by distributing the parameters.
High speed rural roads use 6% as the maximum e thatshould be applied.
Intermediate speed rural roads of 80 to 100km/h, use amaximum e of up to 7%.
Low speed rural roads may use up to a maximum e of10%. Superelevation of 10% should not be used wherethere are vehicles with high centres of gravity.
In addition, the rationalisation of both desirable andabsolute maximum f values has been used forsuperelevations of 6% to zero. For superelevations of7%, 8% 9% and 10%, the maximum values of f as perTable 9.1 have been used.
This rationalisation will provide high-speed rural roads with thebest practice control, over the variation of centripetalacceleration. This gives the best overall consistency in themargin of safety, which is defined as the difference betweenthe speed at which the maximum permissible design sidefriction would be called upon and the design speed (Ref. 82).
In New Zealand the practice has traditionally been to reducethe side friction demand at radii less than the minimum forany design speed using the factor as a constant. Thismethod is described in Transit New Zealand’s State HighwayGeometric Design Manual and is the only method to be usedin New Zealand.
RURAL ROAD DESIGN 39
V2emax
127R(emax + fmax )
V2
127R
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Figure 9.1(a): Relationship between Speed, Radius and Superelevation Based on Desirable Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
Figure 9.1(b): Relationship between Speed, Radius and Superelevation Based on Desirable Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
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Figure 9.2(a): Relationship between Speed, Radius and Superelevation Based on Absolute Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
Figure 9.2(b): Relationship between Speed, Radius and Superelevation Based on Absolute Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
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9.7.1 Maximum Values of Superelevation
Use of maximum superelevation will need to be applied insteep terrain or where there are constraints on increasing theradius of an individual curve in a group. The current designpractice shows that superelevation exceeding 7% is rarelyused. In mountainous terrain there is normally insufficientdistance to fully develop steep (more than 7%) superelevationand in less rugged terrain the use of steep superelevations isquestionable considering the potential adverse effect on highcentre of gravity vehicles. Therefore, the absolute maximumsuperelevation should be 7% with 6% being the normalmaximum superelevation for high-speed rural roads. Themaximum superelevation (low speed <90 km/h) inmountainous terrain should be 10%. Other factors that mustbe considered for 10% maximum super are:
● Driver expectation;● Driver comfort;● Slide off road;● Stability;● Should not be used where there are vehicles with very high
center of gravity;● Erosion; and● Icing.
9.7.2 Minimum Values of Superelevation
At low and intermediate ranges of operating speeds (belowabout 100 km/h), it will usually be found desirable tosuperelevate all curves at least to a value equal to the normalcrossfall on straights. On very large curves, adverse crossfallmay be considered, refer Table 9.7
9.7.3 Application of Superelevation
On straights, the pavement has normal crossfall to shed water.This crossfall is provided both ways from the centre onundivided rural roads. On a divided rural road eachcarriageway usually has one-way crossfall away from themedian on straight alignments.
A change from normal crossfall to full superelevation occurs asthe road changes fall from a straight to a curved alignment(except where adverse crossfall is adopted), or from a verylarge curve with adverse crossfall to a lower radius curve.
The adopted position of the axis of rotation, the point aboutwhich the crossfall is rotated to develop superelevation,depends upon the type of road facility, total road cross sectionadopted, terrain and the location of the road. On a two-lanetwo-way road, the superelevation is developed by rotatingeach half of the cross section (including shoulders) about thecarriageway centreline (axis of rotation).
On divided rural roads where the median is relatively narrow,less than 5 m, the two carriageways may be rotated about thecentreline of the median. Where the median is wide, the axisof rotation is usually along each median edge of carriageway(particularly in flat each country).
9.7.4 Length of Superelevation Development
The length required to develop superelevation should beadequate to ensure a good appearance and givesatisfactory riding qualities. The higher the speed or widerthe carriageway, the longer the superelevationdevelopment will need to be to meet the requirements ofappearance and comfort.
The length of superelevation development is the transition ofcrossfall from a normal roadway on straight alignment to thatof a fully superelevated crossfall on a circular curve. The totallength required to develop superelevation is called the overalllength of superelevation development (Le). It consists of twomain elements:
● Superelevation Runoff (Sro) the length of roadway neededto accomplish a change in crossfall from flat crossfall to afully superelevated crossfall; and
● Tangent Runout (Tro) is the length of roadway required toaccomplish the change in crossfall from a normal crownsection to a flat crossfall.
Lengths of superelevation development are determined fromthe two design criteria of:
● Rate of Rotation of the pavement crossfall; and
● Relative Grade of the axis of rotation to the edges ofcarriageway grades being rotated.
Superelevation runoff and tangent runout lengths arecalculated by proportioning the normal crossfall to fullsuperelevation using design values for superelevationdevelopment shown in Table 9.6.
RURAL ROAD DESIGN42
9.7.1 Maximum Values of Superelevation
9.7.2 Minimum Values of Superelevation
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Table 9.3: Rate of Rotation Criterion Length of Superelevation Development (Lr r )
Operating Length (m) of superelevation developmentSpeed from normal crossfall to required superelevation
-ve 3% to -ve 3% to -ve 3% to -ve 3% to
+ve 3% +ve 5% +ve 7% +ve 10%
50* 24 32 40 52
60* 29 38 48 62
70* 33 44 56 72
80# 53 71 89 116
90# 60 80 100 130
100# 67 89 111 -
110# 73 98 122 -
120# 80 107 - -
130# 87 116 - -
Notes:* = Rate of Rotation 3.5 % per second# = Rate of Rotation 2.5 % per second
Assumed normal crossfall - 3.0%
Sro = Le – Le
Tro = Le – Sro
where:
Le = superelevation development length (m)Sro = superelevation runoff (m)Tro = tangent runout (m)e1 = normal crossfall (%)e2 = full superelevation crossfall (%)
A vertical curve may be used to ease the grade changes fromcrossfall to superelevation at the edges of the pavement andformation.
9.7.4.1 Rate of Rotation
The rate of rotation of the pavement desirably should notexceed 2.5% per second of travel time at the operatingspeed, but should have an absolute maximum rate of 3.5%per second.
The minimum superelevation development length to satisfythe appropriate rate of rotation criterion can be derived fromthe following expression (Ref. 90).
The rate of rotation of 3.5% (0.035 radians/sec) per second isappropriate for operating speeds < 80 km/h:
The rate of rotation of 2.5% (0.025 radians/sec) per second isappropriate for operating speeds ≥ 80 km/h:
Lrr =
where:Lrr = superelevation development length (m) based on the
rate of rotation criterione1 = normal crossfall (%) e2 = full superelevation crossfall (%)V = operating speed (km/h)r = rate of rotation (% per second).
Table 9.3 shows values of superelevation development lengthsatisfying the rate of rotation criterion.
9.7.4.2 Relative Grade
The relative grade is the percentage difference betweenthe grade at the edge of the carriageway and the grade ofthe axis of rotation. This difference should be kept belowthe values shown in Table 9.4 to achieve a reasonablysmooth appearance.
RURAL ROAD DESIGN 43
e1
e1 + e2
0.278(e1 – e2)Vr
(km/h)
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Table 9.4: Maximum Relative Grade Between Edge ofCarriageway and Axis of Rotation in SuperelevationDevelopment
Relative Grade %
More than One Lane(1) Two Lanes(2) Two Lanes(3)
(WR=3.5) (WR=7.0) (WR=10.5)
40 or under 0.9 1.3 1.7
60 0.6 1.0 1.3
80 0.5 0.8 1.0
100 0.4 0.7 0.9
120 0.4 0.6 0.8
130 0.4 0.6 0.8
Note: (1) Applies to normal two lane two way road with the axis at
rotation on the centerline.(2) Applies to two lane two way road with control along one
edge; four lane roadway with control on centreline andtwo lane two way road with climbing lane and control oncentre line of the two lane two way road.
(3) Applies to multilane roadway with more than two lanesbetween the axis of rotation and the edge of running lanes.
The expressions relating to the relative grade criterion are asfollows (Ref. 90):
For a rate of rotation of 3.5% per second, which is appropriatefor operating speeds < 80km/h:
GR =
For a rate of rotation of 2.5% per second, which is appropriatefor operating speeds ≥ 80km/h:
GR =
where:
GR = relative grade (%)WR = width from axis of rotation to outside edge of
running lanes (m)V = Operating Speed (km/h).
The relative grade calculated for the relevant rate of rotation issatisfactory when it is less than relevant maximum relativegrade given in Table 9.4.
The length of superelevation development to satisfy therelative grade criterion is derived from the following formula(Ref. 90):
Lrg =
where:
Lrg = length of superelevation development (m) based on the relative grade criterion
e1 = normal crossfall (%)e2 = full superelevation (%)GR = relative grade (%), from Table 9.4. Use calculated
values for GR if they are < Table 9.4 values.WR = width from axis of rotation to outside edge of
running lanes (m).
Table 9.5 shows values of superelevation development lengthssatisfying the relative grade criterion. These lengths havebeen calculated using GR values from Table 9.4. Thedesigner may consider using the calculated values of GR wherethey are less than the tabulated values.
9.7.4.3 Design Superelevation Development Lengths
The superelevation development lengths (Le) that are to beadopted satisfy both criteria: Rate of Rotation and RelativeGrade. These values are shown in Table 9.6 and combine theprevious tables, Table 9.3 and Table 9.5.
9.7.5 Positioning Of Superelevation Runoff
9.7.5.1 Without Transitions
Normal practice of positioning the superelevation runoff forcircular radius curves without transitions is as follows:
● Tangent to Circular Curve to Tangent
The development of superelevation runoff for tangent tocircular curves is located with the larger proportion of therunoff length on the approach tangent, rather than on thecircular curve.
The proportion of runoff located prior to the circular curve isdetailed in Table 9.7 and shown on Figure 9.3.
In general, theoretical considerations favour the practice ofplacing a large amount of the superelevation runoff on theapproach tangent. The driver may have to steer in a directionopposite to the direction to the curve ahead to stay in line.However, the maximum side friction developed on the tangentis equal to the rate of applied superelevation and is at all timesless than the rate of side friction considered comfortable. Avehicle travelling at the design speed on the minimum radiuscurve (with maximum rate of superelevation) develops the
RURAL ROAD DESIGN44
12.6WR
V
9.0WR
V
WR (e1 – e 2 )GR
Table 9.7: Portion of Superelevation Runoff LocatedPrior to the Circular Curve
Portion of Superelevation Runoff Located Prior to the Circular Curve
No. of Lanes Rotated
1 2 3
20-70 0.80 0.90 0.90
80-130 0.70 0.80 0.85
OperatingSpeed(km/h)
OperatingSpeed(km/h)
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RURAL ROAD DESIGN 45
Table 9.5: Relative Grade Criterion Length of Superelevation Development (Lrg )
Operating Speed Length (m) of superelevation development from normal crossfall to required superelevation
(km/h) -ve 3% to +ve 3% -ve 3% to +ve 5% -ve 3% to +ve 7% -ve 3% to +ve 10%
No. Lanes: 1 2 3 1 2 3 1 2 3 1 2 3
40 23 32 37 31 43 49 39 54 62 51 70 80
50 28 37 42 37 49 56 47 61 70 61 79 91
60 35 42 48 47 56 65 58 70 81 76 91 105
70 38 47 55 51 62 73 64 78 91 83 101 119
80 42 53 63 56 70 84 70 88 105 91 114 137
90 47 56 66 62 75 88 78 93 111 101 121 144
100 53 60 70 70 80 93 88 100 117 - - -
110 53 65 74 70 86 99 88 108 124 - - -
120 53 70 79 70 93 105 - - - - - -
130 53 70 79 70 93 105 - - - - - -
Note: (1) Assumed normal crossfall = 3.0 % and assumed lane width = 3.5 m(2) Lengths based on GR fromTable 9.4.
Table 9.6: Design Superelevation Development Lengths (Le )Satisfying both Rate of Rotation and Relative Grade Criteria
Operating speed Length (m) of superelevation development from normal crossfall to required superelevation
(km/h) -ve 3% to +ve 3% -ve 3% to +ve 5% -ve 3% to +ve 7% -ve 3% to +ve 10%
No. Lanes: 1 2 3 1 2 3 1 2 3 1 2 3
40 23 32 37 31 43 49 39 54 62 51 70 80
50 28 37 42 37 49 56 47 61 70 61 79 91
60 35 42 48 47 56 65 58 70 81 76 91 105
70 38 47 55 51 62 73 64 78 91 83 101 119
80 53 53 63 71 71 84 89 89 105 116 116 137
90 60 60 66 80 80 88 100 100 111 130 130 144
100 67 67 70 89 89 93 111 111 117 - - -
110 73 73 74 98 98 99 122 122 124 - - -
120 80 80 80 107 107 107 - - - - - -
130 87 87 87 116 116 116 - - - - - -
Note: Final figures should be rounded up to the nearest 5 m.
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Figure 9.3: Tangent to Circular Curve
RURAL ROAD DESIGN46
maximum side friction considered safe and comfortable. Toapply rates of superelevation less than maximum at any pointon the circular curve means that vehicles travelling at thedesign speed develop side friction factors in excess of thedesirable minimum. While the side friction developed on theapproach tangent is undesirable, the development on thecircular curve of friction factors greatly in excess of the designbasis, results in a worse condition.
However, some form of transition path of travel can beexpected on the approach tangent and onto the early part ofthe circular curve. What can be considered lack ofsuperelevation at the beginning of the circular curve iscompensated to some extent by the vehicle travelling acurvilinear path that is flatter than the roadway circular arc.
● Reverse Curves
Reverse curves are horizontal curves turning in oppositedirections. Desirably, reverse curves should have sufficientdistance between the curves to introduce the full superelevationdevelopment for each of the curves without exceeding thestandard rate of change of superelevation for the particularoperating speed. When this length cannot be achieved,superelevation development length may extend up to 20 to 30%or a maximum of 25m into the circular curves. The OperatingSpeed will have to be managed to suit the curve geometry.
● Compound Curves
Compound curves are horizontal curves of different radiiturning in the same direction with a common tangent point.
Where compound curves are provided, the full superelevationon the smaller curve should be developed on the larger radiuscurve prior to the common tangent point.
9.7.5.2 With Transitions
Normal practice of positioning the superelevation runoff forcircular curves with transition is as follows:
● Tangent to Transition Curve to Circular Curve to TransitionCurve to Tangent
For circular curves with transition curves, it is normalpractice to make the lengths of superelevation runoff equalto the length of the transition curve. The superelevationrunoff is then contained solely within the transition curvelength.
A typical example of the development of superelevation onhorizontally transitioned curves on two-lane roads is shown inFigure 9.4. The superelevation runoff commences at thetangent to spiral point (flat cross fall) along the straight andends at the spiral to circular curve point.
● Reverse Transitional Curves
On reverse transitioned curves, the reversal of superelevation isimplemented uniformly and linearly.
The only occasion that superelevation runoff might encroachinto the circular curve is when the road alignment is in aconstricted location. In this case, the shorter than normal
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RURAL ROAD DESIGN 47
Figure 9.4: Typical Superelevation Runoff Profile on Two Lane Two Way Roads (Tangent to Transition Curve to CircularCurve to Transition Curve to Tangent)
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transition curves with large superelevations, may be used toproduce an acceptable alignment. The proportion of runofflocated within the circular curve is detailed in Table 9.7.
However, in the case of long transition curves and smallsuperelevations, it is necessary to increase the rotation rate inthe vicinity of the point of zero superelevation to promoteimproved pavement drainage.
It is undesirable to use long transition curves in other thanhigh-speed curvilinear alignments because of the potential tomislead drivers as to the radius of the following circular curve.
9.7.6 Superelevation on Bridges
Special conditions that apply to bridges include:
● The maximum superelevation shall not exceed 6%;
● The absolute minimum crossfall on structures shall be 2%(drainage requirement);
● The maximum grade on structure, taking the vectorial sumof longitudinal grade and crossfall into account, shall notexceed 8 per cent;
● Changes in crossfall on structure create difficulties both fordesign and construction of bridges and increase costs.Where varying crossfall or superelevation on the bridge isunavoidable, the changes should occur uniformly from oneend of the bridge to the other. This also applies withchanges from two-way to one-way crossfall; and
● Where parallel bridges are in close proximity,superelevation changes on the structures need to allow forany future widening and the possibility that the spacebetween structures may be bridged in the future.
9.8 Curves With Adverse Crossfall
Adverse crossfall on curves should normally be avoided excepton curves of large radius that can be regarded as straights.Table 9.8 gives minimum radius curves for various operatingspeeds for which adverse crossfall may be considered.
9.9 Minimum Horizontal Curve Length
Minimum curve length guidelines are required to avoid kinksin the road alignment and maintain a satisfactoryappearance. Table 9.9 shows the maximum deflection anglesfor which a curve is not required and the correspondingminimum curve length.
9.10 Pavement Widening on Horizontal Curves
Pavements may be widened on curves to maintain the lateralclearance between vehicles equal to the clearance available onstraight sections of road. Widening is required for two reasons:
● A vehicle travelling on a curve occupies a greater width ofpavement than it does on a straight as the rear wheels at lowspeeds track inside the front, and the front overhang reducesthe clearance between passing and overtaking vehicles. (Athigh speeds the rear wheels track outside the front.); and
● Vehicles deviate more from the centreline of a lane on acurve than on a straight.
The amount of widening required depends on:
● The radius of the curve;● Width of lane on a straight road;● Vehicle length and width; and● Vehicle clearance.
Other factors such as overhang of the front of the vehicle,wheelbase and track width play a part. However, there is alower practical limit to widening due to construction feasibilityand for a two-lane road curve widening should be omittedwhen the total widening is less than 0.5m.
There may be a requirement to widen the pavement onhorizontal curves for vehicles that occupy a greater width ofpavement than the design vehicle (19.0m semi trailer).
There is no additional steering allowance component fordifficulty of driving on curves. This has been the Austroadspractice since 1979 and has been based on the followingassumptions:
● There is less steering variation with the design vehiclesince it is a large commercial vehicle that is driven by aprofessional driver;
RURAL ROAD DESIGN48
Table 9.8: Minimum Radii with Adverse Crossfall
Speed Minimum Radii (m) (km/h) 3.0% adverse crossfall
50 400
60 600
70 900
80 1250
90 1700
100 2250
110 3000
120 4000
130 5000
Note: Does not apply to intersections where higher demand may be required.
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RURAL ROAD DESIGN 49
Table 9.9: Minimum Horizontal Curve Lengths
Operating Max. Deflection angle where Min curve length Speed (km/h) curve not required (Ref. 98) (TS to ST) (1) (Ref. 90)
2 lane pavement 4 lane pavement
50 1.5 N/A 70
60 1 0.5 100
70 1 0.5 140
80 1 0.5 180
90 1 0.5 230
100 1 0.5 280
110 0.5 0.25 340
120 0.5 0.25 400
Note: (1) Minimum length of circular arc where transition curves not required
Table 9.10: Lane Widths on Curves in Mid-Block Sections
Vehicle Type 19m Semi Trailer
Vehicle Width, u (m) 2.5
No. of rigid units, n 2
Wheelbase lengths (m) 5.4 & 9.5
Ave. vehicle wheelbase, L (m) 7.45
Front overhang, A (m) 1.6
2 lane-2 way Multi-Lane
Operating speed, V (km/h) 60 > 70 60 > 70
Radius, R (m)
75 4.3 4.0
100 4.1 3.8
100 – 200 3.8 3.8
> 200
Notes:
All lane widths have been calculated using 0.6m for the lateral clearance, C, and have been rounded up to the nearest 0.1 m
Radii below absolute minimum radii for operating speed – not to be used. Refer Table 9.2
Lane widening is not required. A standard lane width of 3.5m is adequate.
Where the operating speed is substantially < 60 km/h, lane widening should be calculated using the formula for Wc.
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Figure 9.5: Horizontal Stopping Sight Distance
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RURAL ROAD DESIGN 51
● The swept path width of the design vehicle accommodatesthe swept path width of smaller vehicles plus providesroom for steering variation (and driver skill variation) withthe smaller vehicles; and
● The now common use of full width or part width pavedand sealed shoulders compensates for not having asteering allowance component for the design vehicle.
Table 9.10 shows the width of traffic lane, including wideningfor a range of circular curves and design vehicles.
For lane widening with transitioned curves, it is normalpractice to apply half of the curve widening to each side of theroad. However, this means that the shift associated with thetransition (shift = LP
2/24R, where LP is the length of transitioncurve, and R is the radius of the circular arc) must be greaterthan the curve widening that is applied to the outer side of thecurve so that the design vehicle will make use of the wideningand for appearance. This will usually only be a problem whenthe curve widening has to suit a road train and a greaterproportion of the total widening will have to be applied on theinside of the curve. The painted centreline will then be offsetfrom the control line in order to provide equal lane widths.
For untransitioned curves, it is normal practice to apply all thecurve widening to the inside of the curve with the paintedcentreline then being offset from the control line in order toprovide equal lane widths. This practice aids drivers in makingtheir own transition.
For more information refer to Section 11.2 Traffic Lane Widthand to Guide to the Geometric Design of Major Urban Roads(Ref. 40). Refer Section 12.2, Traffic Lane Width.
9.11 Sight Distance on Horizontal Curves
Horizontal curves with minimum radii shown in Table 9.2 donot necessarily meet the sight distance requirements describedin Section 8. Where a lateral obstruction off the pavementsuch as a bridge pier, cut slope or natural growth restricts sightdistance, the stopping sight distance appropriate to the designspeed of the curve determines the minimum desirable radiusof curvature.
Figure 9.5 shows the relationship between horizontal sightdistance, curve radius and lateral clearance to the obstructionand is valid when the sight distance at the appropriate designspeed is not greater than the length of curve. This relationshipassumes that the driver’s eye and the sighted object are abovethe centre of the inside lane, 1.75m in from the outer edge oflane based on a standard 3.5 m lane width. When the designsight distance is greater than the length of curve, a graphicalsolution is appropriate.
For alignments on lower speed roads, particularly in difficultterrain, it may not be feasible to achieve the 2.5 secondsreaction time stopping sight distances shown in Section 8.Increasing curve radius to improve the sight distance mayincrease the operating speed so that longer, and stillunavailable, design stopping sight distances are required. Inthese situations, the designer should provide the maximumsight distance practicable, and ensure that it is not less thanthe stopping sight distance corresponding to a 2.0 secondreaction time.
Where sight benches in side cuttings are required onhorizontal curves or a combination of horizontal and verticalcurves, the horizontal and vertical limits of the benching aredetermined graphically or by modeling.
9.11.1 Benching for Visibility on Horizontal Curves
Benching is the widening of the inside of a cutting on a curveto obtain the specified sight distance. It usually takes the formof a flat table or bench over which a driver can see anapproaching vehicle or an object on the road. In plan view, theenvelope formed by the lines of sight fixes the benching. Thedriver and the object he is approaching are assumed to be inthe centre of the inner lane and the sight distance is measuredaround the centre line of the lane, the path the vehicle wouldfollow in braking. Benching adequate for inner lane trafficmore than meets requirements for the outer lane.
Where a horizontal and crest vertical curve overlap, the line ofsight between approaching vehicles may not be over the topof the crest but to one side and may be partly off theformation. Cutting down the crest on the pavement will notincrease visibility if the line of sight is clear of the pavement,and the bottom of the bench may be lower than the shoulderlevel. In these cases, as well as in the case of sharp horizontalcurves, a better solution may be to use a larger radius curve sothat the line of sight remains within the formation. However,this will tend to increase the operating speed, which in turnwill increase the sight distance required.
9.11.2 Other Restrictions to Visibility
There are other minor constraints on sight distance that mustbe kept in mind by the designer:
● In avenues of trees, visibility can be reduced at a sag owingto the line of sight being interrupted by the foliage. Thesame may happen where a bridge crosses a sag and theline of sight is obstructed;
● Guard fencing, bridge handrails, median kerbs and similarobstructions can restrict the visibility available at horizontaland vertical curves;
9.11.1 Benching for Visibility on Horizontal Curve
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● There is a sizeable difference between the length of sightdistance available to a driver depending on whether thecurve ahead is to the left or the right.
9.12 Curvilinear Alignment Design in Flat Terrain
9.12.1 Introduction
The traditional approach to the design of road alignment in theflat terrain has been to use long tangents with relatively shortcurves between them. In some cases, the length of straight hasbecome exceptionally long, resulting in monotonous drivingconditions leading to fatigue and reduced concentration.
The problems of the long tangent/short curve alignment havebeen recognised for some time. A general conclusion has beenthat the ideal alignment is a continuous curve with constant,gradual, and smooth changes of direction. This has led to theconcept of curvilinear alignment which has been defined asconsisting of long, flat circular curves, simple and compound,connected by fairly long spiral transitions, about two thirds ofthe alignment being on the circular arcs and one third onspirals. Inherent in this definition is the premise that thealignment is made up of a range of curves varying in radiusfrom about 10,000 metres to a maximum of 30,000 metres. Ifthe whole alignment can be made up of curves of the 10,000metres to 30,000 meters radii, the need for spiral transitions isessentially removed.
9.12.2 Theoretical Considerations
The basis for using curvilinear alignment is found in theconsideration of visual requirements and the effect of speedon perception and vision. As speed increases:
● Concentration increases;● The point of concentration recedes;● Peripheral vision diminishes;● Foreground detail begins to fade; and● Space perception becomes impaired.
Thus the higher the speed, the further ahead the driverfocuses his vision and the more concentrated the angle ofvision becomes. This restriction of vision (called “tunnel view”by some) may induce fatigue unless the point of concentrationis made to move around laterally by means of a curvilinearlayout of the road.
Space perception is achieved with the help of memory, and byassessing relative changes in the size and position of objects. Itis therefore necessary to have a lateral component to enable adriver to discern movement and its direction. This lateralcomponent is provided on curves, the rate of such movementdepending on the radius of the curve.
The radius that should be adopted depends on several factorsincluding the type of topography and the expected speed oftravel, the desired radius depending on how far ahead thedriver can see the road. At high speeds, a driver looks from300 metres to 600 metres ahead and a curve should be at leastthis long to be visually significant when the driver is on it.
It is desirable to design on the basis of at least 30 degrees ofdeflection angle as a minimum, which will result in the
adoption of curve radii of from 3,000 to 30,000 metresdepending on how far ahead the road can be seen. A furtherconsideration is the requirement of overtaking sight distance.It is desirable that overtaking sight distance be provided ifpossible and in flat country this can easily be achieved. A15,000 m radius curve allows overtaking sight distance for120 km/hour to be achieved. The optimum radius range isabout 16,000 to 18,000 metres.
The larger the radius, however, the closer the alignmentcomes to a straight line and the less the advantages becomeand in this respect further consideration may need to begiven to the desirable maximum length of curve in onedirection. There is no point in using radii larger than 30,000metres for this reason.
9.12.3 Advantages of Curvilinear Alignment
A road with curvilinear alignment is much more pleasant todrive on than one with long straight tangents since it unfoldsitself smoothly with no unexpected checks. The driver is moreable to judge the distance to an approaching vehicle, and toassess its rate of approach since the driver sees it to one side,the lateral component of its movement providing thenecessary information for the driver assessment. Judgementson the safety of overtaking manoeuvres are easier to makeunder these circumstances.
Because of the continuously curving alignment, the viewahead is constantly changing and it is also possible to directthe road towards interesting features of the countryside forshort periods. This removes much of the monotony of the longstraight alignment and can create a sense of anticipation in thedriver for what is beyond.
At night, curvilinear alignment removes much of theapproaching headlight glare problem common to long straightroads in flat country. On long straights, headlights becomevisible from a very long distance away and can be annoyingand distracting from a distance of over 3 kilometres. Wherevehicles approach each other on curvilinear alignment, theglow of the approaching vehicle headlamps can be seen wellbefore the lamps become visible, and the rate of approach ofthe vehicle can be assessed.
In the daytime when driving in the direction of the suncurvilinear alignment removes much of the approachingglare problem caused by the sun’s rays common to longstraight roads running in a westerly/easterly direction in flatcountry.
Conditions for both day and night driving are therefore muchmore comfortable on a road with curvilinear alignment.
On treeless plains, some of the effect of the curvilinearalignment is lost. It may be that in such circumstances, thesmaller (optimum) radii would be more effective in that it willincrease the driver’s perception of relative change.
The principles of curvilinear alignment can be applied in a widerange of conditions using a wide range of curve radii togetherwith spirals. Considerable improvements in the quality of ourroad system can be achieved at no extra cost by theapplication of these principles.
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9.13 Bridge Considerations
Bridge carriageway width and width of road on theapproaches to the bridge are based on providing a consistentlevel of service along a section of road. The following factorsshould be considered:
● Road geometry;● Traffic volumes and composition;● Terrain;● Climatic conditions; and● Bridge location.
The traffic lane widths provided on the bridge should not beless than the widths provided on the approach roadway. Onshort bridges (20m long or less for most rural roads), it isnormal practice to carry the full width of shoulders andpavement, including auxiliary lanes, across the bridge.
Where necessary, additional bridge width should be provided:
● To carry a kerbed footway on the bridge and on theapproaches; and
● To achieve satisfactory sight distance and curve widening.
Auxiliary lane lengths and, in particular, tapers should not be
reduced in order to avoid widening on bridges. If possible, itmay be preferable to relocate the auxiliary lane.
The following principles are to be adopted for the alignmentof elevated structures on major rural roads:
● Avoid multiple and varying geometrics on the structure,including superelevation transitions, where possible;
● Skew angle should not exceed 35o;
● Avoid curve radii below 500 m;
● Avoid short end spans on bridges;
● Provide a constant crossfall on bridges;
● If curvature is unavoidable, the bridge should lie fullywithin the circular arc and the radius should be as large aspossible with maximum 6% superelevation; and
● The designer should seek advice from bridge engineers inrelation to construction economies, provision for futureduplication and the location of tangent points.
Further consideration of geometric requirements for bridges isset out in the Austroads Bridge Design Code (Ref. 29).
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9.13 Bridge Consideration
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1 0 . V E R T I CA L A L I G N M E N T
10.1 Introduction
Vertical alignment is the longitudinal profile along thecentreline of a road. It is made up of a series of grades andvertical curves.
The profile is determined by a consideration of the planning,access, topographic, geological, design controls earthworksand other economic aspects.
The grades are generally expressed as a percentage of onevertical divided by the horizontal component.
The vertical curves are parabolic in shape and are expressedas a K Value. The K Value is the vertical curve constant, usedto define the size of a parabola. It is the length (m) requiredfor a 1% change of grade.
For design purposes the K value concept also has theadvantage of easily determining the radius at the apex of aparabolic vertical curve: R = 100K. Within the range ofgrades used for road design there is little variation betweenthe parabola and the extended arc of the apex radius.Therefore, the apex radius value yields a suitableequivalent radius and an alternative vertical curve constantthat can be used to define the size of a parabolic vertical curve.
10.2 Grades
10.2.1 General
Generally, grades should be as flat as possible, consistent witheconomy and longitudinal drainage requirements (wherekerbing is to be incorporated). Flat grades permit all vehicles tooperate at the same speed. Steeper grades produce variationin speeds between vehicles with varying power to weightratios both in the uphill and down hill direction. This speedvariation:
● Leads to higher relative speeds between vehicles producingthe potential for higher rear end vehicle accident rates; and
● Results in increased queuing and overtaking requirementswhich gives rise to further safety problems, particularly athigher traffic volumes.
In addition, freight costs are increased due to the slower speedof heavy vehicles.
Table 10.1 shows the effect of grade on vehicle performanceand lists road types that would be suitable for these grades.Vehicles can tolerate relatively short lengths of steeper gradesbetter than longer lengths of less steep grades.
10.2.2 Vehicle Operation on Grades
There are three aspects to the design of grades that can beadopted in difficult terrain:
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Grade Reduction in Vehicle Speed as compared to Flat Grade % Road Type Suitability
Uphill Downhill
Light Vehicle Heavy Vehicle Light Vehicle Heavy Vehicle
0-3 Minimal Minimal Minimal Minimal For use on all roads
3-6 Minimal Some Minimal Minimal For use on low-moderate speed roads reduction on (incl. High traffic volumes roads)high speed
roads
6-9 Largely Significantly Minimal Minimal for For use on roads in mountainous terrainunaffected slower straight alignment. Usually need to provide auxiliary lanes if
Substantial for high traffic volumeswinding alignment
9-12 Slower Much slower Slower Significantly slower Need to provide auxiliary lanes for for straight alignment. moderate – high traffic volumes. Need to
Much slower for consider run-away vehicle facilities ifwinding alignment proportion of commercial vehicles is high
12-15 10-15 km/h 15% max. 10-15 km/h Extremely Satisfactory on low volume roads (very slower Negotiable Slower slow few or no commercial vehicles)
15-33 Very slow Not Very slow Not Only to be used in extreme cases and be negotiable negotiable of short lengths (no commercial vehicles)
Source: Ref. 66
Table 10.1: Effect of Grade on Vehicle Type
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● The poorer performing vehicles using the road (generallytrucks in the lower power ranges) must be able to climbthe grade. This limits the maximum grade that can beconsidered for roads open to the public. It only becomesan acceptable limit in low volume situations, or for specialpurpose roads, eg to a specific tourist vantage point.
● Grades cause the need for speed variations, gearchanges and braking for all vehicles. This is a quality ofservice consideration. Flatter grades, which enable amore consistent travel speed, make fewer demands onboth vehicle and driver and generally reduce vehicleoperating costs.
● Grades cause speed disparities between vehicle types,leading to increased queuing and overtaking requirements.This is a level of service problem. The increased overtakingrequirements and reduced service volumes can give rise tooperational and safety problems at higher traffic flows. Theproblem can arise from cars towing caravans and trailers aswell as from heavy commercial vehicles.
10.2.3 Maximum Grades
Grades used in design are, therefore, only controlled at theupper end by vehicle performance. In most designs, thegeneral maximum grade to be sought will be based on level ofservice and quality of service considerations, modified asappropriate by the severity of the terrain and the relativeimportance of the road. Table 10.2 shows maximum gradesover long lengths of road in various terrain types.
The adoption of grades steeper than the general maximummay be justified in the following situations:
● Comparatively short sections of steeper grade which canlead to significant cost savings;
● Difficult terrain in which general maximum grades are notpractical;
● Where absolute numbers of heavy vehicles are generallylow; and
● Less important local roads where the costs or impact ofachieving higher standards are difficult to justify.
In any case, design options for the road include, on one hand,flattening the grade, and on the other, the provision ofauxiliary lanes and/or special facilities for safely controllingrunaway vehicles on downgrades (refer Section 13.7).
“When adopting maximum grades, side drains need to beconsidered in respect to the maximum velocity of flow forscour protection. Special lining of the drains may be requiredto limit damage to the drain and the environment.”
10.2.4 Length of Steep Grades
To achieve a quality-balanced design, it is necessary to considerthe length of the grade. Most standards do not explicitly limitthe length of grades, but suggest that it is desirable to limit thelength of sections with maximum grades. AASHTO (1994)proposes limiting the maximum length to that which will notexceed the critical length of grade. “The critical length is thatwhich will cause a typical loaded truck (300pound/horsepower) [5.5 kW/tonne] to operate without anunreasonable reduction in speed. A reduction of 10 mph [16km/h] is recommended, the reason being the significantincrease in accident involvement rate at higher speedreductions.”
The length of steep grades is considered in the design ofauxiliary lanes with the help of Figure 13.3.
However it must be remembered that length of grade canaffect safety and capacity. On both the upgrade and downgrade, the lower operating speed of trucks may causeinconvenience to cars. Long gradients, for example 5km at4%, could result in a high risk of serious accidents involvingdescending vehicles as a result of brake failure. Such gradientscould also cause climbing vehicles to slow down to well belowthe 85th percentile speed.
All short sections of grade should be checked for appearance.
10.2.5 Steep Grade Considerations
Although speeds of cars may be reduced slightly on steepupgrades, large differences between speeds of light and heavyvehicles will occur and speeds of the latter will be quite slow.It is important, therefore, to provide adequate sight distance toenable faster vehicle operators to recognise when they arecatching up to a slow vehicle and to adjust their speedaccordingly. Key considerations are as follows:
● On any generally rising or falling section of the road, steepgrades should be avoided as much as practicable, as thesegrades reduce vehicle operating efficiency.
● Where possible, it is preferable to introduce a flatter gradeat the top of a long ascent, particularly on low speedroads, but this must not be achieved by steepening thelower portion of the grade.
● On steep downgrades, it is desirable to increase the 85thpercentile speed of the individual geometric elementsprogressively towards the foot of the steep grade. Wherethis cannot be achieved and where percentages of heavy
RURAL ROAD DESIGN 55
Operating Speed Terrain
(km/h) Flat Rolling Mountainous
60 6-8 7-9 9-10
80 4-6 5-7 7-9
100 3-5 4-6 6-8
120 3-5 4-6 -
130 3-5 4-6 -
Note: Values closer to the lower figures should be aimed foron primary highways. Higher values may be warrantedto suit local conditionsFor unsealed surfaces the above value should bereduced by 1%.
Table 10.2: General Maximum Grades (%)
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vehicles are high, consideration should be given toconstruction of runaway vehicles facilities. Refer toFigure 13.5, 13.6 and Section 13.7 for runaway vehicle facilities.
10.2.6 Minimum Grades
The minimum grade may be zero except in the followingsituations:
● In cut:In cut, the minimum grade shall normally be 0.5%(absolute minimum 0.33%) for unlined drains. Thisminimum grade in cut is required to provide adequate fallin table drains. In exceptional cases, where for any reasonit is necessary to have a grade flatter than 0.5% this wouldbe acceptable provided that a minimum grade of 0.5% isretained in the table drains. This is done by uniformlywidening the drains at their standard slope, therebydeepening them progressively or, alternatively, lining thetable drains to permit a flatter grading to be adopted.
● In medians:On divided roads the type of median drainage proposedmay control the minimum grade of the carriageways.
10.3 Vertical Curves
10.3.1 General
The vertical alignment of a road consists of a series of straightgrades joined by vertical curves. In the final design, the verticalalignment should fit into the natural terrain, consideringearthworks balance, appearance and the maximum andminimum vertical curvature allowed expressed as the K value.Large K value curves should be used provided they arereasonably economical. Minimum K value vertical curvesshould be selected on the basis of three controlling factors:
● Sight distance:Is a requirement in all situations for driver safety.
● Appearance:Is generally required in low embankment and flattopography situations.
● Riding comfort:Is a general requirement with specific need on approachesto floodway where the length of depression needs to beminimised.
10.3.2 Forms and Types of Curve
There are various curve forms suitable for use as verticalcurves. The parabola has been traditionally used because ofthe ease of manual calculation and is adopted throughout thisGuide. Other forms are equally satisfactory.
There are two types of vertical curves. Convex vertical curvesare known as summit or crest curves, and concave verticalcurves as sag curves.
Vertical curve theory and formulae are presented in AppendixB. However, in summary, most vertical curves can be designedusing the following equations:
L = KA
K = when S < L
and K = – when S > L
where:L = length of vertical curve (m)K = is the length of vertical curve in meters for 1% change
in gradeA = algebraic grade change (%)S = sight distance (m)h1 = driver eye height, as used to establish sight distance (m)h2 = object height, as used to establish sight distance (m)
For design purposes the K value may be used to determine theequivalent radius of a vertical curve using R (radius m) = 100K.
10.3.3 Crest Vertical Curves
Curvature of crest vertical curves is usually governed by sightdistance requirements. However, the appearance of the roadmay dictate larger values to provide satisfactory appearance ofthe curve. These criteria are discussed below.
10.3.3.1 Appearance
At very small changes of grade, a vertical curve has littleinfluence other than appearance of the profile and may beomitted. At any significant change of grade, minimum verticalcurves detract from the appearance. This is particularly evidenton high standard roads.
RURAL ROAD DESIGN56
10.3.3(a) Crest Vertical Curve
S2
200 (√h1 + √h2 )2
200 (√h1 + √h2 )2A2
2SA
10.3.3(b) Crest Vertical Curve
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Table 10.3 gives minimum K values for satisfactoryappearance. Larger K value curves may be preferred wherethey can be used without conflict with other designrequirements, eg overtaking, drainage and where they give abetter fit to the topography.
The designer should avoid large crest curves for longitudinaldrainage reasons (to prevent water ponding near the apex).Large crest curves increase the length of road subject torestricted sight distance.
The values in Table 10.3 are subjective approximations andtherefore the lack of precision is intentional.
10.3.3.2 Sight Distance Criteria (Crest)
The minimum crest vertical curve and K value are calculatedusing expressions from Appendix B and values of car stoppingdistance from Table 8.3(a) and Formulas from Section 8.3.1and 10.3.2.
Minimum crest vertical curve K values are shown in Table 10.4for various operating speeds, reaction times, and verticalheight constraints.
10.3.4 Sag Vertical Curves
10.3.4.1 Appearance and Comfort
Appearance is important when considering small and largerchanges in grade (the same as for crest curves).
Sag vertical curves are generally designed to achieve thecomfort criterion as a minimum.
A person subjected to rapid changes in vertical accelerationfeels discomfort. To minimise such discomfort when passingfrom one grade to another, it is usual to limit the verticalacceleration generated on the vertical curve to a value lessthan 0.05g where g is the acceleration due to gravity. On lowstandard roads and at intersections, a limit of 0.10g may beused.
The minimum sag vertical curve K value for comfort criteriacan be calculated by the following equation.
K =
Note:(i) Length of sag curve L = KA(ii) Equivalent Radius R = 100K
where
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Operating Minimum grade change Minimum length Minimum K Speed requiring a crest of crest vertical Value (4)
(km/h) vertical curve, % (1, 2) curve, m (3) S < L
50 0.9 30 – 40 33 – 44
60 0.8 40 – 50 50 – 62
70 0.7 50 – 60 71 – 86
80 0.6 60 – 80 100 – 133
90 0.5 80 – 100 160 – 200
100 0.4 80 – 100 200 – 250
110 0.3 100 – 150 333 – 500
Note:(1) In practice, crest vertical curves are frequently provided at all changes of grade.(2) Ref. 98(3) Ref. 90(4) Round resultant L values up to nearest 5 m.
10.3.4(b) Sag Vertical Curve
10.3.4(a) Sag Vertical Curve
Table 10.3: Length of Crest Vertical Curves – Appearance Criterion when S < L
V2
1296a
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K = length of vertical curve in metres for 1% change in grade
a = vertical acceleration (m/sec2)V = speed of the vehicle (km/h)R = sag curve radius (m)A = algebraic grade change (%)L = length of curve (m)g = gravitation force m/sec2 = 9.81 m/sec2
Values of minimum K for sag curves are shown in the Table 10.5.
10.3.4.2 Sight Distance Criteria (Sag)
(a) Headlight
Sight distance on sag curves is not restricted by the verticalgeometry in daylight conditions or at night with full roadwaylighting, unless overhead obstructions are present. Undernight conditions on unlit roads, limitations of vehicleheadlights restrict sight distance to between 120 m and 150 mon crest curves. On high-speed roads not likely to be providedwith roadway lighting, consideration may be given toproviding headlight sight distance. Nevertheless, horizontalcurvature would cause the light beam to shine off thepavement (assuming 3o lateral spread each way), and little isgained by increasing the K value of the sag curve.
For headlight sight distance see Figure 10.1
K = when S < L
and
K = – when S > L
where:
h = mounting height of headlights (m)S = stopping sight distance (m), Table 8.3(a)q = elevation angle of beam 10 (+ upwards)
(tan 10 = 0.01746)
The minimum sag curve K values for a headlight mountingheight of 0.60 m and one degree of light beam elevation arepresented in Table 10.6.
Overhead Obstructions
Overhead obstructions such as road or rail overpasses, signgantries or even overhanging trees may limit the sight distanceavailable on sag vertical curves. With the minimum overheadclearances normally specified for roads, these obstructionswould not interfere with minimum stopping sight distance.They may, however, need to be considered with the upper limitof stopping distance (including sight distance to intersections)and overtaking provision. Refer Figure 8.3.
For overhead obstruction sight distance:
K = when S > L
where:
H = height of overhead obstruction (m)h1 = truck driver eye height (2.4) (m), h2 = object height (0.60) (m),S = stopping sight distance (m), Table 8.3(a).
10.3.5 Reverse/Compound/Broken Back Vertical Curves
Upright vertical curves with common tangent points areconsidered quite satisfactory. It is necessary to check that the
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h1 = 1.05m h2 = 0.20m h1 = 2.4m h2 = 0.20m h1 = 1.05m h2 = 1.05m Car to Car
K value based on Stopping K value based on Stopping K value based on Overtaking Sight Distance for Cars Sight Distance for Trucks Sight Distance for Cars
RT = 2.5sec RT = 2.0sec RT = 2.5sec RT = 2.0sec RT = 2.5sec
50 7 5 6 5 120
60 12 10 10 8 210
70 20 16 17 14 321
80 31 25 25 22 488
90 46 38 37 32 706
100 67 57 55 48 1008
110 98 84 85 75 1440
120 139 - - - 2010
130 197 - - - 2680
Note: (1) Correction of Stopping Sight Distance for Grade Refer Table 8.3(a)(2) Overtaking zones rarely occur on a single vertical curve, so the corresponding K value are rarely relevant
Table 10.4: Minimum Crest Vertical Curve K Values, S < L. Refer Table 8.3(a) and 8.4
S2
200 (h+S tan q)
200 (h + S tan q)A2
2SA
S2
200 (√H – h1 + √H – h2)2
OperatingSpeed(km/h)
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sum of the radial accelerations at the common tangent pointdoes not exceed the tolerable allowance for riding comfort, a<0.05m/sec2. There are situations where reverse vertical curvescan produce pleasing, flowing grade lines which are morelikely to be in harmony with the natural landform:
where:
a =
and
0.005 >
It would be desirable to provide a short length of gradebetween the reverse vertical curves. The desirable length isequal to 0.2V in metres. Where less than the desirable bufferlength is available the minimum vertical curves are to conformto the following empirical formula:
K = ≤ (1+b)
where:
K1 & K2 = K values of the two curves being tested
K = minimum K values listed in Table 10.5 (comfort criteria)
b = fraction, being the ratio of the actual length between TP’s of the adopted curves to the normally required buffer length, 0.1Vm (absolute) or 0.2Vm (desirable), as the case may be.
Broken Back vertical curves consist of two curves, both sag orboth crest curves, usually of different K value, joined by a shortlength of straight grade. Their use should be avoided when thelength of straight grade between curves is less than 0.4Vm (V= operating speed in km/h). Where the length of straightgrade exceeds 0.4V m the curves are not then deemed to bebroken-backed.
Compound curves are made up to two curves in the samedirection with the length of straight grade equal to zero.
RURAL ROAD DESIGN 59
h = 0.60 m, q = 1o
Stopping Sight Distance
K value
Des. Min. Abs. Min RT = 2.5 sec RT = 2.0 sec
50 10 8
60 14 12
70 19 17
80 25 22
90 32 29
100 41 37
110 50 46
120 62 57
130 72 66
Table 10.6: Minimum Sag Vertical Curve K Value forHeadlight Criteria when S < L
Operating K valueSpeed(km/h) a = 0.05g a = 0.1g
50 4 2
60 6 3
70 8 4
80 11 6
90 14 7
100 17 9
110 20 10
120 24 12
130 28 14
Figure 10.1: Car Headlight Sight Distance on Curves
K1 + K2
10,000K1K2
V2
1256K
V2
12561K1
1K2
+
Table 10.5: Minimum K Values for Sag Vertical Curves
OperatingSpeed(km/h)
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11. CROSS SECTION
11.1 General
The selection of cross-section elements for rural roads is aniterative process that considers various criteria: safety,environmental impact, economy and aesthetics. The majorelements of a cross section are illustrated in Figure 11.1 anddiscussed below.
11.2 Traffic Lane Width
A traffic lane is that part of the roadway set aside for one-waymovement of a single stream of vehicles. Refer Table 11.1.
Traffic lane width is based on consideration of:
● Traffic: Annual average daily traffic (AADT) of the road, and peakhour traffic figures where relevant. Traffic is usuallypredicted for a future design year. Heavier traffic volumeson a road means frequent passing and overtakingmanoeuvres and the path of vehicles as a result is furtherfrom the centre line. In these circumstances, wider trafficlanes are preferred. When the AADT increases above 500(two lane two way), lane width increases from 3.1 to 3.5m.
● Vehicle Dimensions: Commercial vehicles are commonly the full legal width of2.5m. Normal steering deviations as well as tracking errorsand pavement imperfections reduce the clearancebetween vehicles in adjacent lanes. The wider the vehiclesand the narrower the lanes, the more significant thesereduced clearances become. There is a consensus that3.5m lanes are appropriate for cars and the 19m primemover and semi trailer, however, a lane width of 3.6 to3.75m may be required for significant volumes of largertrucks. The use of 3.5m lanes plus shoulder seals is a moreeffective use of a given total seal width with regard to boththe pavement structure and roadside design.
● Combinations of Speed and Traffic Volume: When both the operating speed and the traffic volume arehigh, narrower lane widths should be avoided. When onlyone of these factors is high, an economic design mayfrequently dictate narrower lanes. This can be justified onlower volume roads because passing by opposing vehiclesoccurs less frequently. If the operating speed is high on alow volume road, it would normally be associated withlonger sight distances and drivers would have time toadjust speed and position slightly or to increase the level ofconcentration when passing other vehicles. Such eventsare relatively infrequent and do not overtax the driver. Evenhere, however, wider pavements do improve the quality ofservice of the road.
RURAL ROAD DESIGN60
Figure 11.1: Typical Cross Sections
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The desirable lane width on rural roads is 3.5m. Thiswidth allows large vehicles to pass or overtake without eithervehicle having to move sideways towards the outer edge ofthe lane.
The lane width and the road surface condition have asubstantial influence on the safety and comfort for users of theroadway. In rural applications the additional costs that will beincurred in providing wider lanes will be partially offset by thereduction in long-term shoulder maintenance costs. Narrowlanes result in a greater number of wheel concentrations in thevicinity of the pavement edge and will also force vehicles totravel laterally closer to one another than would normallyhappen at the design speed.
Drivers tend to reduce their travel speed, or shift closer to thelane/road centre (or both) when there is a perception that afixed hazardous object is too close to the nearside or offside ofthe vehicle. When there is a perceived fixed hazard, there is amovement by the vehicle towards the opposite lane line.
Alternative lane widths may be considered in somecircumstances. Wider traffic lanes should be considered whereany of the following apply:
● There is a higher volume of trucks (greater than 80 per day)for the middle lanes of a carriageway as sealed shouldersprovide enough space for lanes abutting shoulders;
● There is a need for widening on horizontal curves;
● The left lane is to be used by cyclists; or
● Operation of Type 2 (triple) road trains (or even largervehicles) is anticipated.
Narrower lanes (suggest down to 3.0m – Ref. 18) should beconsidered where any of the following apply:
● The road reserve or existing development form stringentcontrols preventing wider lanes;
● The road is in a low speed environment;
● There is little or no truck traffic;
● Finance for road construction is limited; or
● The alignment and safety records are satisfactory in thecase of a reconstructed arterial.
This lane width framework should be supplemented by theconsideration of local practice and experience.
For prime mover and semi-trailer operation, radii above 300mshould be used to avoid lane widening. The use of lanes widerthan 4.6 metres as a result of lane widening is not favouredbecause of the possibility of two cars travelling side-by-sidewithin the lane. If greater width is required for truck tracking, anedge line should be placed at 3.5 m and full pavement depthwidening should be provided for the remainder of the width.
11.3 Traveled Way
Traveled way is that portion of a carriageway ordinarilyassigned to moving traffic (excludes shoulders and parkinglanes). Its width depends on design traffic volumes (AADT) andadopted level of service.
Where operating speeds are over 80km/h or where the heavyvehicle volume in the traffic flow is high, traveled way widthshould be based on 3.5m wide traffic lanes.
11.3.1 Single Carriageways
On many roads in Australia, traffic is less than 150 vehiclesper day. Some of these are arterial roads passing throughsparsely settled flat country where the terrain leads to a highoperating speed.
Where traffic volumes are less than 150 vehicles per dayand, particularly, where terrain is open, single lanecarriageways may be used. The traffic lane width adoptedon such roads should be at least 3.5m. A width of less than3.5m can result in excessive shoulder wear. A width greaterthan 4.5m but less than 6.0m may lead to two vehiclestrying to pass with each remaining on the seal. This
RURAL ROAD DESIGN 61
Element Design AADT
1-150 150-500 500-1,000 1,000-3,000 >3,000
Traffic Lanes 3.5 6.2 6.2-7.0 7.0 7.0(1 x 3.5) (2 x 3.1) (2 x 3.1/3.5) (2 x 3.5) (2 x 3.5)
Total Shoulder 2.0 1.5 1.5 2.0 2.5
Shoulder Seal 0.5 0.5 0.5 1.0 1.5
Note:● Traffic lane widths include centre-lines but are exclusive of edge-lines.● Shoulder beyond the seal can be lightly constructed, gravel surface suitable for supporting occasional heavy wheel load.● Short lengths of wider shoulder seal or lay-bys to be provided at suitable locations to provide for discretionary stops.● Wider shoulder seals may be appropriate depending on requirements for cyclists, maintenance costs, soil and climatic conditions
or to accommodate the tracked width requirements for Large Combination Vehicles.● Full width shoulder seals may be appropriate beside guard barrier and on the high side of superelevation.
Table 11.1: Single Carriageway Road Widths
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potentially increases head-on accidents. The width of 3.5mensures that one or both vehicles must have the outerwheels on the shoulders while passing.
On two lane sealed roads, total width of seal should desirablybe not less than 7.2m to allow adequate width for passing.
11.3.2 Divided Carriageways
A divided rural road has two carriageways separated by amedian. The median width is defined as exclusive of any roadshoulders where provided.
Each of the two carriageways should have at least two trafficlanes so that overtaking is possible. With each carriageway, theshoulder remote from the median should be at least 2 m wide,
but preferably wider to accommodate a broken-down vehicle.Where the shoulder is less than 2 m, opportunity should betaken to provide wider standing areas at regular intervals,by flattening fill slopes on low formations or by wideningshoulders at the transition from cut to fill. The wideningshould be sufficient to allow traffic to pass a stoppedvehicle without having to change position in the lane. Atthe least, the widening should be sufficient to allow trafficto pass a stopping vehicle by changing position in the lanewithout encroaching into the adjoining lane. Although fewrural roads in Australia carry traffic volumes sufficient torequire more than four lanes, in designing a rural road it iscommon to assume that wider carriageways may berequired at some future time and to reserve the landrequired. Table11.2 contains the widths of cross sectionelements for rural roads.
RURAL ROAD DESIGN62
Element Design AADT
< 20,000 > 20,000
Traffic Lanes (1) 3.5 3.5
ShoulderLeft 2.5 3.0Median 1.0 1.0
Shoulder SealLeft 1.5 (2, 3) 3.0Median 1.0 1.0
Median (4)
Wide, no barrier protection 15mNarrow, barrier protected (5) 3m rigid barrier, 8m flexible barrier
Verge Refer Table 11.6
Note:(1) Traffic lane widths include lane lines but are exclusive of edge lines.(2) Wider shoulder seals may be appropriate depending on requirements for cyclists, maintenance costs, and soil and climatic
conditions.(3) Full width shoulder seals are appropriate beside guard barrier and on the high side of superelevation.(4) The median widths are exclusive of median shoulders. Refer Figure 11.5.(5) A greater median width will be required to accommodate at-grade intersections.
11.3.2 Divided Carriageway
Table 11.2: Divided Carriageway Road Widths
11.3.1 Single Carriageway
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11.3.2.1 Independent Design of Carriageways
When land is available, economies can be achieved in earthworkvolumes on divided roads when the two carriageways are farenough apart for them to be partly or wholly aligned andgraded independently of one another.
As carriageways are moved further apart, the width ofbridging over the road may reduce if bridging of the mediancan be omitted.
Close parallel carriageways, both at the same level, can appearmonotonous and have a sleepy effect on the driver. Whenparallel carriageways are to be relieved, a change in thedirection of one relative to the other is best arranged to takeplace at a curve, either vertical or horizontal, so that anyapparent kink resulting from the change can be hidden.
Although the distance between carriageways should be suchthat traffic on one carriageway would not influence drivingbehaviour on the other, a regular glimpse of the secondcarriageway is desirable to reassure drivers that they are on aone-way carriageway.
11.3.2.2 Superelevation Issues
On straights, each carriageway may have a single crossfall or thecarriageways may be individually crowned. The carriagewaysmay have a common grading such that each is at the samelevel or they may be individually graded. The two carriagewaysmay be parallel or individually aligned with median widthvarying. On curved sections, the superelevated lengths of thetwo carriageways may be in one plane or be in parallel planesor they may be far enough apart to be independent.
When the numerous combinations are considered, it becomesimpracticable to identify all the issues for the application ofsuperelevation on divided roads. However, the most commonissues are discussed for independent and related carriageways.
Independent Carriageways
Usually, carriageways, which are independently aligned andgraded, are widely separated with an undisturbed medianarea. In such cases, a carriageway may be designed asthough it were a normal two-lane two-way road or as a two-lane road with an auxiliary lane where three lanes percarriageway are proposed.
Related Carriageways
Where the median is relatively narrow, it is usual for thecarriageways to be parallel and at the same level, avoidingdifficulties in significant level differential in the narrow median.Commonly, the median would be depressed.
Transition curves can be developed along the same principlesas for two-lane two way roads, but the superelevationdevelopment length will still vary in relation to a number offactors. Such factors would relate to each carriageway:
● Crowning;
● Single crossfall;
● Control line location;
● Axis of rotation.
Figure 11.2 illustrates typical developments of superelevationfor parallel carriageways with single grading and a narrowdepressed median. Similar forms of treatment could apply fora raised median.
As the median becomes wider, there is more latitude to absorblevel differential at the edges of the median over the transitionand superelevated lengths, as median slopes can be variedwithin reasonable limits to maintain a uniform invert grading.
The selection of the type of crossfall and the choice of acontrol line for grading may be influenced by the general roadalignment. A section of roadway containing long lengths ofstraights and few curves may be better suited to carriagewayswith individual crowns, with the control lines along each of thecrowns. This method is generally not used as it is consideredthat each carriageway should fall away from the median tominimise cross median incidents. A section with a highpercentage of curved alignment might be better suited tocarriageways with single crossfalls with the control lines alongthe inner shoulder edges, or even along the centre line of theformation overall.
11.3.2.3 Transitions Between Divided and Undivided Carriageways
A number of situations can arise, either temporarily orpermanently, where a transition is made between a dividedand an undivided carriageway. This commonly occurs wherean existing two-lane two-way road is being duplicated instages due to varying traffic or level of service conditions alongthe route, such as a strategy to provide increased overtakingopportunity, or due to funding or construction expedience.
A number of short lengths of dual carriageway in closeproximity can cause confusion to drivers and special attentionneeds to be given to traffic signing and road markingprovisions. In situations where short lengths of duplication arebeing used to provide increased overtaking opportunity, aduplication length of at least 3 km is desirable.
The transition between divided and undivided roadwaysshould take place in an area where there is good sight distancein both directions. For details of the design of the transitionsee Ref. 18.
11.4 Pavement Crossfall and its Considerations
Crossfall is the slope of the surface of a carriageway measurednormal to the centre line. The purpose of crossfall is to drainthe carriageway on straights and curves and to providesuperelevation on horizontal curves.
Crossfalls flatter than 2% do not drain adequately, and even2% should only be prescribed for concrete pavements wherelevels and surface finish are tightly controlled. Unlesscompaction and surface shape are well controlled duringconstruction, pavements with less than 2.5% crossfall will holdsmall ponds on the surface, which may cause potholes todevelop and hasten pavement failure. Rutting of the pavement
RURAL ROAD DESIGN 63
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RURAL ROAD DESIGN64
Figure 11.2: Changes of Crossfall on Related Carriageways
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is also more likely to hold water, increasing the risk ofpavement deterioration and vehicle aquaplaning when thepavement crossfall is less than 3% (Ref. 67).
The pavement crossfall on straights for various pavement typesis given in Table 11.3.
Generally, on divided roads, two-lane carriageways onstraights have a uniform one-way crossfall with the high pointof the pavement at the edge nearest the median. Two-waycrossfall, with the crown in the middle of the pavement, maycome about through one of the carriageways having been orbeing intended for an initial two-way road. Other factors,which could influence the choice between crowned and one-way crossfalls, would include median treatment and mediandrainage. One-way crossfalls would be more likely when themedian was narrow. A crowned crossfall directs more watertowards the median.
The build up of sheet flow across a wide carriageway canbecome a safety problem, and three-lane carriageways onstraights are usually crowned with one lane falling towardsthe median.
At intersections, the crown position may have to be varied tosuit drainage and the grading of the intersecting road. Thewhole pavement surface area has to drain while retainingsatisfactory riding qualities for all traffic movements, havingregard to vehicle speeds. Usually, it is desirable to preparepavement surface contours or profiles to assist design andsubsequent construction.
On straight sections of divided roads where the crossfall of thepavement is away from the median and the shoulders are notsealed, it would be usual for the crossfall of the medianshoulder to be towards the median. With this arrangement,reversed crossfall where pavement meets shoulder, the slopeof the median shoulder may be reduced as necessary to give atotal change of crossfall, pavement to shoulder, of not morethan 7%. Desirably, on curves with superelevated pavements,shoulder crossfall should match that of the running lanes.Where design constraints make this difficult, the frictiondemand of a vehicle passing onto the shoulder at the designspeed should be checked.
With superelevated curves on divided roads, the twocarriageways may be both in the one plane, or they may be inparallel planes with the difference in levels taken up with themedian. Where the two carriageways are far enough apartthat they may be graded independently of one another, there
may be a natural or undisturbed median area between them.Where the two carriageways are closer together, the crossfalland drainage of the median may begin to be a control on therelative levels of the two inner carriageway edges.
11.5 Shoulder
11.5.1 Function
Road shoulder carries out two functions:● Traffic; and● Structural.
The traffic functions of the shoulder are:
● An initial recovery area for any vehicle which may get outof control;
● A refuge for stopped vehicles on a firm surface at a safedistance from traffic lanes;
● A trafficable are for emergency use;
● Space for cyclists;
● Clearance to lateral obstructions; and
● For road train routes, the shoulder has the additionalfunction of providing for the additional tracked widthassociated with road trains. Refer Section 11.2
The structural function of the shoulder is to provide lateralsupport to the road pavement layers.
11.5.2 Width
Shoulder width is measured from the outer edge of the trafficlane to the edge of usable carriageway and excludes any berm,verge, rounding or extra width provided to accommodateguideposts and guard fencing. Wide shoulders have thefollowing advantages:
● Space is available for a stationary vehicle to stand clear ofthe traffic lanes; a vehicle standing partly on a shoulderand partly on a traffic lane may be a hazard.
● Space is available on which vehicles may deviate to avoidcolliding with other vehicles and on which a driver mayregain control of his vehicle.
● The resulting wider formations increase driver comfort andthe quality of service of the road.
● They contribute to improved sight distance across theinside of horizontal curves.
Table 11.1 lists shoulder width values for two lane rural roadsbased on AADT volumes. These widths allow a vehicle to stop,or a maintenance vehicle to operate, with only partialobstruction of the traffic lanes. Provided volumes are not highor sight distances are sufficiently long, this will not present anundue hazard to traffic.
A width of 2.5m is needed to allow a passenger vehicle to stopclear of the traffic lanes.
RURAL ROAD DESIGN 65
Type of Pavement Crossfall (%)
Earth, Loam 5
Gravel, Water bound Macadam 4
Bituminous Sprayed Seal 3
Bituminous Concrete (asphalt) 2.5-3
Portland Cement Concrete 2-3
Table 11.3: Pavement Crossfall on Straights
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A width of 3.0m allows a passenger vehicle to stop clear of thetraffic lanes and provides an additional clearance to passingtraffic. It also allows a commercial vehicle to stop clear of thetraffic lanes.
The cost of maintaining road shoulders does not rise inproportion to their width. However, the cost of the initialconstruction involves additional earthwork and pavementcosts. In reconstruction of older pavements, the provision ofwider shoulders may increase the costs extensively.Therefore, an economic balance must be achieved inshoulder width, and in the case of upgrading work thiselement can be very significant.
The aim should be to provide shoulders of 1.5 m to 2.0 mwherever possible, and up to 2.5 to 3 m on higher volumeroads. Because most vehicles standing on road shouldersexercise some choice as to the stopping place, it is desirable totake every opportunity to provide areas at intervals wherevehicles can stop completely clear of the traffic lanes, such ason low fills where flattening the slopes automatically providesthis, or at the transition from cut to fill where minor additionalearthworks involved can be made at low cost.
On a divided road, refer Table 11.2; with two lanes in eachdirection, it is desirable to provide shoulders at least 2.5m wideon the left side of each carriageway and 1.0m wide on themedian side of each carriageway. If the divided road has threelanes in each direction, it is preferable to have wide shoulderson both sides of both carriageways. This limits the number oflanes a vehicle may have to cross in the event of breakdownsto stop clear of the traffic lanes.
11.5.3 Shoulder Sealing
Shoulders may be wholly or partially sealed. Sealing ofshoulders is frequently done to reduce maintenance costs andto improve moisture conditions under pavements, especiallyunder the outer wheel path.
However, from the geometric design point of view, theshoulder is regarded as being usable by traffic. Partial sealingensures this by protecting the lane edge against thedevelopment of the broken edges or ‘drop offs’ that occuradjacent to the traffic lanes and results in the whole shoulderwidth remaining usable to traffic up to 2.5m wide.
The desirable width of sealed shoulder depends on manyfactors including:
● Traffic composition;● AADT;● Access;● Operating speed;● Rainfall; and ● Shoulder pavement.
While 0.5m wide seals on the shoulders should be consideredthe minimum when the predicted AADT is less than 2000,more sealed width is often warranted. In some instances,partial shoulder sealing is widened to full width adjacent toconcrete gutters and on the topside of superelevated curves.In wetter areas where moisture control is required, shoulderwidth of 0.5 m is desirable and 1.0 m is preferable. In the caseof full or partial sealing of shoulders, longitudinal edge lines
are desirable at the edge of the traffic lanes. Otherwise, in thecase of narrow partial sealing, usage of the additional seal aspart of the traffic lanes merely transfers the problem to thenew edge.
To minimize the effect of wind erosion on shoulder material, a1.0m seal is often used on roads carrying AADT over 2000 vpd(with 10% heavy vehicles (Ref. 90)).
The widths required for the various functions are set out inTable 11.4.
A full width seal should be considered under the followingconditions:
● Adjacent to a lined table drain, kerb or dyke;● Where a safety barrier is to be provided;● On the outer shoulder of a superelevated curve;● On floodways;● Where rigid pavement is proposed’● Where environmental conditions require it;● Where needed to reduce maintenance; and● In high rainfall areas.
A contrast in texture or colour between the sealed shouldersand the pavement would assist in defining the limits of thetraffic lanes and supplement the edge lines. Where medianshoulders are not sealed, depending on median configuration,
RURAL ROAD DESIGN66
Function of Shoulder Sealed Width
(m)
Lateral support of pavement 0.5
Control of water flow path on outside curves 1.0
Initial recovery area 0.5
Discretionary stopping- Cars 2.5- Trucks 3.0
Bicycle demand 2.0/3.0
Source: Ref. 99
Table 11.4: Shoulder Width
11.5.3 Shoulder Sealing
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it may be found that the width of 1 m is not suitable formaintenance using mechanical equipment. A width of 1.5 mor 2 m may, therefore, be adopted.
11.5.4 Crossfalls
Shoulders generally should be steeper than the adjacent trafficlanes to assist surface drainage (marginal increase of 1%).However, where the shoulder consists of full depth pavementand is sealed, its slope may be the same as the adjacentpavement in order to facilitate construction.
On straights the shoulder crossfall is shown in Table 11.5
On superelevated sections of roads, the shoulder on the highside and low side must have the same crossfall as the trafficlanes. A cross fall of 5% or more extended across the verge maylead to more frequent maintenance and should be monitored.
11.6 Verge
The main functions of the verge are to provide:
● Traversable transition between the shoulder and the batterslope;
● A firm surface for stopped vehicles at a safe distance fromtraffic lanes;
● Support for the boxing edge and shoulder material;
● Space for installation of guide posts and road safetybarriers; and
● Provide rounding between the formation cross slope andembankment batter slope to assist controllability ofvehicles, which encroach the formation and to reducescouring due to road storm water run off.
The minimum widths for these functions are shown in Table 11.6.
It is not intended that verge widths should vary continuously.Designers should apply long sections of appropriate minimumverge width with short transitions where greater or lesserwidths are required.
Verge and batter toe rounding are of critical importance inminimizing rollover accidents. Verge rounding (see Figure11.3) enables tyre contract to be maintained and decreasesthe likelihood of rollover. An errant vehicle may becometemporarily airborne where the verge is only 0.5m wide, andthe change in slope is greater than 7 per cent. Verges andverge rounding should be provided on unkerbed medianswhere the lateral change in grade is greater than 10 per cent.Also, rounding at the toe of batter reduces the potential tooverturn due to tripping.
11.7 Batters
Batters are surfaces, commonly but not always of uniformslope, which connect carriageways or other elements of crosssections to the natural surface. Batters may:
● Provide a recovery area for errant vehicles;● Be used as part of the landscaped area; and● Be used for access by maintenance vehicles.
Batter slopes are usually defined as the ratio of one vertical on“x” horizontal and are shown as, for example, 1 on 4.
The following factors should be considered when selectingbatter slopes:
● The results and recommendations of geotechnicalinvestigation;
● Batter stability;● Batter safety (economics of eliminating safety barriers);● Future costs of maintaining the adopted slope;● Appearance and environmental effects;● Earthworks balance;● Available width of road reserve; and● Landscaping requirements.
Slopes flatter than the desirable maximum (see Table 11.7)should be used where possible.
In shallow cuttings (up to about 3 meters depth) it is commonpractice to flatten cut batters beyond that required for stabilitypurposes for improved appearance. In areas where the batterstransition from cut to fill, a catchline treatment (a constant
RURAL ROAD DESIGN 67
Shoulder Material Crossfall %
Earth, Loam 5 – 6
Gravel and crushed rock 4 – 5
Full depth pavement with bitumen seal or asphalt as wearing course Match traffic lane
Concrete Match traffic lane
Table 11.5: Shoulder Crossfall
Table 11.6: Verge Width
Function Width (m)
1 Shoulder support and locate guide posts 1.0
2 Traversable transition between the shoulder and the batter slope (depending on how steep the superelevation and/or batters might be and what batter rounding is required) 1.0 to 6.0
3 To provide a space for installation of road safety barrier (extra for terminals) 1.5
4 To achieve horizontal sight distance, or to balance cut and fill Where required, 3m to 5m.
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RURAL ROAD DESIGN68
Figure 11.3: Verge Rounding
Table 11.7: Design Batter Slopes (without safety barriers)
CLASSIFICATION CUT SLOPES FILL SLOPES
Height Desirable Maximum Height Desirable Maximum
ARTERIAL RURAL DIVIDED
Batter: Earth H < 3m 1 on 3 1 on 2 H < 3m 1 on 6 1 on 4 H > 3m 1 on 2 1 on 1.5 H3 - 12m 1 on 4 1 on 2(2)
H > 12 1 on 2(2) 1 on 2(2)
Batter: Rock 1 on 0.5 1 on 0.25 - - -
Table Drain Batter 1 on 6 1 on 2 - -
Median Batter 1 on 10 1 on 6 1 on 10 1 on 6
RURAL UNDIVIDED
Batter H < 3m 1 on 3 1 on 2 H < 3m 1 on 6 1 on 3H > 3m 1 on 2 1 on 1.5 H > 3m 1 on 4 1 on 4
Table Drain Batter 1 on 4 1 on 2 - -
LOCAL
Batter H < 3m 1 on 2 1 on 1/5(1) H < 3m 1 on 4 1 on 2H > 3m 1 on 2 1 on 1/5(1) H > 3m 1 on 4 -1 on 2(2)
Table Drain Batter 1 on 4 1 on 2 1 on 6 1 on 4
Notes:(1) May be steeper in rock cut. Source: Ref. 99(2) Batter with roadside safety barrier installed.(3) A benched fill slope batter of 1 on 1.5 may be considered in specific cases.(4) Batter slopes may vary depending on height and geotechnical reports.
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batter offset) may be used to smooth the transition fromadjoining cut to fill. It also blends the batters into thesurrounding terrain as it follows the natural slope of the surface.Catchlines, or constant batter widths are also applicable, on thegrounds of aesthetics, in flat and gently undulating terrain.
Where shoulders are near the minimum widths given in Table11.4, opportunity should be taken to provide pull-off areas atintervals, on low fills (0.5 m) and at the transition from cut tofill. Catchline treatment assists this provision.
Where earthwork volumes are significant, maximum batterslopes are dictated by the angle at which the material will standcut, or at which it can be shaped for a stable embankment.While solid rock cuttings might be stable when vertical, it isunusual to adopt a slope steeper than 1 on 0.25, as otherwisethe cutting walls can give the impression of leaning inwards.
Accidents can occur where vehicles run off the road and thedriver loses control on a steep embankment or the vehicle runsinto a cutting wall or drain. The severity of this type of accidentmay be reduced if the batter slopes are sufficiently flat for thedriver to recover control of the vehicle. However, where truckvolumes are high (10% and more), embankment slopes flatterthan 1 on 6 are desirable, refer also Section 17.3.1.
For maintenance purposes (grass mowing) a maximum batterslope of 1 on 2 for side boom slashers is to be used. However,this cannot be achieved in some areas due to geotechnicalrestraints. A 1 on 4 is the preferred maximum batter slope fora slasher (the most widely used maintenance machine).Mowers and slashes are likely to overturn on a 1 on 3 orsteeper batter. Irregularities in the batter face may contributeto overturning. The steepest slope preferred for plantingpurposes is 1 on 3 and will assist revegetation.
11.7.1 Benches
On high batters (generally exceeding 10m vertical height) orwhere batters are constructed on unstable material,consideration should be given to the provision of benches.
Benches can have the beneficial effects of:
● Eliminating the need to flatten the batter slope in theinterests of stability;
● Minimizing the possibilities of rock falling on to thepavement;
● Reducing scour on the batter face;
● Reducing the amount of water in cuttings to be carried bythe table drain;
● Providing easier access for maintenance of the batter face;
● Improving the appearance of the cutting;
● Assisting the re-establishment of vegetation;
● Improving sight distance on horizontal curves.
Benches should be sloped away from the roadway andlongitudinally so that stormwater can be drained towards the
ends of the bench and discharged on to the natural ground. Insome instances, the invert so formed may require lining.
The minimum width of bench should be 3m (see Figure 11.4)with a maximum crossfall of 10%. The desirable width ofbench for maintenance and drainage purposes is 5m.
11.7.2 Batter Rounding
Rounding of the tops of all cut slopes is essential in order toreduce erosion, especially riling. The size of the rounding is inthe range of 1m x 1m minimum up to 6m x 6m maximum,proportional to the height of the batter.
Rounding of 1m x 1m shall be applied to the base of all fillbatters steeper than 1 on 3, to avoid tripping of errant vehicles.
11.8 Medians
A median may be defined as a “strip of road not normally usedby vehicular traffic, which separates opposing traffic lanes”. Itsmain function is to separate opposing streams of traffic and tolimit conflict areas for turning traffic, thereby significantlyreducing the risk of severe collisions and increasing the safetyof the road. In addition, medians can:
● Reduce conflict with vehicular traffic waiting to turn right(by provision of protected turning lanes);
● Provide space to shelter crossing traffic at unsignalisedintersections;
● Reduce headlight glare;
● Provide a recovery area for out of control vehicles;
● Provide emergency stopping areas;
● Reduce air turbulence between opposing traffic;
● Accommodate level differences between carriageways;
● Provide scope for improvement of visual amenity bylandscaping;
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11.7.2 Batter Rounding
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Figure 11.4: Benches
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● Provide areas for the location of road furniture on the righthand side of carriageways.
Medians are usually incorporated in all rural roads of fourlanes or more. They may be raised or depressed as shown inFigure 11.5.
A depressed median should be of sufficient width to place theinvert of the median drain below subgrade level to facilitatedrainage of pavement layers. If this cannot be achieved,pavement subsurface drains shall be provided. Subsurfacedrains may be required as a result of the fill material type, evenif a median drain below subgrade level is provided. Theabsolute minimum width of a depressed median is 10 meters(for drainage reasons), and 15m are a desirable minimum.
Numerous studies have shown that wider medians improvesafety and that 90% of run off the road incidents deviate lessthan 15 m from the edge of the carriageway. However, themarginal effectiveness of increased width drops rapidly (80%of these incidents deviate less than 10 m) and, where land isexpensive, it is hard to justify widths greater than theminimum. In most rural areas, the additional cost of a widemedian is small and widths of 15 m (and more) can bewarranted. For Medians less than 15m, roadside safety barriersneed to be used to minimise cross-median incidents.
Raised medians are sometimes used, especially in cuttings, andhave some advantages with headlight glare and a reduction inearthwork costs; however, the cost advantage is somewhatmitigated by additional drainage and safety barrier costs.
The width of a median need not be constant and independentlyaligned and graded carriageways have much to commend,provided that the opposing carriageway is not out of sight forextended periods. Local widening at intersections may benecessary to accommodate crossing or turning heavy vehicles.
Due to numerous factors, usual practice is to widen on themedian side for extra lanes and the median width adoptedshould include provision for future widening.
Further discussion on the function and design of medians isprovided in Road Medians, AASHTO 1996 Roadside DesignGuide, Ref. 2.
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11.8 Median
Figure 11.5: Typical Median Cross Sections
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11.9 Roadside Drains
Roadside drains remove water from the road and itssurroundings in order to maintain the traffic safety and strengthof the pavement. The basic types of roadside drains are:
● Table drains;● Catch drains; and ● Median drains.
11.9.1 Table Drains
Table drains are located on the outside of shoulders in cuttingsor alongside shallow raised carriageways in flat country. Anunsealed table drain should have its invert level below the levelof pavement subgrade for effective drainage of the pavement.This becomes less important where a subsurface drain isprovided at the edge of the pavement.
Where scour is likely because of the nature of the material orbecause of the longitudinal grading, some type of protectionof the drain invert would be required. This protection couldtake the form of loaming and grassing, rock lining or concrete.Lining is generally applicable where the material is likely toscour due to velocity. The terminal treatment at the bottom ofa steep drain is also important.
Consideration may also be given to sealing the outer edges ofthe pavement, the shoulder verges and the drain lining wheresiltation or scour could be a problem. Typical table drain detailsare shown in Figure 11.6.
In flat country, the table drain is sometimes used as a source ofborrow material. Flat bottom inverts may be adopted wherethere is a shortage of materials, and this has the additionalbenefit of reducing scour of the invert. The use of “V” drainsshould be discouraged due to adverse scouring potential. Tabledrains in flat country can hold water and cause damage to thepavement in some areas.
The side slopes of table drains should be flat enough tominimize the possibility of errant vehicles overturning. Sideslopes not steeper than 1 on 4 with a desirable slope of 1 on6 are preferred.
11.9.2 Catch Drains
Catch drains are located on the high side of cuttings clear ofthe top of batters to intercept the flow of surface water andupper soil seepage water (Figure 11.6). Their purpose is toprevent overloading of the table drain and scour of thebatter face.
They are generally located at least 2.0 m from the edge ofthe cuttings in order to minimize possible undercutting of thetop of the batter. Catch banks are sometimes used instead ofdrains to reduce effects of seepage on stability of the batterslopes.
11.9.3 Median Drains
Where depressed medians are adopted, the median will berequired to perform functions similar to those of a table drain.
There are no special considerations required when raised
medians (kerbed) are adopted; normal design practice applieswhere the kerb acts as a channel.
11.10 Noise Barriers
Traffic noise and the need to protect the abutting environmentare discussed in Section 6.3.
Cross-sectional detail to provide for noise barriers is shown onFigure 11.7.
11.11 Right of Way
The clearance to the right of way boundary can be measuredfrom either the batter line or the edge of traffic lanes. It isdependent upon several factors including:
● Class of road;● Landscape planting;● Drainage requirements;● Access for maintenance vehicles;● Batters;● Batter rounding;● Requirements for services; and● Cost, etc, in obtaining additional right of way.
Generally, a clearance of at least 5m to the batter line and 10mto the pavement edge is desirable.
Extra clearance may be needed adjacent to high cut batters toprevent the erosion of batters affecting adjacent property.
11.12 Widths of Bridges
A guide to the width of traffic lanes on bridges and theclearance between the outer edge of traffic lanes andstructures such as retaining walls, bridge handrails, guardfencing, and subways are set out in the Austroads BridgeDesign Code (Ref. 29). Refer also to Section 9.13.
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11.9 Roadside Drain
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Figure 11.6: Catch Drains and Table Drains
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Figure 11.7: Noise Barrier Cross-Section Detail
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12. PRINCIPA L FACTORS
The principal factors influencing the choice of design standardfor a road are as follows.
12.1 Financial Level
The appropriate design standard for a particular road dependson both the overall availability of finance and the state ofdevelopment of the road network. When the overall networkis substantially adequate and finance is available, improvementprojects will be directed at operational safety and efficiency,and higher geometric standards are appropriate. When thenetwork is inadequate in terms of traffic demand and fundsare limited, geometric standards may be lowered selectively onparts of the road system. The state of the network and thefunding position are partly dependent on population, and thedeveloped area over which the network must spread. InAustralia, these vary between geographical and administrativeregions, and it is reasonable that the appropriate design forindividual roads should differ somewhat between regions.
12.2 Safety
Whatever design standard is adopted, safety is a major goal ofroad design. The theme of enabling the driver to perceivehazards in time to take appropriate action, and of providinggeometric parameters appropriate to the likely speed ofoperation, runs throughout the Guide. Further, vehicles canget out of control, and items like traversable batter slopes,roadside safety barriers, breakaway light poles and signsupports are desirable attributes of what has been described asa ‘forgiving’ roadside.
12.3 Energy
The total road fleet makes considerable use of liquid fuels andother products derived from crude oil. Grades exceeding about5% cause greater consumption of fuel by heavy vehicles in theuphill direction than they save in the downhill direction.However, the greatest changes in the energy consumptionrelated to transport spring from questions of appropriatemodes for long freight haulage, and from land usedistributions in urban areas. At the present stage, flattening ofgrades can rarely be justified on the basis of energy savingalone.
12.4 Stage Construction
In a situation of changing land use and growing traffic, noroad can ever be regarded as ‘final’. There will always berequirements for future augmentation or modification. Whereit is obvious that medium term requirements would alter thebest-staged design for a particular road, it is often possible tomodify the design slightly to provide better options for futureaction. While this ties up some funds and prevents their use on
other current projects, the effect can be much less than if thelonger-term design was adopted in the first instance.
One area where this approach is relevant is the high functionalclass low volume road. Even here, however, one must beaware of committing large amounts of current funds for verylong-term options.
13. AUXILIARY LANES
13.1 General
Auxiliary lanes are adjacent to the through traffic lanes. Theyare added to maintain the required level of service on the roadand for other purposes supplementary to through trafficmovement. They are used to remove traffic that is causingdisruption to the smooth flow of traffic in the through lane, toa separate lane. Auxiliary lanes improve the safety, capacityand level of service on the road in question.
13.2 Types of Auxiliary Lanes
Traffic speed and congestion on rural arterial roads are largelydetermined by two factors:
● Alignment and standard of a road affects the magnitudeand the spread of operating speeds;
● Interactions between faster and slower vehicles determinethe extent of traffic delay and congestion. The effect ofthese interactions is greatest when the spread of speeds(the difference between the operating speeds of thefastest and slowest vehicles) is largest.
Of these two, traffic interactions have an increasinglydominant effect on delay and congestion as traffic flowsincrease. Overtaking opportunities, therefore, have a largeeffect on traffic operations on rural roads. These can beimproved in varying degrees by the following methods:
● Speed change lanes;● Improved overtaking sight distance;● Overtaking and climbing lanes;● Wide full depth paved shoulders;● Four lane wide cross sections;● Dual carriageway cross sections;● Slow vehicle turnouts; and● Descending lanes.
The types of auxiliary lanes discussed in this section are as follows.
● Speed change lanes (acceleration and deceleration);● Overtaking lanes/climbing lanes;● Slow vehicle turnouts; and● Descending lanes.
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In addition, passing bays and emergency escape ramps(runaway vehicle facilities) are included in this category. In thisguide, weaving lanes are not treated as auxiliary lanes but aspart of the required cross section of a motorway whereweaving conditions occur.
13.3 Speed Change Lanes
13.3.1 Acceleration Lanes
Acceleration lanes are provided at intersections andinterchanges to allow an entering vehicle to access the trafficstream at a speed approaching or equal to the 85th percentilespeed of the through traffic. They are usually parallel to andcontiguous with the through lane with appropriate tapers atthe entering point. The warrants for this type of auxiliary laneand the desirable road layouts are discussed in Austroads“Guide to Traffic Engineering Practice”, Part 5 – “Intersectionsat grade” (Ref, 18).
13.3.2 Deceleration Lanes
Deceleration lanes are provided at intersections andinterchanges to allow an exiting vehicle to depart from thethrough lanes at the 85th percentile speed of the throughlanes and decelerate to a stop or to the 85th percentile speedof the intersecting road, whichever is appropriate for thecircumstances. These lanes are usually parallel to andcontiguous with the through lanes with appropriate tapers atthe departure point on the through lane.
At intersections, the deceleration lane can be placed on eitherthe right or the left of the through lanes, depending on thetype of turn being effected. At interchanges, it is preferredthat the exit be from the left side for most ramps and thedeceleration lane will therefore be on the left in most cases.
Details of the requirements for deceleration lanes are given inAustroads “Guide to Traffic Engineering Practice, Part 5 –“Intersection at grade” (Ref, 18)
13.4 Overtaking Lanes/Climbing Lanes
13.4.1 Overtaking Lanes
On two lane two-way carriageways, overtaking laneconfigurations are shown on Figure 13.1. These overtakinglanes are provided to break up bunches of traffic and improvetraffic flow over a section of road. They provide a positiveovertaking opportunity and are sometimes the only realchance for overtaking to occur.
The desirable layout is based on the start or end of the lanemerge location being separated by a 3 second distance oftravel time. This distance is to minimise the possibility ofconflict between opposing merging vehicles.
An acceptable layout, when the geometric considerations donot provide for an alternative is to allow the start of themerges to be opposite one another.
The undesirable and unacceptable configurations are shownto highlight the possible conflict areas of late merging vehiclesif these two were to be considered. These are not to be used.
13.4.1.1 Overtaking Demand
The demand for overtaking occurs each time a vehicle catchesup with another and the driver wishes to maintain the speed oftravel. Provided there is no approaching traffic, this manoeuvrecan occur at where there is adequate sight distance.
As traffic volume increases the approaching traffic will restrictthe available places where overtaking can occur and these willbe further limited by the road geometry.
If demand is not met the results are: enforced following, thegrowth of traffic bunches, and driver delay and frustration. Inextreme no-overtaking situations very long queues candevelop behind the slowest vehicles in the traffic stream. Thedelay and frustration experienced on grades may be greaterdue to the slow speed of travel. The proportion of the journeytime spent following in bunches is a useful measure of qualityof service as seen by the driver.
The type of slow vehicle influences the nature of overtakingdemand. Some vehicles can be overtaken easily anywherealong a route, while for others an upgrade overtakingopportunity is desirable. In evaluating the need for auxiliarylanes, attention should be given to the type of slow vehiclesinvolved and whether the overtaking demand is continuousalong a route or confined to specific problem locations.
Types of slow vehicles are:
● Vehicles with fairly high speeds, that slow down markedlyon grades;
● Vehicles with low speeds, not affected by grades; and● Vehicles with average speeds, that are seen as slow by
those wishing to travel faster.
13.4.1.2 Overtaking Opportunities
On two-lane roads, the availability of overtaking opportunitiesdepends on sight distance and gaps in the opposing trafficstream. As opposing traffic volume increases, overtakingopportunities become restricted even if sight distance isadequate. Sight distance that appears adequate may also beunusable on occasions due to the size of the vehicle in front,particularly on left-hand curves.
On an existing road, overtaking opportunities can be increasedeither by improved alignment or the provision of overtaking
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13.4.1 Overtaking lane
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Figure 13.1: Overtaking Lane Configurations
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Figure 13.2: Development of Overtaking Lane
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lanes. Of the two options, overtaking lanes will generally proveto be the most cost-effective in reducing the level of trafficbunching. This is because realignment to provide overtakingopportunities is likely to be a much more expensive option,and even then the opportunities are only available whenopposing traffic permits. This has been demonstrated by ARRBsimulation studies, which showed that the provision ofovertaking lanes at regular spacings often led to greaterimprovements in overall traffic operations than even majoralignment improvements (Ref. 58).
A two-lane two way road with overtaking lanes at regularintervals provides an intermediate level of service between twolane two way roads and four lane roads, undivided anddivided. The overtaking lanes may delay the need for theprovision of dual carriageways. Where a four-lane road hasalready been provided, and traffic volumes are consistentlyhigh, the need for auxiliary lanes on grades may still arisewhen there are a high proportion of heavy vehicles.
13.4.1.3 Warrants
In deciding whether an overtaking lane is warranted, theevaluation needs to be carried out over a significant routelength and not be isolated to the particular length over whichthe additional lane may be constructed.
Overtaking opportunities outside the particular length canaffect the result considerably. On multi lane roads, this may notapply since the reason for the extra lane will usually beconfined to a specific location.
The following guidelines are based on initial ARRB researchusing traffic simulation and benefit-cost analysis (Ref. 59).Alternatively, the need for an additional lane can be evaluatedin terms of level of service. In special circumstances, a moredetailed evaluation may be undertaken using traffic simulationor the results of prior ARRB research (Ref. 58).
The basis for adopting an overtaking lane is the traffic volume,the percentage of slow vehicles including light trucks and carstowing, and the availability of overtaking opportunities onadjoining sections. The percentage of road allowingovertaking is described in section 8.4 of this Guide.
Table 13.1 gives the current-year design volumes (AADT) atwhich overtaking lanes would normally be justified. Theseguidelines apply for short low-cost overtaking lanes atspacings of 3 to 10 km or more along a road in a givendirection. If spacing is less than this a specific cost benefitanalysis will be needed to justify the construction at theshorter spacing.
Development of an overtaking lane is shown in Figure 13.2.
13.4.1.4 Length
Table 13.2 (a) presents the adopted lengths of overtaking lanelengths that are appropriate for both grades and level terrain.On long grades, the values for a lower operating speed shouldbe used. The minimum lengths provide for the majority ofmovements as single over takings, but may not allow manymultiple over takings, or over takings between vehicles withonly a small difference in speed. Minimum lengths aregenerally only appropriate for lower operating speeds orconstrained situations.
Overtaking lanes may be extended up to the normal maximumlength to allow start and termination points to fit in with theterrain. However since bunches generally break up in the firstsection of the overtaking lane, the additional length is not aswell utilised.
As a general rule, it is more cost-effective to construct twoshort overtaking lanes several kilometres apart rather than toconstruct one long one in excess of the normal maximumlength. Even when very long bunches occur at the start of an
Overtaking Opportunities Current-year DesignOver the Preceding 5 km (1) Volume (AADT)
Description Percent Length Percentage of Slow Vehicles (3)Providing Overtaking (2)
5 10 20
Excellent 70-100 5670 5000 4330
Good 30-70 4330 3670 3330
Moderate 10-30 3130 2800 2470
Occasional 5-10 2270 2000 1730
Restricted 0-5 1530 1330 1130
Very Restricted (4) 0 930 800 670
Table 13.1 Traffic Volume Guidelines for Providing Overtaking Lanes
Note:(1) Depending on road length being evaluated, this distance could range from 3 to 10 km.(2) See Section 8.4.4.(3) Including light trucks and cars towing trailers, caravans and boats.(4) No overtaking for 3 km in each direction.
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Operating Overtaking Lane Lengths (excluding taper lengths) (m)*Speed(kmh) Minimum Desirable Minimum Normal Maximum
50 75 225 325
60 100 250 400
70 125 325 475
80 200 400 650
90 275 475 775
100 350 550 950
110 420 620 1070
overtaking lane, it is generally preferable to provide severalovertaking lanes at regular spacings rather than one very longone. This should break up traffic bunches before they becomevery long.
The length of an overtaking lane on a grade is largelyconstrained by the choice of appropriate locations for start andtermination points. These should be clearly visible toapproaching drivers, and be located to minimise speeddifferences between slow and fast vehicles. These constraints,however, sometimes lead to quite long and/or expensiveclimbing lane proposals.
The sight distance to the termination of the overtaking lane isbased on the distance for the vehicle in the fast lane tocomplete or abandon the overtaking manoeuvre. The sightdistances required to overtake the various types of MCV’s areshown in Table 13.2 (b).
Situations may exist however, where an overtaking lane mightend where the sight distance is less than that required tocomplete an overtaking. In such cases drivers will have to relyon adequate signage of the termination.
13.4.1.5 Location
The location of overtaking sites should be determined afterconsidering the following:
● Strategic planning of the road in question and the longterm objectives of that link – the spacing and consequently,expenditure, must be in accord with the strategy to obtainthe best use of funds over the whole network;
● Nature of traffic on the section of road – if queuing occursall along the route, then overtaking lanes at any locationwill be useful; if they occur at specific locations where slowvehicles cause the queue, then specific locations shouldbe chosen;
● Location of grades – may be more effective to takeadvantage of the slower moving vehicles;
● Costs of construction of the alternative sites – may get amore cost effective solution by locating on the sites whereconstruction is cheapest;
● Geometry of the road – when the sites are not on grades,sections with curved alignment and restricted sightdistances are generally preferable to long straight sections.These locations will make the location appear appropriateto the driver. However, sections with curves with reducedsafe speeds are not suitable for overtaking lanes.
If the conclusion is that the overtaking lane should be locatedon a grade, the length will be tailored to fit the grade. If thecosts of the lane on the grade outweigh the benefits of beingon the grade, the lane should be located to minimise the costs.Alternatively, a partial climbing lane could be considered (see“Climbing Lanes” Section 13.4.2).
13.4.1.6 Spacing
The factors already discussed must be taken into account indeciding the spacing of the overtaking lanes on a section. Ananalysis of the operating conditions over the whole link in thenetwork, combined with the strategy for that link will establishthe desired locations and therefore the spacing of theovertaking lanes. In general, if no auxiliary lanes exist,establishing the first ones at a larger spacing will providebetter service than placing two lanes in close proximity.
In the first instance, a spacing of up to 20km (Ref 98) may beappropriate, depending on the available overtakingopportunities. A more desirable spacing would be from 10 to15km with the objective of providing overtaking opportunitiesevery 5km in the long term. The intermediate lanes will beprovided between the initial installations as required as thetraffic grows.
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Table 13.2 (a) Overtaking Lane Lengths
Note:* (1) Derived from Table VI Ref. 59(2) Refer Table 13.8 for diverge and merge taper lengths(3) For road train routes, the normal maximum should be the minimum and lengths 1.5 times the ‘normal maximum’ are desirable.
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There may be cases where the spacing is closer (3km) becauseof the proximity of long grade sections requiring treatment. Afurther case where the spacing may be close is where twopartial climbing lanes are provided on the same long grade toreduce the total costs involved. In all these cases, theavailability of overtaking opportunities on adjacent sectionsmust be taken into account.
Further research is needed into the effect of variouscombinations of configurations, length and spacing, on thetraffic operations and level of service of overtaking lanes.
13.4.1.7 Improvement Strategy ForOvertaking Lanes
The goal of any improvement strategy is to identify and planfor staged development that will keep pace with increases intraffic demand, ensuring the availability of overtakingopportunities at regular intervals. A strategy for improvingoperational performance of two-lane two-way rural roadsshould consider overtaking lane strategy in the context ofpotential future road duplication.
With an overtaking lane strategy, overtaking lanes should beprovided to maintain the desired level of service. Fullduplication of the road will not normally be anticipated duringthe economic life of these improvements. This period of time,typically 20 years, will be used to recover the cost of theimprovements. This strategy should be applied when there areno existing overtaking lanes. The proposed spacing (for eachdirection) will typically be 3 to 10 km.
The upper limit for an overtaking lane strategy is 800 veh./hr,if the desired level of service is C. If the desired Level of Servicewere B, 500 veh/hr would be the upper limit (Ref. 66).
For hourly traffic volumes above the suggested limits, astrategy that is compatible with future road duplication shouldbe adopted. In this situation, full duplication will normally
occur within the economic life of the overtaking lane pavement.Sections of duplication 2km long and at 5km spacings areusually warranted. This strategy does not necessarily precludethe use of some overtaking lanes, particularly at the initialstages. However, it is highly desirable to use all improvements inthe final road duplication.
Further analysis of a particular section of road will be requiredto determine the optimum combination of overtaking lanelength and spacing.
13.4.2 Climbing Lanes
13.4.2.1 General
Climbing lanes can be considered as a special form ofovertaking lane but they are only provided on inclines. Wherethey are provided, they form part of the network of overtakingopportunities and will therefore have an effect on decisions onthe location of other overtaking lanes.
On multi lane roads, there is no need to take account of theoverall overtaking situation, as the effect is limited to thespecific location of the grade in question. The decision onwhether to add a climbing lane is based on level of serviceconsiderations only. Climbing lanes on multilane roads arespecifically provided for slow moving vehicles and aretherefore treated differently for signing and line marking.Refer Section 13.5.
13.4.2.2 Warrants
Climbing lanes are warranted where:Truck speeds fall to 40km/h or less; andTraffic volumes equal or exceed those in Table 13.3.In addition, climbing lanes should be considered where:● Long grades over 8% occur;● Accidents attributable to the effects of the slow moving
trucks are significant;
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Multiple Combination Vehicles
Operating Speed Car & Prime Mover B Double Type 1 Type 2(kmh) Semi-Trailer Road Train Road Train
50 110 120 130 145
60 135 145 160 180
70 165 180 195 225
80 200 220 245 285
90 250 270 305 355
100 300 330 345 400
110 375 410 410 435
120 430 430 430 435
130 450 450 450 450
Table 13.2 (b) Merge Sight Distance at End of Overtaking Lane for Cars Overtaking MCV’s
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Table 13.3: Volume Guidelines for Partial Climbing Lanes
Table 13.4 (b): Merge Sight Distance at end of Climbing Lane for Cars Overtaking MCV’s
Overtaking Opportunities Over Current Year the Preceding 5km (1) Design Volume (AADT)
Description Percent Length(2) Percentage of Slow Vehicles(3)
Providing Overtaking
50 110 120 130 145
5 10 20
Excellent 70-100 4500 4000 3500
Good 30-70 3500 3000 2600
Moderate 10-30 2500 2200 2000
Occasional 5-10 1800 1600 1400
Restricted 0-5 1200 1000 900
Very Restricted (4) 0 700 600 500
Multiple Combination VehiclesOperating
Speed Car & Prime B Type 1 Type 2(km/h) Mover Semi Trailer Double Road Train Road Train
50 100 100 105 120
60 130 130 135 155
70 150 160 175 205
80 185 200 220 260
90 230 250 280 325
100 285 305 345 400
110 350 350 350 400
120 385 385 385 400
130 400 400 400 400
Approach Speed (km/hr) +ve Grade%
4 5 6 7 8 9 10
100 - - 1050 800 650 550 450
80 630 460 360 300 270 230 200
60 320 210 160 120 110 90 80
Note:(1) Depending on road length being considered, this distance can range from 3 to 10km.(2) See section 8.4.4.(3) Including light trucks and cars towing trailers, caravans and boats.(4) No overtaking for 3km in either direction.
Table 13.4 (a) Grade/Distance Warrant (Lengths (m) to Reduce Truck Vehicle Speed to 40 km/h).
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● Heavy trucks from an adjacent industry enter the trafficstream on the up grade; and
● The level of service on the grade falls two levels below thaton the approach on the up grade or to level “E” (Ref 1.).
Development of a climbing lane is shown in Figure 13.2.
13.4.2.3 Length
The length of the grade and the start and end points of thelane dictate the length of the climbing lane. The theoreticalstart point is taken as the point at which the speed of the truckfalls to 40km/h and decelerating. The point at which the truckhas reached a speed equal to operating speed minus 15km/hand is accelerating determines the end of the lane. Thestarting and ending points of the lane should be clearly visibleto drivers approaching from that direction.
Table 13.4 (a) indicates the lengths on constant individual gradesneeded to produce a reduction in truck speed to 40km/h.
Truck speeds on grades can be assessed using the curvesincluded in Figure 13.3 and the longitudinal section of theroad. These curves assume an entrance speed to the grade of100km/h. This is conservative as modern trucks can operate athighway speeds approaching those of cars. If more precisedesign is required, the conditions should be analysed usingsoftware designed to simulate truck performance and usingentrance speeds based on the operating speed at the site.
The sight distance to the termination of the climbing lane isbased on the distance for the vehicle in the fast lane tocomplete or abandon the overtaking manoeuvre. The sightdistances required to overtake the various types of MCV’s areshown in Table 13.4 (b).
The starting point should be located at a point before thewarrant is met to avoid the formation of queues and possiblyhazardous overtaking manoeuvres at the start of the lane.
If the length of climbing lane exceeds 1200m, the designshould be reconsidered. Options include:
● Partial climbing lane;● Passing bay(s) in extreme conditions;● Overtaking lane prior to the grade (where the delays on
the grade are not excessive); and● Retention of the climbing lane where traffic volumes are
sufficiently high.
13.5 Slow Vehicle Turnouts
13.5.1 Partial Climbing Lanes
A turnout is a very short section of paved shoulder or addedlane that is provided to allow slow vehicles to pull aside and beovertaken. It differs from an overtaking lane in its short length,different signing, and the fact that the majority of vehicles arenot encouraged to travel in the left lane.
On dual carriageways a partial climbing lane for slow vehiclescan be provided as shown on Figure 13.4.
While climbing lanes should preferably be designed to spanthe full length of the grade, there may be circumstances where
it will be satisfactory to use a turnout on part of the up grade.A turnout may be appropriate if traffic volumes are low orconstruction costs are very high.
Turnout lengths of 60 to 160 m for average approach speeds of30 to 90 km/h respectively and a width of 3.7 m is to be used.
If a turnout is used, care must be taken to provide adequatesight distance. Signing at the start and merge points arerequired to better indicate diverge and merge locations. Theminimum sight distance should be stopping distance for theOperating Speed.
13.5.2 Passing Bays
On two lane two-way roads a passing bay may be provided asshown on Figure 13.4, for slow vehicle turnouts.
On steep grades where truck speeds can reduce to a “crawl”speed less than 20km/h and a full climbing lane can not beprovided, passing bays may provide an improvement to trafficflow. A passing bay is a very short auxiliary lane (of the order
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13.5.2a Passing bay (sequential)
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Figure 13.3: Determination of Truck Speeds on Grades.
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of 100m) that allows a slow vehicle to pull aside to allow afollowing vehicle to pass. The passing bay provides for theovertaking of the slowest vehicles and is only appropriate if allof the following conditions are met:
● Long grades over 8%;● High proportion of heavy vehicles;● Low overall traffic volumes; and● Construction costs too high for full climbing lanes.
Passing bays must be properly signed to ensure theireffectiveness. Normally, 300m advance warning of the locationof the bay is required to allow heavy vehicle drivers to preparefor the overtaking manoeuvre and to alert other drivers to theapproaching facility.
13.6 Descending Lanes
On steep down grades the speed of trucks will be as low asthat on equivalent up grades as shown on Figure 13.3 with asimilar effect on traffic flow if overtaking opportunities are notavailable. A descending lane will be appropriate in thesecircumstances.
If overtaking sight distance is available overtaking will bereadily accomplished and a descending lane will not beneeded. Similarly, if a climbing lane is provided in the oppositedirection, and the overtaking sight distance is adequate,overtaking slower down hill vehicles can be safely achieved anda descending lane will not be needed. Where the downgrade iscombined with tight horizontal curves, a descending lane willbe appropriate to provide satisfactory traffic operation. Designdetails are similar to those of climbing lanes.
13.7 Runaway Vehicle Facilities
13.7.1 General
Where long steep grades occur it is desirable to provideemergency escape ramps at appropriate locations to slowand/or stop an out-of-control vehicle away from the maintraffic stream. Out-of-control vehicles result from drivers losingcontrol because of loss of brakes through overheating ormechanical failure or because the driver failed to change downgears at the appropriate time. Experience with the installationand operation of emergency escape ramps has led to theguidelines described below.
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Figure 13.4: Development of Slow Vehicle Turnouts
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13.7.2 Types of Escape Ramps
Figure 13.5 illustrates four types of escape ramps.
13.7.2.1 Sand Pile
The sand pile types are composed of loose, dry sand and areusually no more than 130m in length. The influence of gravityis dependent on the slope of the surface of the sand pile. Theincrease in rolling resistance to reduce overall lengths issupplied by the loose sand. The deceleration characteristics ofthe sand pile are severe and the sand can be affected byweather. Because of these characteristics, the sand pile is lessdesirable than the arrester bed. It may be suitable where spaceis limited and the compact dimensions of the sand pileare appropriate.
13.7.2.2 Descending Grade
Descending grade ramps are constructed parallel and adjacentto the through lanes of the highway. They require the use ofsingle sized or uniform graded aggregate to preventcompaction in an arrester bed to increase rolling resistanceand, therefore, slow the vehicle. The descending-grade rampscan be rather lengthy because the gravitational effect is notacting to help reduce the speed of the vehicle.
13.7.2.3 Horizontal Grade
For the horizontal-grade ramp, the effect of the force ofgravity is zero and the increase in rolling resistance has to besupplied by an arrester bed composed of single sized oruniform graded aggregate to prevent compaction. This type oframp will be longer than those using gravitational force actingto stop the vehicle.
13.7.2.4 Ascending Grade
The ascending-grade ramp uses both the arresting bed and theeffect of gravity, in general reducing the length of rampnecessary to stop the vehicle. The loose material in thearresting bed increases the rolling resistance, as in the othertypes of ramps, while force of gravity acts downgrade,opposite to the vehicle movement. The loose bedding materialalso serves to hold the vehicle in place on the ramp grade afterit has come to a safe stop. Ascending grade ramps without anarresting bed are not encouraged in areas of moderate to highcommercial vehicle usage as heavy vehicles may roll back andjack-knife upon coming to rest.
Each one of the ramp types is applicable to a particularsituation where an emergency escape ramp is desirable andmust be compatible with the location and topography. Themost effective escape ramp is an ascending ramp with anarrester bed. On low volume roads of less than approximately1000 vehicles per day, clear run off areas without arrester bedsare acceptable.
13.7.3 Location of Runaway Vehicle Facilities
Runaway vehicle facilities should not be constructed where anout of control vehicle would need to cross oncoming traffic.On divided roadways where adequate space is available in themedian, safety ramps can be located on either side of the
carriageway with adequate advance warning sings prior to thesafety ramp exit.
For safety ramps to be effective their location is critical. Theyshould be located prior to or at the start of the smaller radiuscurves along the alignment. For example, an escape ramp afterthe tightest curve will be of little benefit if trucks are unable tonegotiate the curves leading up to it. Vehicle braketemperature is a function of the length of the grade, thereforeescape ramps are generally located within the bottom half ofthe steeper section of the alignment.
Lack of suitable sites for the installation of ascending typeramps may necessitate the installation of horizontal ordescending arrester beds. Suitable sites for horizontal ordescending arrester beds can also be limited, particularly if thedownward direction is on the outside or fill side of theroadway formation.
13.7.4 Arrester Beds and Escape Exits
An arrester bed is a safe and efficient facility used todeliberately decelerate and stop vehicles by transferring theirkinetic energy through the displacement of aggregate in agravel bed. An escape exit consists of any surfacing used in theevent of an emergency that will allow a runaway vehicle to exitthe downgrade off the road and decelerate to a lower speed.For example, escape exits can be side streets, sidetracks oraccesses that are not normally signed as a safety ramp. Anarrester bed is a particular kind of escape exit. The following
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Figure 13.5: Types of Vehicle Escape Ramps
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section lists broad guidelines for the design of arrester bedsand escape exits.
13.7.4.1 Arrester Beds
From field tests and other research studies, rounded particlessuch as uncrushed river gravel with uniform gradation producehigher deceleration than the more angular crushed aggregate.This is because the vehicles sink deeper in to the river gravel,transferring more energy to the stones over a shorter length.The use of a material with low shear strength is desirable inorder to permit tyre penetration. Sand is not ideal because itconsolidates with time and moisture ingress. Crushed stonehas been used but is not considered effective as it will requirelonger beds and will need regular ‘fluffing’ or de-compaction.
Nominal 10mm river gravel has been used satisfactorily intesting. The gravel should be predominantly rounded, ofuniform gradation, free from fine fractions and with a meanparticle size ranging between 12mm and 20mm. In general,
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13.7.4 Arrester bed and Escape Exit Note: One-way carriageway (sequential)
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Figure 13.6: Typical Arrester Bed Layout
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gravels with a smaller internal friction angle will perform betterthan those with larger internal friction angles.
An appropriate crush test such as the Los Angeles abrasiontest (or equivalent) should be used to evaluate durability of thestone. Stones with a high crush test will not deteriorate andwill therefore not produce fines.
A typical arrester bed is shown in Figure 13.6.
A gradual or staged increase in the depth of the bed should beprovided on the entry ramp. This is to ensure a gradual rate ofdeceleration when entering the ramp. The first 50 metres ofthe bed acts as the entry ramp and should increase in depthfrom 50 mm to 350 mm of suitable material. Over the first 50m of the arrester bed length the depth increases to 450mmand remains at that depth for the rest of the bed length. A bedconstructed to this design would accommodate low speedentries within the 350mm deep section of the bed. Vehiclesentering at higher speeds will slow down significantly as theyreach the deeper section of the bed, thus reducing the chancesof the vehicle being damaged.
The average deceleration achieved in sand or gravel bed is:
● Sand 350mm deep 2.8m/sec2;
● Sand 450mm deep 3.4m/sec2;
● Gravel 350mm deep 3.0m/sec2; and
● Gravel 450mm deep 3.7m/sec2 (Ref 91)
These decelerations may be used in the following formula tocalculate the length of an arrester bed.
L = V2 / (26a + 2.55g1)
where:
L = length of full depth arrester bed excluding 50mtransition at start (m)
V = entry speed (km/h)
a = deceleration (m/sec2)
g1 = grade (%) (positive for upgrade, negative fordowngrade).
A 50m entry ramp provides a satisfactory and safe means ofentering the full depth of the arrester bed; this entry ramp isnot included in calculations for bed length.
Where insufficient length is available at a particular site forstopping the vehicle at the anticipated entry speed, the beddepth should be increased in stages from 350mm up to450mm. The increasing depth will provide greater decelerationtoward the end of the bed allowing the vehicle to stop withinthe available length. However, each case should be designedon its merits.
Sand has problems of drainage, compaction andcontamination and should not be used unless alternativematerials are unavailable. Beds using sand will require a strict
maintenance regime to ensure their continued effectiveness.
13.7.4.2 Escape Exits
Lengths will vary depending on the gradient of the facility andthe surface material used (specific to the site). Wambold et al(Ref 101) recommend the following formula to determine thelength of a truck escape ramp exit.
L = 0.004V2 / (r + G)
where
L = Distance to top, the escape exit (m)
V = Entering velocity (km/h)
G = Grade (g1) divided by 100 (m/m)
r = Rolling resistance expressed as equivalent grade (%)divided by 100.
Values of r for several materials given in Table 13.5
The design of arrester beds and escape exits is site dependent,and careful consideration of all of the factors discussed inSection 13.7.4.4. For escape exits, careful consideration of theland use adjacent to the exit is required. Local streets shouldonly be used at the top of steep exit grades where the truckhas decelerated to a speed equal to the posted speed limit.Existing roads and streets used for property access should onlybe used where the traffic volume is very low and there is a verylow probability of an escaping truck meeting another vehicle.
13.7.4.3 Spacing
For new projects Table 13.6 may be used as a guide whenconsidering the need for escape exits on grades greater than6% and with numbers of commercial vehicles exceeding 150per day.
The distances in Table 13.6 are not absolute and greaterdistances could be acceptable, as site location is dependent onfactors discussed in Section 13.7.4.4. The need for a facility
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Surfacing RollingMaterial Resistance (r)
Portland Cement Concrete 0.010
Ashphalt Concrete 0.012
Gravel Compacted 0.015
Earth, sandy and loose 0.037
Crushed Aggregate, loose 0.050
Gravel, loose 0.100
Sand 0.15
Pea Gravel (uniform grading) 0.25
Table 13.5: Rolling Resistance Values
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will be increased if the number of commercial vehicles is morethan 250 per day and the maximum decrease in OperatingSpeed between successive geometric elements is approachingthe limits set in Table 13.7.
13.7.4.4 Summary of Design Considerations
● The length of the escape ramp must be sufficient todissipate the kinetic energy of the vehicle;
● The alignment of the ramp should be straight or of verygentle curvature to relieve the driver of undue vehiclecontrol problems;
● The width should be wide enough to accommodate twovehicles if it is considered likely that a second vehicle willneed to use the ramp soon after the first one;
● The arrester bed material should be clean, not easilycompacted or consolidated and have a high coefficient ofrolling resistance;
● The full depth of the arrester bed should be achieved in thefirst 50m of the entry to the bed using a tapering depthfrom 50mm at the start to the full depth at 50m;
● The bed must be properly drained;
● The entrance to the ramp must be designed so that avehicle travelling at high speed can enter it safely. A 5ºangle of departure or less is required, and as much sightdistance as possible should be provided. The leading edge
of the arrester bed must be normal to the direction of entryto ensure that the two front wheels of the vehicle enter thebed simultaneously;
● Comprehensive signing is required to alert the driver to thepresence of the escape ramp;
● Vehicles that enter the ramp will have to be retrieved, as itis likely that they will not be able to remove themselvesfrom the arrester bed. An appropriate service road adjacentto the ramp is required to effect retrieval. An alternativeand/or enhancement to the service road is the provision ofanchorage points/blocks for winching vehicles out;
● When the location of the ramp is such that the length isinadequate to fully stop an out-of-control vehicle, apositive attenuation (or ‘last chance”) device may berequired. Care is required to ensure that the device doesnot cause more problems than it solves – sudden stoppingof the truck can cause the load to shift with potentiallyharmful consequences to the driver and the vehicle.Judgement will be required on whether the consequencesof failing to stop are worse than these effects. Crashcushions or piles of sand or gravel have been used as “lastchance” devices.
13.7.5 Brake Check and Brake Rest Areas
A Brake Check Area is an area set aside for commercialvehicles at the top of a steep descent. A Brake Rest Area,however, is an area set aside part way down or at the bottomof the decent.
These facilities should be provided, at least to an unsealedgravel condition, on routes that have long steep downgradesand commercial vehicle numbers of around 100 per day,especially on National Highways and principal traffic routes.These areas, when used, will ensure that drivers begin thedescent at zero velocity and in a low gear that may make thedifference between controlled and out-of-control operation onthe downgrade. It also would provide an opportunity todisplay information about the grade ahead, escape ramplocations and maximum safe descent speeds.
These areas may need to be large enough to store severalprime mover and semi-trailer combinations, the actualnumbers depending on volume and predicted arrival rate.
The location will need good visibility with acceleration anddeceleration tapers provided, as discussed in Section 8 andSection 13.8.2. Adequate signage will be required to advisedrivers in advance of the facilities. Special signs, specific to thesite, will need to be designed for these areas.
13.8 Geometry of Auxiliary Lanes
13.8.1 Starting and Termination Points
The start and termination points of an auxiliary lane should beclearly visible to approaching drivers from that direction. Thestart point should be prior to the point at which the warrant ismet to avoid potentially hazardous overtaking manoeuvres.Visibility to this point should be sufficient for the driver toassess the situation and make a decision on the course ofaction to take.
RURAL ROAD DESIGN90
Table 13.6: Approximate Distance from Summit to Safety Ramp
Note:• Actual distances will depend on site topography, horizontalcurvature and costs.
Table 13.7: Maximum Speed Decrease betweenSuccessive Geometric Elements
Maximum Decrease in SpeedGrade (%) between Successive
Geometric Elements (km/hr)
< 6 10
6 – 10 8
> 10 6
Approximate DistanceGrade (%) from the Summit
to Ramp * (km)
6-10 3
10-12 2.5
12-15 2.0
15-17 1.5
17 1.0
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The termination of the auxiliary lane should only be at a pointwhere there is sufficient sight distance for the overtakingdriver to decide whether to complete or abandon theovertaking manoeuvre. The overtaking sight distances given inSection 8.4.2 Table 8.4 may be used. These distances wereadopted from the research (Ref 95) carried out in 1981.
It is desirable for the termination point to be on a straight to givedrivers a better visual appreciation of the approaching merge.Termination on a left-hand curve should be avoided becauseslow vehicles are seriously disadvantaged by reduced rear vision.It is also desirable that the termination point be on a downgradeto minimise the speed differential between vehicles.
There are however, some examples in the state ofQueensland where an auxiliary lane must end where thesight distance is less than that required to complete anovertaking. In such cases, drivers have to rely upon signing.Eye height and object height requirements at least must beachieved. This method should only be used when all otheroptions have been considered.
13.8.2 Tapers
Diverging TaperThe widening of the pavement at the start of the auxiliary laneis achieved with a taper. The length of the taper should besufficient to permit easy diverging of traffic with the slowertraffic moving to the left and the faster traffic going to theright lane. This length depends on the speed of theapproaching traffic and the width of the through lane. Therate of the lateral movement is assumed to be 1.0m/sec, givingthe following formula for taper length:
TD = VW/3.6
where: TD = Diverge taper length (m)V = Operating speed (km/h)W = Amount of pavement widening (m)
If convenient, developing the widening around a horizontalcurve can improve appearance and contribute to an easierdivergence of the traffic into the fast and slow streams
Merging TaperAt the termination of the auxiliary lane, a taper that allows thetwo streams to merge into one should reduce the pavementwidth. Since this situation is equivalent to the dropping of alane, drivers will be less prepared for the merging action thanthey would be if merging from an acceleration lane. It istherefore necessary to adopt a lesser rate of merging than forthe tapers on acceleration lanes and a rate of 0.6m/sec is used.The minimum length depends on the speed of theapproaching traffic and the width of the lane and isdetermined from the following formula:
TM = VW/2.16
where:TM = Merge taper length (m)V = Operating speed (km/h)W = Amount of pavement widening (m)
(This formula has been derived on the basis of a merging rate of 0.6m/sec2 of lateral movement)
A “run out” area should be provided through the merge areato accommodate those vehicles prevented from merging asthey approach the narrowed section. This can be achieved bymaintaining a total pavement width in the direction of travelequal to at least the sum of the full lane width plus a shoulderwidth of 2.0m over the full length of the taper plus 30m (seeFigure 13.2).
13.8.3 Cross Section
13.8.3.1 Pavement Width
The width of the auxiliary lane should not be less than thenormal lane width for that section of road.
13.8.3.2 Shoulder Width
A shoulder width of 1.0m is often satisfactory because thepavement has been widened over the section with an auxiliarylane. This width will have to be increased in areas of restrictedvisibility (eg. around curves) and in the merge area at the endof the lane.
13.8.3.3 Crossfall
The crossfall of the auxiliary lane will usually be the same asthe adjacent lane. Because of the additional width ofpavement, the depth of water flowing on the pavementshould be checked to ensure that aquaplaning does not occur.It may be necessary to change the crown line to overcome thistype of problem.
13.8.3.4 Lane Configurations
The specific circumstances of each design will dictate thepreferred treatment for individual locations but the followingconsiderations should be taken into account when deciding onthe layout of the design:
● If duplication is a longer term goal, providing a section offour lane divided road may be a logical first stage;
● Providing a four lane section of divided road is applicablewhen the analysis of the road shows that a spacing lessthan 5km is required and the topography is suitable;
● The merge areas of opposite overtaking lanes should be inaccordance with Figure 13.1;
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Table 13.8: Tapers for Diverges and Merges
Operating Taper Length (m)Speed Diverge Merge(km/h) (TD) (TM)
60 60 100
70 70 115
80 80 130
90 90 150
100 100 165
110 110 180
120 120 200
130 130 210
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● Diverges may occur opposite each other without anyspecial requirements.
13.8.4 Line marking and Signing
13.8.4.1 Signs
All forms of auxiliary lane should be signed as OvertakingLanes and the sign Keep Left Unless Overtaking should beused as specified in relevant standards (see Ref 8). This form ofsigning encourages maximum use of the auxiliary lane andallows overtaking even between vehicles travelling at similarspeeds.
The alternative signs for Slow Vehicle Lane and Slow VehiclesKeep Left should only be used in exceptional circumstanceswhere it is specifically desired to encourage a lesser use of theadded lane. Passing bays should be specifically signed to alertdrivers of their existence.
The provision of advance signs for auxiliary lanes promotesroad safety and improves the quality of service as perceived bythe driver. Having seen such a sign, drivers wishing to overtakemay relax their search for overtaking opportunities and are lesslikely to accept gaps with low safety margins. Advance signsare particularly appropriate when significant bunching occursfor 3 minutes of driving time (at the slow vehicle's speed)before the commencement of an auxiliary lane.
13.8.4.2 Linemarking
General practice for marking overtaking barrier lines on ruralroads is described in the relevant standards (see Ref 8). Forauxiliary lanes constructed as three-lane road sections, threeparticular aspects are of relevance:
● In (direction 1 of the auxiliary lane traffic) it is normal practiceto provide a continuous barrier line over the full auxiliarylane length – including tapers – to prohibit any use bydirection 1 vehicles of the third or opposing traffic lane. Thisalso serves to define the centreline of the road and indicatethat the centre lane is primarily for direction 1 traffic.
● For direction 2 (opposing traffic) a barrier line is generallyprovided adjacent to the auxiliary lane diverge andmerge tapers.
● For direction 2 traffic adjacent to an auxiliary lane indirection 1, AS 1742 (Ref. 8) recommends that thedirection 2 lane separation line marking follow normalpractice for two lane roads. This means that, if sightdistance permits, direction 2 vehicles may be permitted touse the centre lane as an opposing traffic lane provided novehicles are encountered in that lane.
Some use of auxiliary lane sections by opposing traffic isallowed, particularly when traffic volumes are low. Howeverthere may be cases where more restrictive line marking isappropriate. These will generally arise when there exists acombination of the following factors:
● Short auxiliary lane length● Moderate to heavy traffic volumes● Sight distances only marginally adequate for overtaking,
and
● Perceived operational or safety problems on a given roadsection.
The use of more restrictive line markings should not be toowidespread, since the presence of apparently unnecessarybarrier lines can lead to driver frustration and a reduced qualityof service on a road.
1 4 . V E H I C L E S TO P P I N G A R E A S
14.1 General
Vehicle stopping areas are roadside facilities that are placedbeyond the edge of shoulder along a roadway, allowingtravellers to safely stop and rest, well clear of the through traffic.
Provision of vehicle stopping areas is important for maintainingan efficient and safe movement of vehicles along a route.
Vehicle stopping areas can be divided into:
Service Centre● Highway Service Centre; and● Highway Service Town.
Major Rest Area● Major rest area, catering for light and heavy vehicles
combined;● Major rest area, catering for light and heavy vehicles
separated; and● Welcome Centres.
Basic Rest Area● Basic rest area catering for light vehicles only;● Driver Reviver sites;● Truck Parking areas; and● Truck Changeover areas.
Other Areas● Lay-bys;● Breakdown bays;● Bus Bays;● Telephone bays; and● Enforcement areas for speed and for overloaded heavy
vehicles.
Depending on the facility provided, the use will vary for eachof these facilities, but will generally take the form of:
● Stopping for fuel and food;● Carry out emergency repairs;● Change drivers;● Rest to alleviate fatigue;● Seek emergency assistance; and● Pick up and/or set down passengers.
14.2 Service Facilities
14.2.1 Rest Areas
Rest areas are areas clear of the road carriageway, wherevehicles may park and where basic facilities such as toilets,
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picnic tables, etc. are provided. There are two types of restareas:
● Major rest areas; and● Basic rest areas.
14.2.1.1 Major Rest Areas
Major rest areas include combined major rest areas, separatedmajor rest areas and welcome centres.
a) Combined Major Rest Areas● These are major rest areas that cater for both heavy and
light vehicles. These rest areas should be separated by adistance of three to four hours driving time.
● Preferably these should be placed at the crest of a hill orin flat areas to allow trucks to enter and leave the siteeasily.
● Heavy vehicle parking should be separated from the lightvehicle parking areas, and any recreation facilities. Treesor sound absorbing walls should be used for theseparation.
● Major rest areas should include the following facilities:– Parking for cars, cars and caravans and trailers;– Sheltered parking for heavy vehicles;– Covered tables and seats;– Toilets;– Shelter;– Rubbish and recycle bins, if viable;– Water;– Children’s play/exercise areas;– Shade;– Lighting;
– Barbecues or fireplaces, if practical;– Emergency telephones;– Access to facilities for disabled people;– Parking area for more than 10 cars;– Parking area for 5 prime movers and semi-trailers;– Information board, including local geographic and
historical information (no advertisements);– Sealed access and parking areas;– Acceleration and deceleration lanes on approach and
exit respectively; and – Turning lanes where site services both carriageway
directions.
b) Separated Major Rest Areas● These rest areas should be spaced at three to four hours
driving distance.
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14.2.1.1 Major Rest Area
14.2.1.2 Basic Rest Area
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● Where possible a rest area should cater for both directionsof travel.
● These sites should provide 10 or more spaces for lightvehicles and 5 or more separate spaces for heavy vehicles.
c) Welcome Centres● These are centres that are designed to attract tourists and
are established at the gateway to a major tourist region,featuring tourist and accommodation information.
● These centres offer services to motorists including toilets,telephones, food and information.
● Sites should provide a minimum of 15 or more parkingspaces for cars (6m x 2.4m) and a minimum of 5 spaces forcar and caravans (14m x 2.4m).
14.2.1.2 Basic Rest Areas
Basic rest areas are provided for light vehicles only.
● Basic rest areas should be provided at 50km intervalswhere the AADT exceeds 1000 and the distance betweentowns having comparable facilities exceeds 50km. Theyshould be provided at reducing intervals of 30 km wherethe AADT exceeds 2500 and the distance between townshaving comparable facilities exceeds 30km (Ref. 78).
● These sites can be built so that access is from onedirection only.
● Sites should include:– Off road parking for cars, caravans, and trailers;– Covered tables and seats;– Rubbish bins;– Potable water;– Electricity;– Toilets; and – Five or more car parking spaces.
a) Driver ReviverA formal approach for the placement of these sites must beinstituted with the following measures suggested:
1. Placement of driver reviver sites within rest areas;
2. Spacing driver reviver sites at 45 minutes driving timeseparation.
b) Truck Parking Area● A truck parking area can be any large site separated from
the roadway by shrubs or trees to block the headlight glarefrom passing vehicles but taking into account driversecurity. The only facility needed at truck parking areas is aregularly emptied covered bin.
● Truck parking areas should accommodate two or moreprime mover and semi-trailer parking spaces.
c) Truck Change-over Area● A truck changeover area is a small sealed or unsealed area
where trucks can safely pull over to change drivers. Theseare larger than breakdown bays as they mustaccommodate the largest vehicle using any route.
● Sign posting is discretionary for these sites.
14.2.1.3 Other Areas
a) Lay-bys and Breakdown Bays● The provision of wide shoulders for discretionary parking
is both expensive and unwarranted. However, there maybe a need to provide lay-bys at regular intervals forvehicles to stand clear of the carriageway, and provisionshould be made accordingly.
Passenger vehicle lay-bys should be a minimum of 4.5mwide from the edge line and 20m long to accommodatetwo vehicles. Where the predicted AADT exceeds 1000they should be approximately 10km apart, staggered onalternate sides of the road at 0.5km intervals. Wherevolumes are less than 1000 AADT, the spacing may beextended to a maximum of 15km. Preferably, lay-bysshould be sealed, however a gravelled surface isacceptable. Desirable locations for lay-bys include sags,flat areas near cutting/embankment lines, pick-up pointsfor country school buses, and adjacent to property accesspoints.
● For heavy vehicles, an area of at least 4.5m in width and50m in length is to be provided on the near side of eachcarriageway at intervals of approximately 10km, to allowtrucks to stop. The design of these areas is to includemeasures for the capture of all surface drainage runofffrom the lay-by. It is desirable for lay-bys for heavyvehicles to be located on or near a crest.
b) Bus BaysA bus bay is an indented storage area that is provided forbuses to pull clear of the through traffic flow in order tostop and to pick up or set down passengers.
Shoulders should be widened to provide sufficient widthto enable buses to stand clear of the pavement,particularly where sight distance is restricted or wherespeeds are generally high enough that a stopped vehiclewill create a hazard.
When a bus bay is provided, it should be designed in sucha way to allow free flowing, passenger comfortablemovements and ease of manoeuvring for the largestdimensional bus that is likely to use the facility.
Bus stops and/or bus bays should be provided at regularintervals along a recognised bus route within rural townsso that users generally do not have to walk more than400 metres from their dwelling to the bus stop.
Adequate provision must be made behind the kerb line,especially at indented bus bays, for sufficient waiting areato allow passengers to assemble and disperse. This maynecessitate local widening of the formation/footway areato satisfy pedestrian standing.
At schools, where the safety of children is of paramountimportance, consideration should be given to theprovision of a one-way movement bus zone.
c) Emergency Telephone BaysEmergency telephones are installed to provide a
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communication facility for the benefit of road usersrequiring assistance. Controlled access roads create anenvironment where the availability of outside assistancefor a vehicle breakdown, road accidents, etc is restricted.Therefore, emergency telephones are desirable oncontrolled access roads despite other warrants not beingsatisfied.
There are advantages associated with the provision ofemergency telephones, provided they are installed andused correctly.
• They will result in early attention to reported roadusers’ problems.
• They will reduce the delay in getting medicalattention to injured people, thus reducing thepossibility of loss of life.
• They will reduce the time of exposure to danger byoccupants of disabled vehicles.
Location of emergency telephone facilities should be onthe near side of each carriageway, approximatelyopposite one another. This alleviates the tendency and/ornecessity for road users to cross multiple lanes of high-speed traffic to access a facility.
On routes where there are three lanes or more, and thereis an inner shoulder of sufficient width to accommodatea broken down vehicle, median placed emergencyphones can be installed to provide a facility for use byboth carriageways, reducing the necessity to crossmultiple lanes to use the emergency telephone facility.
As a guide, on rural routes, desirable spacing is 2km witha maximum spacing of 5km.
Emergency telephone facilities should be easilyidentifiable both during the daylight hours and darkness.If lighting is inadequate, provision must be made toenable night-time use of the facility by road users.
Emergency telephones should be placed to allow easyaccess to the facility from the carriageway. Normally,emergency telephone facilities are to be provided justoutside the shoulder, and not in a position that isvulnerable to errant vehicles.
Careful consideration must be given to the requirementsof road users with a disability when determining thelocation and the height of the installation.
14.2.2 Location of Vehicle Stopping Areas
The appropriate locations for vehicle stopping areas should beplanned in the design stage so that earthworks, pavementdesign, conduits, etc., can be installed during the constructionstage.
When considering suitable spacing for these facilities,predetermined distances cannot be strictly adhered to.Consideration of road user safety, isolation of a strandedvehicle, effect of a disabled vehicle on through traffic, sightdistance to the facility, associated earthworks, etc., will
determine the actual distance between each installation.
Vehicle stopping areas between towns should complementstopping opportunities provided by towns, aiming to provideadequate, signposted stopping opportunities at intervals of80km or less on routes with medium traffic volumes (2000-5000 AADT), and at intervals of 50km or less on higher volumeroutes (>5000 AADT). A signposted rest area or service centrefacility should be available at not more than twice theseintervals.
Factors to be considered in locating new vehicle stopping areasshould include:
● Topography (preference to stop on crests in hilly areas);● Width of road reserve;● Scenic or aesthetic value (presence of natural features);● Potential environmental impact;● Volume and type of traffic;● Sight distance (to permit safe access to the facility);
14.2.3 Heavy Vehicle Considerations
Road transport drivers and representative groups should beconsulted on the location and facilities for planned newroadside rest areas, changes to existing areas to allow heavyvehicle use or changes affecting heavy vehicle use.
Stopping opportunities suitable for heavy vehicles should beprovided at 10km intervals. Because of the exposure of longdistance heavy vehicle drivers to the dangers of driverfatigue, and legal obligation for heavy vehicle drivers to restfrom driving, stopping opportunities for these drivers shouldbe first priority when providing for vehicle stopping. Areaswhere greater than minimum provision is required should beidentified in consultation with road transport industryrepresentatives.
Modifications to vehicle stopping areas must be driven byuser needs but may include provision for heavy vehicleaccess, with parking separate from other vehicles to preventconflict during manoeuvring, reduce the disturbance ofheavy vehicle drivers’ rest by holiday travellers and meet therequirements for parking of dangerous goods carryingvehicles. Separated vehicle stopping areas may be analternative. However, driver security should also beconsidered so that a potentially isolated driver with avaluable cargo does not feel vulnerable.
Sealed, bypassed sections of road can be made usefulparking areas for heavy vehicles, provided that connectionswith the through road are designed appropriately.
For more information refer to “Guide to the provision andsignposting of service and tourist facilities” AS/NZ1742.6 1990.
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1 5 . C O M M U N I T Y C O N S U LTAT I O N
The planning and design process for rural road projects shouldinclude consultation with local community and otherstakeholders.
The objectives of such consultation should be to:
● Collect and analyse information on local conditions anditems of importance to the local community;
● Provide information on the proposed project;
● Obtain the views and responses from the local community;and
● Identify areas of agreement or disagreement and possiblecompromises.
A variety of consultation methods can be used including:
● Public meetings;● Discussions with land owners;● Direct discussion with affected owners;● Meetings with stakeholder groups;● Public displays and exhibitions at various stages of the
project with provision for community comments; and● Distribution of project bulletins.
The following basic principles should be employed to ensureeffective community participation:
● Clear statements should be made at the beginning of aproject on the:– Purpose, nature and extent of the project;– Project timetable, indicating community participation;
and– General project procedure.
● Affected parties should be given the opportunity toparticipate and to be heard;
● Flexible project procedures should be able toaccommodate the community input, as required;
● Alternatives, developed and presented in a simple andclear fashion, should be used for discussions with thecommunity;
● Alternatives agreed to should be developed further;● Results and conclusions should be presented to the
community;● Adequate time for effective participation should be
allowed;● Quick responses should be provided on community
comments; ● Participation at community activities should be adequately
resourced; and ● Status of all presented materials should be provided to the
community.
The extent of community participation will depend on socialand environmental factors involved, significance of the road,and on the possible degree of controversy of any proposalslikely to result from the project.
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16. DRAINAGE
16.1 General
Any road should have an adequate drainage system to:
● Maintain the natural flow of water that existed prior toroad construction;
● Collect water from the road pavement and convey it tosuitable discharge points;
● Protect the road from overland flow from adjacent areas;and
● Provide an appropriate level of service.
Drainage structures can also provide access across roadcorridors for both terrestrial and aquatic fauna.
An effective design must balance a number of factors againstthe construction cost and the proposed level of protection,such as:
● Flooding effects on adjacent properties as a result of roadconstruction;
● Traffic delays or extra travel distance caused by roadclosures during floods greater than the design AverageRecurrence Interval;
● Possible structural damage to the road or adjacentfacilities due to floods greater than the design ARI;
● Service life of the proposed drainage systems and thecosts of its replacement, improvement, or extension; and
● Road maintenance cost.
The prime sources of data and methodology for this sectionare:
● Australian Rainfall and Runoff (Ref. 60);● Metric version of technical memorandum No. 61, Water
and soil Division, Ministry of Works and Development NZ(Ref. 74);
● Waterway Design, A Guide to the Hydraulic Design ofBridges, Culverts and Floodways (Ref. 34);
● Guide to the Design of Road Surface Drainage (Ref. 80);and
● Road Runoff & Drainage: Environmental Impacts andManagement Options, 2001 (Ref.104).
References more suited to local characteristics and practicesmay supplement or substitute the above reference list.
16.2 Flood Estimation
Runoff flowing towards a road should be returned to itsnatural course as soon as possible.
Estimates of design floods can be based upon either streamflow or rainfall records. Stream flow records are usually heldby the regional water authorities and provide the largest flowrate in each year. In the absence of stream flow records,flood flows can be estimated by using mathematicalprocedures incorporating rainfall data.
Australian Rainfall and Runoff (Ref. 60) is recognised inAustralia and NZ as the primary reference for the estimation
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of design flood flows. In addition to the discussion of thetheory of catchment analysis and the estimation of floodflows, it presents rainfall intensity, storm frequency andduration data. Most road authorities have manuals andguides to supplement Ref. 60 for local conditions.
For rural catchments flood estimation procedures available tothe designer can be divided into those used for gauged andungauged catchments.
For gauged catchments the following methods are generallyused:
● Flood frequency analysis – for catchments with long streamflow records, where the recorded floods are statisticallyanalysed to estimate design floods of a selected probabilityof exceedance.
● Unit hydrograph methods – for catchment with limitedstream flow records, where the recorded floods andassociated rainfall are used to construct a unit hydrograph.Design storms, less losses are applied to the unithydrograph to obtain the design flood of the same ARI asthe design storms.
● Runoff routing method – for catchments with limitedstream flow records, where the recorded floods andassociated rainfall are used to derive the catchment modelparameters. Design storms, less losses are applied to themodel to produce design flood hydrographs of the sourceARI as the design storms.
For ungauged catchments, the following methods, commonlyknown as regional methods, are generally used:
● Rational Method – as a probabilistic or statistical method inwhich a peak flow of a selected ARI is estimated from anaverage rainfall intensity of the source ARI.
● Regional Flood Frequency Methods – such as the Indexflood method and multiple regression method.
● Synthetic Unit Hydrograph Methods - using regionalrelationships for the parameters required to construct theunit hydrograph.
● Runoff Routing Methods – using regional relationships toestimate the model parameters.
Ref. 60 gives descriptions of each method and details ofthe factors to be considered when choosing a flood estimationprocedure. It also provides guidance on when flood estimationprocedures based on rainfall should be used in preference toflood frequency analysis.
16.3 Rational Method
The “Rational Method” is the most commonly used method toestimate design flood flow in road surface catchments, whichare generally well defined and relatively small. The design flowestimated using the Rational Method has about 25% accuracyand is described by the following equation:
Q =
where:Q = peak discharge (m3/s)C = coefficient of runoffI = average rainfall intensity over the time of
concentration for the particular catchment and theselected storm recurrence interval (mm/h)
A = catchment area (ha).
However, this method has a number of deficiencies. Theyinclude:
● Assumption of uniform rainfall over a catchment;● Use of a constant value of C, which assumes that runoff is
a fraction of rainfall, rather than the residual after losseshave been accounted for; and
● Inability to take storage effects into account.
These deficiencies mainly apply in large rural catchments withlarge proportions of pervious areas. Most road surfacedrainage catchments are generally:
● Small enough for the assumption of uniform rainfall to bereasonable;
● Relatively impervious; and● Surface storage is not a major issue.
As the equation does not take channel storage into account,this may require consideration.
Earley (Ref 46) examined channel storage. He found thatchannel storage lessened peak flow prediction, using theequation, by about 2 to 7 percent. It is concluded thatrefinements to take account of this relatively small difference arenot worthwhile, and the deterministic interpretation of theRational method represents the most appropriate method ofestimating peak flows in road surface drainage design (Ref. 80).
The coefficient of runoff is the ratio of the peak rate of runoffto the average rainfall intensity during the critical rainfallperiod for the catchment area under consideration. It is themeasure of the peak rate at which water drains from aparticular area compared to the average rate at which rain fallson the area.
The coefficient of runoff adopted must account for theultimate future development of the catchment as depicted inthe strategic plan of the relevant local authority, but shouldnot be less than the value determined for the catchment underexisting conditions. The procedure for the determination of Cis in Ref. 60.
In cases where portions of a catchment are significantlydifferent, the percentage impervious of separate areas willprovide an appropriate C value to be used in the calculation ofrunoff.
Intensity is measured in millimetres of rain per hour (mm/h).Data are provided for storm durations (for which the stormcontinues for the given intensity) between 6 minutes and 72hours with frequencies (recurrence intervals or return period)between 1 and 100 years. The data are based on 100 rainfallstations located around Australia.
Intensity data for New Zealand is presented in Ref. 74.
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16.4 Design Considerations
Water must be conveyed away from the road for the followingreasons:
● Maintain adequate pavement skid resistance;● Maintain an acceptable level of road lighting performance
(Ref. 25);● Reduce spray; and● Visibility of pavement markings to be maintained.
The designer must consider the following issues:
● Grading of the roadway with respect to flood levels,ground water levels and tidal levels;
● Estimated runoff;● Maximum permissible flow width on the carriageway;● Minimum size of cross culverts and outlet conditions;● Subsurface drainage; and● Consequences of a storm of greater ARI than the design
storm.
Average recurrence interval (ARI)The average recurrence interval is the average interval of timeduring which a storm event will be equalled or exceeded once.When selecting the average recurrence interval for a design,the following factors should be considered:
● Consequence of flooding (potential damage to property,road and structures);
● Additional cost of providing for a larger ARI;● Capacity of underground or outfall drainage systems into
which the road surface drainage components willdischarge;
● Level of serviceability to traffic; and ● Consistency of flood immunity along other sections of the
road.
Table 16.1 Contains values of ARI, used successfully inAustralia and New Zealand in the past 20 years, and may beused for preliminary design. The ARI to be used for final designmust be selected after evaluation of the factors listed above,and consideration of local road authority practice.
Level of Serviceability to TrafficThe level of serviceability will depend upon the ARI of the floodfor which the stream crossing will be passable to traffic, the
serviceability requirements of the road in question, and theduration of road closure during times of flooding.
The selection of the level of serviceability is generally based onthe following criteria:
● The level of service expected by the community;● The availability of alternative routes and period of closure;● The importance of the road/access to hospitals, airports,
etc; and● Economic considerations. (Ref 34)
In addition, the requirements of local authorities,environmental agencies, and those responsible for navigationand flood control, will also influence type of waterwaystructures and hence impact on the level of serviceabilityprovided.
Typical levels of serviceability are as follows:
● Arterial roads generally designed to pass the 50 or 100years ARI without interruption to traffic. However, forarterial roads in remote areas, a reduced standard iscommonly adopted where traffic densities are low.
● Minor roads are generally designed to pass the 20 (or less)year ARI. The level of their serviceability depends upon:
– The importance of the road;– Interruption to traffic significance; and– Economics of providing a higher level of serviceability.
TrafficabilityTrafficability will depend upon the combination of depth andvelocity of flow over a floodway, when the frictional resistancebetween a vehicle’s tyres and the floodway surface isovercome and the vehicle loses stability.
Road closure is normally assumed when the total head (staticplus velocity) on a carriageway with a two-way crossfall oracross the highest edge of a carriageway with a one-waycrossfall exceeds 300mm (Ref. 34).
Longitudinal drainageA desirable minimum longitudinal grade of 1.0% and anabsolute minimum grade of 0.3% are to drain watereffectively from the traffic lanes. However, where flat terrainprevents these grades being achieved, the carriageway itselfcan have a zero longitudinal fall, provided that water can drainaway from the road formation.
Pavement drainagePavement drainage is achieved by providing minimum crossfallon the pavement of 3.0%. However, on wide smoothpavements with flat grades there may be difficulty inmaintaining satisfactory drainage. Special problems(aquaplaning) may occur at intersections and superelevationtransitions. With flat grades, water is shed directly towardsshoulder but problems can arise on steep grades when watertends to flow longitudinally down the pavement. The resultingsheet of water can cause drivers to lose control.
Further consideration of drainage of wide flat pavements is setout in the publication Drainage of wide flat pavements (Ref77). The formula for depth of flow is:
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Table 16.1: ARI for Road Design
Location ARI, years
Major waterway structures 100
Water bypass around water treatment facilities 100
Cross road drainage 50
Road with landlocked areas (at a sag in cut) 50
Road surface drainage 10
Bridge deck drainage 10
Road surface drainage at wide flat pavement 1
Water quality treatment (wetlands, etc) 1
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d = – (TXD)
where:d = is depth of flow measured from the top of the
surface texture (mm)L = is flow path length (m)I1 = is a rainfall intensity for 1 year ARIS = is the average flow path slope (%) TXD = is the texture depth measured by the sand patch or
silicon putty method (mm)
Indicative values of (TXD) are:
Burlap drag concrete 0.05mmGrooved concrete 1.2mmDense asphalt 0.9mmSize 14 stone seal 3.7mm* Open grade asphalt 1.2mm
Note:* There is some doubt about the drainage properties of opengraded asphalt in the long term. However, it may be used toimprove safety at critical locations in the short term.
AquaplaningAquaplaning is the complete loss of traction and directionalcontrol of the vehicle as a result of a fluid film between thetyres and the road surface. The texture of the road surface andthe tread on vehicle tyres provide drainage channels for waterto escape from beneath a vehicle tyre. If these channels areinadequate and water does not escape there is a risk of partialor full aquaplaning.
Partial aquaplaning occurs at quite low speeds as a result ofsome intrusion of a water film between the tyre and roadsurface. As the vehicle speed increases the water will have lesstime to escape through the tyre and road surface drainagechannels. As a result of this, a tyre’s contact area and skidresistance will be reduced.
Full dynamic aquaplaning occurs when a tyre is completelyseparated from the road surface by a film of water. Fluidpressures can build up within the contact zone between the tyreand the pavement to the point where the hydrodynamic upliftequals the downward force exerted on the tyre. At this point,the tyre is aquaplaning or completely supported by the waterlayer. As a result, there is almost complete loss of traction anddirectional control since the fluid film cannot develop thenecessary shear forces for braking or steering manoeuvres.
Various research projects have been undertaken aimed atpredicting water depth as a function of rainfall intensity andphysical conditions. Guide to the Design of Road SurfaceDrainage (Ref. 80) describes some of these relationships. Oliver(Ref. 84) has concluded that currently available skid resistancespecifications or recommendations are not suitable in theirpresent form for use in calculating a maximum water filmdepth consistent with safety for different classes of road.
Oliver (Ref. 84) also concludes that full aquaplaning will be arare event. It was found that a progressive reduction in tyrefriction occurs as water depth increases from just wetcondition to a film thickness of 4 mm. It was also noted that ifpavement rutting occurs, deep films of water could be presentin the wheelpath area under conditions of light rain.
There are various methods of reducing the depth of water andlength of flow paths that are available to the designer. Thesemay include use of two-way crossfall on one-way pavements,use of artificial crown lines, and modification of superelevationdevelopment. Special drainage provisions such as slottedchannels or grated trenches may also be considered.
More research and trials are required to determine therelationship between speed, rainfall intensity, water depth andthe occurrence of aquaplaning. In the meantime, waterdepths should be limited to 4mm for a rainfall intensityof 1 year ARI, Table 16.1.
Roadside drainsRoadside drains include:● Table drains;● Catch drains; and● Median depressed drains.
Details of depth, width, gradient, and capacity of the drainscan be obtained by reference to “Guide to the design of roadsurface drainage” (Ref 80) and Section 11.9.
16.5 Water Quality
Rural storm water management plans associated with ruralroad projects should consider the treatment of runoff to meetthe water quality requirements as per legislation and of thelocal environment protection agency or catchmentmanagement authority.
Water quality treatment facilities should be provided to meetthe requirements of the catchment management authority.Details of methods to manage erosion and treat storm watercan be obtained by reference to: Urban Storm Water: BestPractice Environmental Management Guidelines (Ref. 43)Austroads document, Road Runoff and Drainage:Environmental Impacts and Management Options, 2001 (Ref.104) and Water Sensitive Road Design-Design Options forImproving Storm Water Quality of Road Runoff (Ref. 103).Refer table 16.1 for ARI for design purposes.
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0.15(TXD)0.11L0.43 I10.59
S0.42
16.5 Water Quality
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17. ROADSIDE SAFETY
17.1 Safety Objectives
Road environment factors are one of the three elements thatcontribute to road crashes, the others being driver behaviourand vehicle characteristics. It is estimated that some 30% ofcrashes relate to roadside environment. While roadenvironment factors are often not the single cause of a crashthey can contribute to their severity.
This section is based on AASHTO “Roadside Safety Barriers”and work done by Troutbeck to adjust AASHTO detail toAustralian conditions. Austroads are currently reviewing the1987 NAASRA “Safety Barriers – Considerations for theProvision of Safety Barriers on Rural Roads”.
Further reading may be obtained by reference to the AASHTO“Roadside Safety Barriers” 1996.
Austroads Guide to Traffic Engineering Practice, Part 4 – RoadCrashes (Ref. 17) outlines various issues to be considered in thedesign process that can reduce the potential for crashes. Inparticular it describes the fundamentals of good design. Theseinclude:
● Designing for all road users;● Pavement surface;● Intersection design;● Intersection control;● Pavement markings and delineation;● Pedestrian crossing facilities;● Street lighting; and● Signing including guide posts.
Therefore, the following safety objectives are to be adoptedwhen designing a road:
● Separate potential conflict points and reduce potentialconflict areas;
● Control the relative speeds of conflicting vehicles;
● Clearly identify the path to be followed;
● Ensure that the needs of all road users are considered;
● Provide a roadside recovery area that forgives a driver’serrant or inappropriate behaviour; and
● Ensure that roadside furniture is located safely.
17.2 On-Road Safety
17.2.1 Intersections
Intersection design and control is a major factor in improvingroad safety. The main factors in intersection safety include:
● Number of legs;● Angle of intersection;● Sight distance;● Alignment;● Auxiliary lanes;
● Channelisation;● Intersection control;● Friction or pavement skid resistance;● Turning radii;● Traffic lane and shoulder widths;● Property access;● Signposting;● Approach speed; and● Lighting.
In general, an intersection should be obvious andunambiguous and allow good visibility of traffic controldevices and other road users (Ref. 83).
It is appropriate to increase intersection control with anincreasing ratio between minor and major flow. Capacityconsiderations will also govern the type of intersection controlrequired. The various types of intersection control in order ofincreasing standard (and safety) are provided in Figure 17.1.
The following actions should be taken in designing for on-roadsafety:
Intersections● Define and minimise the number of conflict points;● Separate conflicts;● Give priority to major movements through alignment,
delineation and traffic control;● Provide a clear indication of priority;● Control the angle of conflict – crossing streams of traffic
should intersect at right angles while merging trafficshould intersect at small angles;
● Define vehicle paths;● Provide adequate sight distance;● Control approach speeds to major intersections through
alignment and geometry; and● Provide suitable lighting.
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Figure 17.1: Scale of increasing safety of intersectioncontrols
Note: *Rural intersections are unlikely to be controlled by traffic signals.
Roundabout SignalsFilter turns
Signals*Fully controlled turns
Uncontrolled IntersectionRely on priority rules
Assigned PriorityGiveway signs
Assigned PriorityStop signs
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17.2.2 Mid Block
Mid block on-road safety should also be considered during the design phase. The factors that influence mid block safetyinclude:
● Pavement surface;● Delineation;● Shoulder width;● Verge rounding;● Horizontal and vertical geometry;● Degree of access control;● Overtaking opportunities;● Sight distance;● Speed differential between vehicles; and● Vehicle speeds.
The following actions should be taken in designing for on-roadsafety:
Mid block● Define vehicle paths, especially where there are changes in
geometry;● Minimise headlight glare;● Provide appropriate access control for the function of the
road;● Provide overtaking opportunities including passing lanes or
bays;● Provide truck escape bays on roads with steep grades;● Minimise major changes in road geometry;● Minimise adverse or severe crossfall; and● Provide a smooth road surface with an appropriate level of
skid resistance.
17.3 Recovery Area
Roadside safety typically relates to the area adjacent to thetraffic lane where an errant vehicle can recover. Providing asafe roadside involves removing or treating likely hazards thatmay contribute to the severity of a crash.
17.3.1 Clear Zone
It is not feasible to provide width adjacent to the carriagewaythat will allow all errant vehicles to recover. Therefore it isnecessary to reach a compromise or level of risk management.The most widely accepted form of risk management forroadside hazards is the ‘clear zone concept’. The clear zone isthe horizontal width measured from the edge of the traffic lanethat is kept free from hazards to allow an errant vehicle torecover. The clear zone is a compromise between the recoveryarea for every errant vehicle, the cost of providing that area andthe probability of an errant vehicle encountering a hazard. Theclear zone should be kept free of non-frangible hazards whereeconomically possible; alternatively, hazards within the clearzone should be shielded. The clear zone width is dependent on:
● Speed;● Traffic volumes;● Batter slopes; and● Horizontal geometry.
It should be noted that the clear zone width is not a magicalnumber and where possible hazards beyond the desirable clearzone should be minimised.
Clear zone widths vary throughout the world depending on landavailability and design policy. The concept originated in theUnited States in the early 60’s and has progressively been refinedand updated. For a typical high-speed road the clear zone widthvaries between 4.0m (France, South Africa) to 10.0m (Canada,USA). More recent studies have found that the first 4.0-5.0mprovides most of the potential benefit from clear zones.
Figure 17.2 provides an indication of appropriate clear zonewidths for a straight section of road with trafficable battersThe clear zone width increases where there is sub-standardhorizontal geometry, especially on the outside of a curve orwhere non-trafficable batter slopes are present.
Non-trafficable batter slopes refers to batter slopes of steeperthan 1 on 4.
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Figure 17.2: Clear Zone Widths on Straights
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RURAL ROAD DESIGN102
The clear zone width on the outside of curves increases by afactor Fc, which depends on the operating speed and theradius of the curve. Fc ranges between 1.0 to 1.9. Figure 17.3provides guidance on adjustment factors for clear zones on theoutside of curves.
Where batter slopes are steeper than 1 on 4 (that is nontrafficable) designers should give consideration to theprovision of a road barrier (refer to Section 17.4).
A guide for the installation of roadside safety barriers onembankment is shown on Figure 17.4.
Figure 17.5 indicates the variation of clear zone widths onbatters steeper than 1 on 6 to give an effective clear zonewidth to be used in design.
17.3.2 Existing Hazards Within a Clear Zone
Common existing roadside hazards in a rural environmentinclude:
● Poles – power poles or sign posts;● Trees;● Batters;● Dams and water courses;● Drainage and associated infrastructure like culverts and
endwalls;● Fences; and● Bridge piers.
The most desirable action is to remove or relocate hazardsalthough this is not always possible due to road reservation oreconomic and environmental constraints. Where hazardscannot be relocated then they should either be shielded ormade ‘more forgiving’.
It is becoming increasingly common for light poles andsignposts to be provided with frangible bases. This is anattempt to provide a forgiving roadside while still providing thenecessary roadside infrastructure. Common types of frangiblepoles include:
● Slip base poles;● Impact absorbent poles;● Steel frangible posts;● Aluminium frangible assemblies; and● Wooden frangible posts.
The support connection of a slip base pole shears on impactwith the pole landing close to the point of impact. Impactabsorbent poles crumple and bend around the vehicle. Slipbase poles can usually be re-used after an impact and for thisreason tend to be more common. However, they can only beused where there will not be a conflict with overhead servicesin the event of an impact, and where the risk to other roadusers, particularly pedestrians, is minimised.
Steel frangible posts fail on impact as a result of shear failureplanes. Aluminium assemblies collapse due to shear pin action.Frangible wooden signposts have holes drilled at the basecreating a plane of weakness that permits the posts to collapseon impact.
Other measures to make roadside hazards more forgiving include:
17.3.1 Clear Zone
17.3.2 1(a) Existing Hazards Within a Clear Zone
17.3.2 1(b) Existing Hazards Within a Clear Zone
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Figure 17.3: Adjustment Factors for Clear Zones on Curves
Figure 17.4: Warrants for Guard Fence on Embankment
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Figure 17.5: Effective Clear Zone Widths on Batters
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Test Vehicle mass (kg) Speed Angle Height of Centre Level and type (km/h) degrees of gravity (mm)
0 820 C 50 20 550
1 600 C 50 25 550
1 820 C 50 20 550
2 000 P 50 25 700
2 820 C 70 20 550
2 000 P 70 25 700
3 820 C 100 20 550
2 000 P 100 25 700
4 820 C 100 20 550
8 000 S 80 15 1 250
5 820 C 100 20 550
36 000 V 80 15 1 850
6 820 C 100 20 550
36 000 T 80 15 2 050
Table 17.1: Test levels for longtitudinal barriers (TL- 0 TO TL- 6) and test levels for terminals and crash cushions (TL- 0TO TL- 3)
Legend:C = small carP = four wheel drive or utility truckS = single-unit van truckT = tanker type semi-trailerV = van type semi-trailer
Note:(1). Refer NCHRP350 for Test Level Procedure(2) TL- 3: High-speed arterial roads
TL- 2: Local and collector roadsTL- 0 and 1: Work zones and low speed roadsTL- 4 to 6: Truck and other heavy vehicles
● Considering the mature trunk size of trees prior toplanting;
● Installing driveable culvert end walls; and ● Extending culvert walls to beyond the clear zone width.
17.4 Safety Barriers
Safety barriers are used to shield hazards that cannot berelocated or made more forgiving. The barrier itself is a hazardand accordingly should only be used when it is less of a safetyconcern than the hazard the designer is trying to shield.
Roadside safety barrier systems may be considered for use onlyafter they have been satisfactorily crash tested, computersimulated or designed by other professionally acceptablemethods that demonstrate acceptability to meet AS/NZS3845:1999.
The crash test procedures to be adopted are based on theAASHTO, National Cooperative Highway Research ProgramReport Number 350. The European Committee forNormalisation (CEN) has established performance criteria forsafety barriers and crash cushions as set out in CENprEN 1317-
1 & 2 procedures. The tests by CEN do provide an equivalentset of tests to compare systems with NCHRP350.
Acceptance of the roadside safety barrier systems is based onan evaluation of its performance in an idealised crash test(vehicle in tracking mode; approach surface flat, paved andfree from obstructions such as kerbs) for a specific weight andtype of vehicle at designated speeds and impact angles.
In accordance with NCHRP350 procedures, there are six testlevels, refer Test Levels in Table 17.1, so as to provide for arange of restraint requirements (vehicle size) and impactseverity conditions (speed and angle). The evaluation criteria,refer Table 17.2, on impact of the vehicle with the barriersystem is based on the:
● Structural adequacy of the barrier system;● Occupancy risk and the impact velocity and ride down
acceleration limits; and ● Vehicle trajectory after impact.
The designer should be aware that the site of installation willoften be different from the test condition, the errant vehicle
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Table 17.2: Safety Evaluation Guidelines
EVALUATION FACTORS EVALUATION CRITERIA
Structural Adequacy
Occupant Risk
Vehicle Trajectory
A. Test article should contain and redirect the vehicle; the vehicle should notpenetrate, under ride or override the installation although controlledlateral deflection of the test article is acceptable.
B. The test article should readily activate in a predictable manner by breakingaway, fracturing or yielding.
C. Acceptable test article performance may be by redirection, controlledpenetration or controlled stopping of the vehicle.
D. Detached elements, fragment or other debris from the test article shouldnot penetrate or show potential for penetrating the occupantcompartment or present an undue hazard to other traffic, pedestrians orpersonnel in a work zone. Deformations of, or intrusion into, theoccupant compartment that could cause serious injuries should not bepermitted.
E. Detached elements, fragments or other debris from the test article orvehicular damage should not block the driver’s vision or otherwise causethe driver to lose control of the vehicle.
F. The vehicle should remain upright during and after collision althoughmoderate roll, pitching and yawing are acceptable.
G. It is preferable, although not essential, that the vehicle remain uprightduring and after collision.
H. Occupant impact velocities should satisfy the following:
Occupant Impact Velocity Limits (m/s)Component Preferred MaximumLongitudinal and lateral 9 12Longitudinal 3 5
I. Occupant ride down accelerations should satisfy the following:
Occupant Ride down Acceleration Limits (G’s)Component Preferred MaximumLongitudinal and Lateral 15 20
J. (Optional Hybrid III dummy. Response should conform to evaluationcriteria of Part 571.208, Title 49 of Code of Federal Regulations, ChapterV (10-1-88 Edition)
K. After collision it is preferable that the vehicle’s trajectory not intrude intoadjacent traffic lanes.
L. The occupant impact velocity in the longitudinal direction should notexceed 12 m/s and the occupant ride down acceleration in thelongitudinal direction should not exceed 20 G’s.
M. The exit angle from the test article preferably should be less than 60% oftest impact angle, measured at time of vehicle loss of contact with testdevice.
N. Vehicle trajectory behind the test article is acceptable.
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will not be in tracking mode and ground conditions for thesupport of posts will be different from the test site. Judgementmust, therefore, be exercised in the application of test resultsand the performance of safety devices monitored in the fieldto ensure they operate as intended.
Test level 3 is considered to be the rating by which roadsidesafety barriers systems are designed. They will perform for thecar and pick-up truck at 100 km/h at a nominal angle of 20degrees. The work zone systems can be designed for test levels0, 1, 2 & 3 at nominal speeds of 50, 70 & 100 km/hrespectively and 20 degrees nominal angle. Roadside safetybarrier systems and the equivalent test level category of eachare listed. The test level rating of a barrier system can beincreased by raising the height of the top of the system andproven by acceptable methods:
Rigid System Test Level● F-Shape concrete barrier 3 to 4
(adopted by AS/NZS 2845/1999)● New Jersey concrete barrier 3 to 4● Sloping face concrete barrier 3 to 5● Vertical face concrete barrier 3 to 5● High containment concrete barrier 5 to 6
Semi-Rigid System Test Level● W-beam steel barrier 3 to 4● Thrie-beam steel barrier 3 to 4● Hollow box steel barriers 3● Wire rope safety barriers - four wire rope 3
Work Zone System Test Level● F-shape concrete precast barriers 3 to 4● Water filled barriers 0 to 4● Truck mounted attenuators 3
All these systems have specialised terminals, which will providecontrol led deceleration. Terminals provide deceleration belowrecommended limits and ensure that the vehicle is not spearedand is not vaulted, snagged or rolled on impact.
Crash cushion systems are also used to shield hazards inconfined locations, such as the junction of concrete barriers, atramp noses and other rigid hazards.
A discussion of issues to be addressed in the specification ofsafety barrier and crash cushion systems is included in AS/NZS3845:1999 – Road Safety Barrier Systems (Ref. 13).
Consideration of these issues and the discussion of variousbarrier systems below provide general guidance on the mostappropriate system for a particular situation. Furtherinformation should be obtained from system suppliers or therelevant road authority.
Concrete safety barriers are best suited to situations where thereis little room between the barrier and the hazard. Typically thisoccurs in narrow medians or in areas of restricted road cross-section. The greatest concern with concrete safety barriers is themethod of termination. Available options include:
● Steel guardrail terminal assembly to shield the end of theconcrete barrier in association with a bridge approachassembly;
● Burying the end of the barrier in an adjacent embankment;and
● Shielding the barrier system with an impact attenuator/crash cushion system.
Site characteristics will determine the most appropriate type oftermination/ attenuation to use.
Concrete safety barriers may be considered on high volume roadsas they retain full functionality after impact, provide excellentwhole of life costs and minimise the risk to maintenance workers,as maintenance is minimal after an impact.
Steel W-beam barriers are perhaps the most common barrierand are used extensively in urban and rural areas. Theeffectiveness of W-beam is dependent on its length and offsetfrom the main carriageway. W-beam termination is also ofconcern and standards are continually developing to improveend terminals. Most road traffic authorities have detailedguidelines on the installation of W-beam and end terminals.Care should be taken in meeting these requirements. Theimpact behaviour of the W-beam and terminals should also beconsidered to ensure that the selected system is appropriatefor the intended location.
Wire rope safety barrier works through high-tension cables. Anerrant vehicle bends the supporting posts and the rope deflectswith the vehicle before directing it back towards the directionof travel. Wire rope safety barriers are the most forgiving on theerrant vehicle of the three methods. The deflection width mustbe a design consideration for the offset of features behind thebarrier. AS/NZS 3845 and relevant road traffic authorityguidelines should be referenced to establish installationrequirements and the acceptability of these systems.
The location of safety barrier in the vicinity of kerb and channelis to be considered very carefully. If kerb and channel is essentialin high-speed locations, the line of kerb shall be located:
● At least 3m from the face of concrete safety barrier types;● At least 3m from W-beam barrier or wire rope safety
barrier for barrier kerbs;● Between 0.0 and 1.0m or at least 3m from W-beam barrier
or wire rope safety barrier for semi-mountable kerbs; and● In areas where the Operating Speed is less than 70km/h an
offset of 200-300mm can be used to minimise nuisancedamage to vehicles.
Note: Semi-mountable kerb should be 100mm maximumheight to minimise dynamic jump.
17.4 Safety Barriers
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Work zone barriers come in various forms and can be precastconcrete with impact attenuator/crush cushion terminals orwater filled plastic systems. These systems must be consideredduring the design phase. Truck mounted attenuators can beused for short term or mobile work areas.
17.5 Landscaping
In relation to safety, landscaping on road verges or within theclear zone should:
● Not obstruct sight distance; ● Be frangible; and ● Not obstruct sight lines to signs, delineation and traffic
control devices.It is common for medians to be landscaped to reduceheadlight glare from opposing traffic.
Generally, trees with a mature trunk diameter less than 100mm (subject to tree species) are considered to be frangible.Trees with small trunk diameters (< 100 mm diameter mature)may be used for medians and borders, while for traffic islands,low level vegetation or trees with high canopies areappropriate (subject to the trunks being frangible or outsidethe clear zone). Provision for adequate sight distance for allroad users must be considered.
The designer must consider the balance between landscapeand road safety objectives.
17.6 Lighting
The benefits of a high level of street lighting, especially atintersections, are well documented with a strong correlation tonight-time accident reduction.
At complex intersections an appropriate level of street lightingshould be considered. The lighting should be substantialenough to provide the driver with a clear view of the roadalignment.
AS/NZS 1158.1.1, Road Lighting – Vehicular Traffic (categoriesV1, V2 and V3) Lighting – Performance and Installation DesignRequirements (Ref. 12) has recently undergone a review ofacceptable lighting levels. This should be the main referencefor street lighting. The Austroads Guide to Traffic EngineeringPractice, Part 12 – Roadway Lighting (Ref. 25) is a usefulbackground document.
17.7 Pedestrians and Cyclists
In designing a safe road environment consideration must alsobe given to non-vehicular road users. The needs of pedestrians(including those with disabilities) and cyclists are discussed inthe relevant Austroads guides (Ref. 26 and 27).
The safety issues to be addressed for on-road cyclists are verysimilar to those relating to motor vehicles. It is important forthe road surface to be smooth, to minimise conflicts and toprovide appropriate delineation. Consideration should begiven to the provision of sealed shoulders on major rural roads.
Pedestrian safety relates to the number of controlled crossingpoints and the provision of footpaths close to the traffic lanes.It is desirable for footpaths to be set back from the kerb to
provide protection for pedestrians from errant vehicles. Localspecific guidelines should be referenced to determine theprovision of crossing facilities.
17.8 Temporary Works DuringConstruction
Appropriate traffic management requirements for constructionsites are described in the Field Guide for Traffic Control atWorks on Roads, SAA HB 81.1 – 816 (Ref. 92).
Construction and maintenance operations should not inhibittraffic and, where possible, separation should be achievedthrough diversion routes. Studies conducted in the UnitedKingdom have identified high accident rates through workzones where proper warning and delineation has not beenachieved.
In reality, there will always be a requirement for some trafficmovement through work zones on existing roads. Where thisis necessary, clear and positive guidance approaching andthrough the work zone is a crucial element in the overall safetyof the site.
Work zone barriers can be used to shield vehicles from hazardsand provide a safer work zone. Work zone barriers need to beapproved by each road authority against the appropriate workspeed zone and NCHRP 350 Test Level before they can be usedon site, refer Section 17.4.
Careful consideration of the following factors is required fortraffic through work zones, particularly in relation to heavyvehicles:
● Alignments should desirably be designed in accordancewith the geometric guidance in this publication. Theyshould be designed to operate safely for the chosenreduced speed limit (i.e. no surprises for the driver such asadverse crossfall, abrupt changes in direction, etc);
● Reduced lane widths;● Median crossovers;● Reduced number of lanes;● Clearances to hazards (trucks travel closer to hazards
because of their overhang);● Short merge zones;● Height of flashing lights (often these affect visibility for
the driver); ● Provision for surface drainage, especially where pavement
works are staged; and ● Provision of work zone safety barriers and truck mounted
attenuators for short term or mobile work areas.
17.9 Road Safety Auditing
Road safety auditing, especially during the design stage, servesto identify opportunities to reduce the incidence and severityof crashes. As defined by Road Safety Audit (Ref. 33), a roadsafety audit is:
"A formal examination of a traffic project, or any project thatinteracts with road users, in which an independent, qualifiedexaminer reports on the project’s accident potential and safetyperformance.”
A road safety audit should be conducted:
RURAL ROAD DESIGN108
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● At the feasibility stage;● At the draft design stage;● At the detailed design stage;● Pre-opening; and● On existing roads.
The earlier a road is audited within the design anddevelopments process the better. The need is as important fornew works as it is for retrofit projects.
1 8 . R A I LWAY L E V E L C R O S S I N G S
This section outlines geometric guidelines for at-graderailway/road level crossings and provides guidance for a safetyreview of existing level crossings. The guidelines have beendeveloped for typical situations. They are intended to aid butnot replace sound engineering judgment based on particularlocal conditions and requirements of the rail authority (Ref. 66).
At-grade railway level crossings present a potential for severeaccidents. Designers should aim to eliminate, improve, orgrade separate existing crossings and to avoid the introductionof any new at-grade railway level crossings where possible.
The derivation of sight distance requirements at railway levelcrossings is discussed in Appendix C.
These requirements do not apply where other factors such asthe level of train and vehicle exposure may require thatflashing lights be installed at the crossing. For crossingscontrolled by lights, the sight distance requirements relate tothe ability of a driver to see the signals, not the train. AS 1742Part 7, 1993 specifies the use of railway crossing warning signswhich prompt drivers to ‘Look for Trains’ when approaching acrossing. Refer to the standard for detail warning signage andvisibility requirements and details indicated in this section.
18.1 Horizontal Alignment
Approach and crossing visibility are the primary featuresaffecting safety of the at-grade railway level crossings. Theapproach visibility is deemed to be adequate when an area ofunrestricted visibility exists for each approach as shown onFigure 18.1.
Approach visibility is adequate when the following conditionsare met:
The driver of an approaching vehicle, travelling at the 85thpercentile speed (VV) can see a train travelling at maximumoperating speed (VT), when the vehicle and the train are atdistances S1 and S2 respectively from the crossing, such thatthe vehicle can either safely stop short of the crossing, orclear the crossing before the train reaches it. Appropriatevalues of VT should be obtained from the rail authority.
Distance S1 shall not be less than truck stopping sight distance.For a given vehicle, the approach visibility must be adequatefor trains approaching from either direction.
For a given vehicle the approach visibility must be adequate fortrains approaching from either direction.
The approach visibility angle must not exceed 95º to the left ofthe crossing and 110º to the right of the crossing as shown inFigure 18.1. Occasional obstructions such as posts, smalltrees and sparse vegetation can be considered acceptable iftheir size and spacing would not obscure the driver’s vision ofa train. (Also refer to Figure C2)
Crossing visibility is deemed to be adequate when an area ofunrestricted visibility exists for each approach and thefollowing conditions are met:
The driver of a stationary vehicle, positioned at a stopline, has a clear view of approaching trains to a distancealong the tracks such that a train appearing in the driver’sfield of view at the point where the vehicle begins tomove would reach the crossing after the vehicle hascleared the crossing.
For the purpose of calculating the visibility triangle, thefollowing figures should be used:
● Distance from the driver’s eye to the rail, whilst at astandstill, is 5.0m;
● Height of the driver’s eye above the road is 1.05m; and● Height of train above the rails is 2.3m.
For a given vehicle, the crossing visibility must be adequatefor trains approaching from either direction. The crossingvisibility angle must not exceed 110º to the left of thecrossing (see Figure 18.2) and 140º to the right of thecrossing (see Figure 18.3). If there is a choice of crossingangle, 90º are preferred. (Also refer to Figure C3)
Many railways run parallel to adjacent roads and motoristson such roads may be unaware of a train travelling justbehind the vehicle in the same direction. In these caseswhere the road then crosses the rail or a side road crosses therail, distances S1 and S2 must be checked (unless there is stopcontrol on the crossing with advance warning signs) at thedesign speed of the main road. It is essential that the visibilityangles for S1 and S2 fall within the prescribed limits (seeFigure 18.4).
18.2 Vertical Alignment
18.2.1 Road Grading
The railway grading is usually a control on the road. As ageneral guide, for rural roads the road surface shall not bemore than 75mm above, nor more than 150mm below, theprojection of the top of the rail pair at a distance of 10m fromthe nearest rail.
The maximum level difference between road and rail when thetrack is below the road level is 10mm. On rural roads, the raillevel should not protrude above the surface, although this maynot always be achievable. The maximum permissibleprotrusion above the road surface is 10mm.
The protrusion of the rail level above the road level is more ofa problem when the angle between the road and the rail isacute, particularly for cyclists and motorcyclists.
Where a road crosses multiple railway lines at a level crossing,a smoother crossing can be achieved by adjusting the relative
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Figure 18.1: Approach Visibility Angles
Figures 18.2/3: Crossing Visibility Angle for Driver Looking Left and right
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Figure 18.4: Road Parallel to Roadway
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grade of the railway lines to more closely match thelongitudinal grade of the road.
18.2.2 Cross Section
WidthThe minimum clear width provided through level crossingsshould be equal to the traffic lanes plus 1.5m each side; thatis, the carriageway width plus 3 m.
On duplicated roads, the 1.5m are added to the outer edge ofeach carriageway.
CrossfallAt the level crossing, the pavement slope should match thegrade line of the railway. This could present a potential hazardwhere the road is on a curved alignment. The road curvatureand superelevation should be selected with superelevationmatching the rail grading, so that crossfall does not reduce inthe direction of travel along the curve.
1 9 . C O M P U T E R S O F T WA R E FORROAD DESIGN
The geometric design of rural roads involves many calculationsthat can be performed by computers. They can quickly andaccurately handle large quantities of data, saving considerabledesign time and cost. Importantly, they enable many morealternatives to be examined and evaluated and as a result theycan assist in producing optimum solutions in a reasonable timeand at reasonable cost (Ref. 97).
Various computer software packages are available and arewidely used for designing roads, bridges and multileveloverpasses. They help designers to work faster or reach proofof concept sooner in the design process. Each softwarepackage has its own advantages and disadvantages and thoseinterested in pursuing this topic further should contact theirrelevant road authority. The commercial suppliers of thesoftware packages will provide specific information.
The primary functions of road design software include:
● Horizontal and vertical alignment design;● Coordination of horizontal and vertical alignments;● Creation and viewing of digital terrain model (DTM);● Automated calculation of quantities; and● Production of plans and profiles.
Software should have a smooth user interface that givesdesigners full access to all geometric design data, non-graphicinformation, and criteria at any point in the project cycle. Thissupports rapid decision-making and design changes. Three-dimensional model-viewing capabilities further assist thedecision process, as well as enhancing the designer’s ability topresent the work.
Various computer design aids can:
● Stratify and organise project documentation;● Record project histories;● Work in 2D and generate 3D;● Edit digital terrain models with real-time movement of
points (rubber banding), allowing contours to movedynamically;
● Generate cross sections automatically from any datasource, for any situation;
● Link cross section elements to plan view elements; and● Alter cross sections and automatically update earthworks
quantities.
The use of three-dimensional models can enhance a project tothe community especially those non-technical persons.
In summary, modern technology opens the door for efficiency,cost savings and better-informed judgement.
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REFERENCES
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48 Donaldson G A Safety of Large Trucks and Geometric (1986) Design of Two-lane Two-way Roads,
TRR 1052, Symposium on GeometricDesign for Large Trucks,Transportation Research Board,Washington D.C., USA.
49 Ervin et al (1986) Ervin R.D., MacAdam C.C. andBarnes M., Influence of theGeometric Design of Highway Rampson the Stability and Control of HeavyDuty Trucks, TRR 1052, Symposiumon Geometric Design for LargeTrucks, Transportation ResearchBoard, Washington D.C., USA.
50 Faber E (1982) Driver Eye Height Trends and SightDistance on Vertical Curves, JournalTransportation Engineering, Vol. 108No.4.
51 Fambro D (1997) Fambro D, Fitzpatrick K, Koppa R,Determination of Stopping SightDistances, Report 400, TransportationResearch Board, Washington D.C.,U.S.A.
52 Fancher P S (1986) Sight Distance Problems Related toLarge Trucks, TRR 1052, Symposiumon Geometric Design for LargeTrucks, Transportation ResearchBoard, Washington D.C., USA.
53 Fildes et at (2000) Fildes B, Corben G, Morris A, Oxley J,Prouk N, Brown L, and Fitzharris M.,Road Safety Environment and Designfor Older Drivers, Austroads.
54 Garber N J and Traffic and Highway Engineering, Hoel L A (1988) West Publishing Co., St. Paul, MN,
USA.
55 Harwood et al Harwood D.W., Mason M.J., Glauz (1990) D.W., Kulakowski T.B., Fitzpatrick K,
Truck Characteristics for use inHighway Design and Operation,Volume 1 & 2, Research Report,FHWA – RD-89.226, USA.
RURAL ROAD DESIGN114
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56 Harwood et al Harwood D.W., Fambro D.B., (1997) Fishburn B., Joubert H., Lamm R. and
Psarianos B., International SightDistance Design Practices,International Symposium on HighwayGeometric Design Practices, DraftProceedings, TRB, USA, January 1997.
57 Hempsey L and Redesigning the Design Hour for Teply S Alberta Highways, ITE Journal, May
1999, Institution of TransportationEngineers, USA.
58 Hoban C J (1983) Guidelines for rural roadimprovements a simulation study,ARRB internal report, AIR 359-10.
59 Hoban C J, Overtaking Lane Practice in Canada Morrall F J (1986) and Australia, ARR 144, Australian
Road Research Board, Australia.
60 Institution of Australian Rainfall and Runoff, Vol 1,Engineers, Australia A guide to flood estimation, Institution(1998) of Engineers Australia, Canberra.
61 Kanellaidis G (1999) Road curve superelevation design:current practices and proposedapproach, Vol 8, No 2, June 1999,Road and Transport Research.
62 Krammes R A, State of the Practice Geometric Brackett R Q, Design Consistency, Final Report, et al (1993) Federal Highway Administration, U.S.
Department of Transportation, U.S.A.
63 Lay M G (1985) Source Book of Australian Roads,Third Edition, ARRB, Melbourne.
64 Lee R E (1963) Driver Eye Height, Australian RoadResearch, Vol 1, No 6.
65 Mai and Sweetman Articulated Vehicle Stability - Phase II (1984) Tilt Tests and Computer Model, ARRB
Internal report, AIR 323-2.
66 Main Roads, QLD Main Roads Design Manual, version 1,August 1999.
67 Main Roads, WA Road Design, Technical Standards, (1997) Volume 1 Part 1, Geometric Road
Design Standards and PracticeGuidelines, Main Roads, WA.
68 McCormick E J and Human Factors in Engineering and Sanders M S (1982) Design (Fifth Edition), McGraw Hill,
New York, USA.
69 McLean J R (1978) Review of the Design Speed Concept,Australian Road Research, Volume 8,No. 1, March 1978, Australia.
70 McLean J R (1983) Speeds on Curves: Side FrictionFactor Considerations, ResearchReport No. 126, ARRB TransportResearch Ltd, Australia.
71 McLean J R (1988) Speed, Friction Factors and AlignmentDesign Standards, Research ReportARR No. 154, Australian RoadResearch Board, Australia.
72 McLean J R (1995) Speed, Curve Speed Prediction Relations,Research Report, Vol. 4, No. 3, ARRBTransport Research Ltd, Australia.
73 Ministry of Works Highway surface drainage. Design and Development guide for highways with a positive
collection system, Roadway Division,Ministry of Works and Development(NZ), November 1977.
74 Ministry of Works Metric version of technical and Development memorandum No. 61, Water and (NZ) Soil Division, Ministry of Works and
Development (NZ), October 1975.
75 NAASRA (1972) National Association of AustralianState Road Authorities, Guide Policyfor Geometric Design of Major UrbanRoads, NAASRA, Sydney, Australia.
76 NAASRA (1973) National Association of AustralianState Road Authorities, Policy forinstallations by public utility authoritieswithin the road reserve (metric units),NAASRA, Sydney, Australia.
77 NAASRA (1974) National Association of AustralianState Road Authorities, Drainage ofWide Flat Pavements, NAASRA,Sydney, Australia.
78 NAASRA (1979) National Association of AustralianState Road Authorities, Guide to theprovision and signposting of serviceand tourist facilities, NAASRA,Sydney, Australia.
79 NAASRA (1984) National Association of AustralianState Road Authorities, RoadMedians, NAASRA, Sydney, Australia.
80 NAASRA (1986) National Association of AustralianState Road Authorities, Guide to theDesign of Road Surface Drainage,NAASRA, Sydney, Australia.
81 NZS 4404 (1981) New Zealand Standard, Code ofPractice for Urban Land Subdivision,Standards Association of NewZealand, New Zealand.
82 Nicholson A (1998) Superelevation, side friction androadway consistency, Journal ofTransportation Engineering, Vol 124,No 5 American Society of CivilEngineers.
83 Ogden K W (1997) Safer Roads, A Guide to Road SafetyEngineering, Avebury Technical,England.
RURAL ROAD DESIGN 115
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84 Oliver V W (1979) Skid resistance reduction in wetweather due to hydroplaning ofvehicle tyres, Australian RoadResearch Board, Australia.
85 Olson et al (1984) Olsen P.L., Cleveland D.E., FancherP.S., Kostyniuk L.P., Schneider L.W.,Parameters Affecting Stopping SightDistance, NCHRP Report No. 270.
86 Pape M (1990) Rural Road Alignment DesignProcedure, Technical Note TN/1,VicRoads, Melbourne, Australia.
87 PIARC (1995) XXth World Road Congress,Interurban Roads, C4, Report ofCommittee, Montreal.
88 PIARC (1995) XXth World Road Congress,Interurban Roads, C10, Report ofCommittee, Montreal.
89 Prem H et al (1999) Prem H, Ramsay E, Fletcher C,George R, Gleeson B, Estimation ofLane Width Requirements for HeavyVehicles on Straight Paths, ResearchReport ARR 342, ARRB TransportResearch Ltd, Australia.
90 RTA (1989) Road Traffic Authority NSW, RoadDesign Guide, RTA, NSW, Australia,9 Sections 1989 to 2000.
91 QLD DoT (1992) Development of Design Standards forSteep Downgrades (DSB02),Queensland, Australia.
92 SAA HB 81.1 – Field Guide for Traffic Control at 81.6 (1996) Works on Roads
93 TRB (1994) Transportation Research Board,Highway Capacity Manual, HCM2000, National Research Council,Washington, D.C., USA, 2000.
94 Triggs T J and Reaction Time of Drivers to Road Harris W G (1982) Stimuli, Human Factors Report HFR-
12, Monash University, Melbourne.
95 Troutbeck (1981) Overtaking Behaviour on Australiantwo-lane rural highways, ARRB,Special Report 20.
96 Underwood R T Traffic Management – An introduction, (1990) Hargreen Publishing Co., Melbourne.
97 Underwood R T The Geometric Design of Roads, (1991) Melbourne.
98 VicRoads (1997) VicRoads, Road Design GuidelinesPart 2 Horizontal and VerticalGeometry, VicRoads, Victoria.
99 VicRoads (1997) VicRoads, Road Design GuidelinesPart 3 – Cross Section Elements,VicRoads, Victoria.
100 VicRoads (1997) Traffic Engineering Manual, Volume1, Chapter 6, Edition 2, Melbourne,Victoria.
101 Wambold et al Wambold JC, Rivera-Ortiz LA, Wang (1988) MC . A field and Laboratory Study to
establish truck escape ramp designmethodology. Commonwealth ofPennsylvania. Department ofTransportation, Report No FHWA-PA-86-032+83-26
102 WITS (1997) Water Industry Technical Standards,Volumes 1 and 2, Melbourne Water,Victoria.
103 Cooperative Water Sensitive Road Design – Research Centre Design Options for Improving Storm for Catchment Water Quality of Road Runoff.Hydrology (2000)
104 Austroads (2001) Road Runoff and Drainage:Environmental Impacts andManagement Options.
105 AASHTO (1993) American Association of StateHighway and Transportation Officials,Recommended Procedures for theSafety Performance Evaluation ofHighway Features.
106 Cox R.L. (1995) Analysis in Traffic and its Effect ofService for the Justification ofOvertaking Lanes and Future RoadDuplication. Queensland Departmentof Main Roads.
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1 . B A S I C P R O P E R T I E S O F T H EC LOT H O I D T R A N S I T I O N C U R V E
Transition curves connecting a circular curve to two straightsare shown in Figure A1. Typical standard notation for transitioncurves is as follows (See Figure A1):
RURAL ROAD DESIGN 117
C H A R A S T E R I S T I C S O F T H E E U L E R S P I R A L( C LOT H O I D )AAPPENDIX
Figure A1: Transition Curve Details
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R = radius of the circular curve in metres;IP = intersection point, or the point at which the two
straights join;TS = start transition, or the point at which a straight and a
transition curve join;SC = start circular curve, or the point at which a transition
and a circular curve join;PC = the point on the circular curve (extended) at which the
radius if extended would be perpendicular to thestraight;
I = intersection angle, or the angle between the twostraights in degrees;
øs = spiral angle in degrees;T = tangent distance in metres;S = secant distance in metres;LP = length of transition curve from TS to SC in metres.Lc = length of circular curve from SC to SC in metres;l = distance in metres along the transition to any point B
and TS;x = abscissa of any point B on transition with reference to
the straight and TS in metres;y = ordinate of any point B on transition corresponding to
the abscissa x in metres;p = the shift, which equals the offset from the PC to the
straight in metres;
2. BASIC RELATIONSHIPS FOR CLOTHOID TRANSITION CURVES
T = (R + p) tan + K
S = (R + p) sec – R
Lc = (I – 2øs) R
The expressions for x, y, p and k are approximations only andnormally are satisfactory for practical use. More preciseexpressions may be seen in any standard books on surveying.
x =
y = –
p = –
øs =
K = –
As the clothoid has a constant rate of change of curvature itgives a constant rate of change of lateral acceleration atconstant speed. For a vehicle travelling at a constant speed ofv m/s, the lateral acceleration increases from zero at the startof the transition to
at the start of the circular curve. This increase in accelerationtakes place over a length LP metres or over a time t (seconds)where:
t =
Thus, the rate of change of lateral acceleration, A m/s3
= /
= m/s3
If v m/s is converted to V km/h, this equation becomes:
A m/s3 = where V is in km/h
RURAL ROAD DESIGN118
l 5
40(RLp)2
l 7
336(RLp)3
l 3
6(RLp)Lp
2
24RLp
4
2688R3
Lp
2RLp
2
v2
R
L3p
240R2
180π
π180
0.0214V3
RLp
v 3
RLp
v 2
R
Lp
V
Lp
V
I2I2
l –
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Figure B1: Vertical Curve Nomenclature
1 . G E N E R A L
The parabola has traditionally been used in road design forcrest and sag vertical curves because:
● the vehicle undergoes a constant vertical acceleration;● the length of curve is directly proportional to the grade
change;● a parabola retains its basic shape when the scale is
changed whereas a circle takes the form of an ellipse whena change is made to one of the scales.
● The calculation of vertical and horizontal ordinates inrelation to any point on a parabola is a simple matter.Gravity makes the use of vertical ordinates moreconvenient in construction.
Other curves such as circular curves may be used if required fora specific reason. The K value equivalent radius R = 100 K.
2. VERTICAL CURVE FORMULAE
Parameters used in formulae for parabolas are shown onFigure B1.
where:
A = g2 – g1 = Algebraic grade change (%)a = Vertical acceleration of vehicles on
parabolas (m/sec2)g1, g2 = Grade (%)e = Middle ordinate (m)h1 = Eye height – for use with sight distance (m)h2 = Object height – for use with sight distance (m)K = Length of vertical curve for a 1%
change in grade (m) L = Length of vertical curve (m)L1 = Length over which the grade is less
than a specified slope SL (m)SL = Slope of the tangent to the curve at
any point (%)Low or high points occur where SL = 0
S = Sight distance (m)V = Speed (km/h)x = Distance from tangent point to any
point on curve (m)xhp = Distance from tangent point to high point (m)xlp = Distance from tangent point to low point (m)y = Vertical offset from tangent to curve (m)
NOTE: A rising grade with increasing chainage carries a plus sign and a falling grade carries a minus sign.
RURAL ROAD DESIGN 119
VE R T ICA L C U RVE FORMULAEBAPPENDIX
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The general formula for the parabola used in road design is:
y = = K =
In road design most parabola can be designed using thefollowing three equations:
L = KA
L = K(g2 – g1)
K =
An explanation of the use of K is included in Section 10.3.
Other equations that may be used include:
a =
e = (g2 - g1)
e = 0.5 ElevIP -
L1 = 2SL
x = L
xhp =
y =
y = 4
RURAL ROAD DESIGN120
Figure B2: Eye Height and Object Height
x2 (g2 – g1)200L
Lg2 – g1
(SL – g1)g2 – g1
Lg1
g2 – g1
S2
200(√h1 + √h2)2
x2
200Kx2
200y
AV2
1300L
ex2
(0.5L)2
ex2
(L)2
ElevTP1 + ElevTP2
2
L800
...
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1 . G E N E R A L
Before detailing the procedures used in the derivation of theformulae used in this Guide, it is important that users note thatsight distance requirements at railway level crossings havehistorically varied from State to State.
It is necessary to consider two scenarios in the evaluation ofsight distance requirements at railway level crossings. Case 1address the sight distances required for an approaching vehicleconsidering two critical situations (necessary to establishwhether the Give Way Control is adequate); and case 2addresses the sight distance along the railway for a vehiclestopped at a STOP sign (necessary to establish the adequacy ofSTOP sign control). The geometry and associated notation forcases 1 and 2 are depicted on Figures C2 and C3 respectively.
2. CASE 1: Sight Distance Required for Give Way Control
Case 1 allows a motorist approaching the crossing at distanceS1 to sight a train at distance S2 from the crossing and either:
Case 1(i) Decelerate and safely stop at the stop or holdingline; or
Case 1(ii) Proceed and clear the crossing with an adequate safety margin.
When motorists reach a crossing and see a train approaching,they must decide whether to decelerate and stop, or proceedand clear the crossing. There is a finite distance requiredbetween the vehicle and the rail in order to reach a decisionand act in safety. This distance, assuming a level grade crossingsite, comprises four components:
● The distance travelled during the perception/reaction time
RTVv = metres
● Braking distance
= = metres
where:
g = acceleration due to gravity = 9.81m/sec2;
● Distance of the driver from the front of the vehicle (Ldmetres); and
● Clearance from the vehicle stop or holding line to thenearest rail (Cv metres).
Thus, to stop on level ground, we require:
S1 ≥ + + Ld + Cv (1)
The influence of slope on the stopping distance component ofthis equation can be derived using simple physics as shown onFigure C1.
The influence of grade on vehicle deceleration can be derivedas follows:
● Braking distance
= = metres
● Component of vehicle mass acting down the slope = mgsin0(g = acceleration due to gravity = 9.81m/sec2);
● For small angles sinq = tanq = x/y = G (m/m)(grade is expressed as ratio, negative for downhill);
● Force acting down the slope . mgsinq . mgtanq = mgG;● Effective deceleration = gF + gG = g(F + G); and● Therefore effective deceleration = g(F + G)
In order to stop on sloped ground, equation 1 subsequentlybecomes:
S1 ≥ + + Ld + Cv (2)
where:
S1 = minimum distance of an approaching road vehiclefrom the nearest rail when the driver of the vehiclecan see an approaching train (m);
RT = perception/reaction time (general case assumption = 2.5 sec);
RURAL ROAD DESIGN 121
RTVv
3.6
Vv2
2a
RTVv
3.6Vv
2
254F
Vv2
2a
(Vv / 3.6)2
2gFVv
2
254F
(Vv / 3.6)2
2gFVv
2
254F
D E R I VAT I O N O F S I G H T D I S TA N C ER E Q U I R E M E N T S AT R A I LWAY L E V E L C R O S S I N G SCAPPENDIX
Figure C1: Influence of Slope on Stopping Distance
RTVv
3.6Vv
2
254(F+G)
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Vv = the 85th percentile road vehicle speed in thevicinity of the crossing. The road speed limit plus10% is a reasonable approximation where the 85thpercentile speed is not known (km/h);
F = coefficient of longitudinal friction (refer to Table8.2);
Ld = distance from the driver to the front of the vehicle(general case assumption = 1.5 m);
Cv = clearance from the vehicle stop or holding line tothe nearest rail (general case assumption = 3.5 m);and
G = grade, negative for downhill, positive for uphill(m/m).
3. CASE 1( i ) : Decelerate and Safely Stopat the Stop or Holding Line
The time required for a motorist (at a distance S1 from thenearest rail) to stop at the stop or holding line, comprises:
● Perception/reaction time (RT); and● Braking time
= = metres
(g = acceleration due to gravity = 9.81 m/sec2).
Therefore, for the motorist to safely stop, the train would haveto be sighted at a minimum distance, S2 from the crossing:
S2 ≥ RT + (3)
where:
S2 = minimum distance of an approaching train fromthe point of impact with a road vehicle, when thedriver of the road vehicle first sees a trainapproaching in order to safely stop at the stop orholding line (m);
VT = the speed of the train approaching the crossing(the allowed operating speed of trains, as advisedby the rail authority) (km/h);
RT = perception/reaction time (general case assumption= 2.5 sec);
Vv = the 85th percentile road vehicle speed in thevicinity of the crossing. The road speed limit plus10% is a reasonable approximation where the 85thpercentile speed is not known; and
F = coefficient of longitudinal friction (refer to Table8.2).
Note that the distance S2 is measured from alternate datumpoints which are contingent upon whether a train approachesfrom the left or right. For a train approaching from the left, thepoint of impact is at the road edge line, whilst, for a trainapproaching from the right, it is at the road centre line. For afield survey, distances S2L and S2R are required to be calculatedseparately as a common datum point is referenced.
Sight Distance S2L Adjustment
For the case of a train approaching the crossing from the left,the sight distance S2 is calculated from the left edge line of theroad (or the road pavement if there is no edge line). In orderto measure distance S2L from the referenced datum point, anadjustment needs to be incorporated in the S2 equation.
The datum point referenced in the field survey is theintersection of the centre line of the road and the mid point ofthe rail tracks at the crossing.
Adjustment for S2L equation =
In the case of a train approaching the crossing from the right,the sight distance S2R is equal to that adopted for S2, as thepotential point of impact is at the datum point.
The minimum distances, S2L and S2R, where an approachingtrain is first sighted in order for a driver of an approachingvehicle to safely stop at the stop or holding line, are calculatedfrom equations 4 and 5 respectively.
The minimum distance for a train approaching from the left ofthe crossing, to enable the driver of a road vehicle todecelerate and safely stop at the stop or holding line is:
S2L(l)≥ + RT + (4)
The minimum distance for a train approaching from the rightof the crossing, to enable the driver of a road vehicle todecelerate and safely stop at the stop or holding line is:
S2R(1)≥ RT + (5)
The calculated distances S2L and S2R are then compared to thedistances obtained in the case of a driver of a road vehiclesafely proceeding and clearing the crossing ( Case 1 (ii): Thelarger value is adopted as the critical case.
4. CASE 1( i i ) : Proceed and Clear the Crossingwith an Adequate Safety Margin
It is also important to consider the case in which a motorist atdistance S1 from the crossing decides to proceed (even thoughhe/she could safely stop) and attempt to clear the crossingprior to the arrival of the train.
Referring to Figure C2, the distance a motorist has to travel toclear the crossing is:
S1 + + + Cv + CT + L – Ld
Substituting S1 from equation 2, this becomes:
+ + + + 2Cv + CT + L
RURAL ROAD DESIGN122
Vv
aVv / 3.6
gFVv
2
35.3
0.5WR
sinZ
0.5WR
sinZVT
3.6Vv
35.3F
VT
3.6
RTVV
3.6VV
2
254(F+G)WR
tanZWT
sinZ
WR
tanZWT
sinZ
Vv
35.3F
VT
3.6Vv
35.3F
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Therefore, the distance travelled by the train for the motoristto precede and clear the crossing:
S2 = + + + + 2Cv + CT + L (6)
where:
S2 = minimum distance of an approaching train from thepoint of impact with a road vehicle, when the driver ofthe road vehicle can first see the train approaching thecrossing in order to proceed and safely clear thecrossing (m);
VT = the speed of the train approaching the crossing (theallowed operating speed of trains, as advised by the railauthority) (km/h);
Vv = the 85th percentile road vehicle speed in the vicinity ofthe crossing. The road speed limit plus 10% is areasonable approximation where the 85th percentilespeed is not known;
RT = perception/reaction time (general case assumption = 2.5 sec);
Cv = clearance from the vehicle stop or holding line to thenearest rail (general case assumption = 3.5 m);
CT = clearance or safety margin from stop or holding line ondeparture side of the crossing (general case assumption= 5 m);
F = coefficient of longitudinal friction (refer to Table 8.2);L = length of road vehicle, refer to Table C1;WR= width of the travelled way (portion of the roadway
allocated for the movement of the vehicles) at thecrossing (m);
WT= width, outer rail to outer rail, of the rail tracks at thecrossing (1.1 m for single track, 5.1 m for doubletrack); and
Z = angle between the road and the railway at the crossing(degrees).
Table C1: Vehicle Lengths
Vehicle Route Vehicle Type and LengthRoads not on nominated route Medium car 5 m
Prime mover and semi-trailer 19 m
B-double route B-double 25 mRoad train route – Type 1 Type 1 road train 33 mRoad train route – Type 2 Type 2 road train 50 m
As discussed in Case 1(i), distance S2 is measured fromalternate datum points to correspond with the potential pointof impact for the left and right train approaches. In order tocarry out a detailed survey of a crossing, distances S2L and S2Rare required to be calculated separately, as a common datumpoint is utilised.
The minimum distance (S2L) of an approaching train from theintersection of the centre line and the mid point of the railtracks, when the driver of the road vehicle first sees a trainapproaching from the left, in order to safely proceed and clearthe crossing (considering the sight distance S2L adjustmentindicated in Case 1(i)) is:
S2L = + + + + +2Cv+ CT+ L (7)
The minimum distance (S2R) of an approaching train from theintersection of the centre line of the road and the mid point ofthe rail tracks, when the driver of the road vehicle first sees atrain approaching from the right, in order to proceed and clearthe crossing is:
S2R = + + + +2Cv+ CT+ L (8)
In order to obtain the critical sight distances, S2L and S2R, thelarger distances from Cases 1(i) and (ii) should be adopted.
Source: Ref 66
5. CASE 2: Sight Distance Required forSTOP Sign Control
When motorists are stationary at a crossing controlled by aSTOP sign, they require adequate sight distance to determinewhether or not it is safe to cross the tracks before the trainarrives. Referring to Figure C3, it presents a method by whichthe time taken to complete this manoeuvre can beascertained. The time comprises:
● Perception time and time required to depress clutch (J);and
● Time to clear the crossing by a ‘safe’ distance
The distance travelled by the train during this time:
S3 = J +GS (9)
Field testing has confirmed that the influence of grade onvehicles accelerating from a stationary position is notaccurately modelled by the application of simple physicsprinciples (Lay 1990:571). American literature (AASHTO Policyon Geometric Design of Highways quoted in MRD (WA)1991:16) provides the grade correction factors in Table C2.
Equation (9) subsequently becomes:
S3 = J + GS (10)
where:-
S3 = minimum distance of an approaching train from thepoint of impact with a road vehicle, when the driver ofthe road vehicle must first see an approaching train inorder to safely cross the tracks (m);
RURAL ROAD DESIGN 123
0.5WR
sinZ
VV2
254(F+G)WR
tanZWT
sinZVT
Vv
VT
Vv
RTVT
3.6
VV2
254(F+G)WR
tanZWT
sinZRTVT
3.6
VT
Vv
VV2
254(F+G)WR
tanZWT
sinZRTVT
3.6
WR
tan ZWT
sin Z
VT
3.6
VT
3.6
+
a2
1/2
1/2
1/2
+ 2CV + CT + L
WR
tan ZWT
sin Z+
a2
+ 2CV + CT + L
WR
tan ZWT
sin Z+
a2
+ 2CV + CT + L
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VT = the speed of the train approaching the crossing (theallowed operating speed of trains, as advised by the railauthority (km/h);
J = sum of the perception time and time required todepress clutch (general case assumption = 2 sec);
GS = grade correction factor, refer to Table C2;L = length of road vehicle, refer to Table C1 (m);CV = clearance from the vehicle stop or holding line to the
nearest rail (general case assumption = 3.5m);CT = clearance or safety margin from stop or holding line on
departure side of the crossing (general case assumption= 5m);
WR = width of the travelled way (portion of the roadwayallocated for the movement of the vehicles) at thecrossing (m);
WT = width, outer rail to outer rail, of the rail tracks at thecrossing (1.1m for single track, 5.1m for double track);
Z = angle between the road and the railway at the crossing(degrees); and
a = average acceleration of vehicle in starting gear (generalcase assumption = 0.5 m/sec2, refer to Table C3).
Table C2: Grade Correction Factors (AASHTO Policy onGeometric Design of Highways)
Percentage Grade Grade Correction Factor GS– 4 0.8– 2 0.9+ 2 1.2+ 4 1.7
Sight Distance S3L Adjustment
A sight distance adjustment is necessary to calculate S3L forthe common datum point used in the field survey. The datumpoint referenced in the field survey is the intersection of thecentre line of the road and the mid point of the railway tracksat the crossing.
Adjustment for S3L equation =
Therefore, the minimum distance of an approaching train fromthe intersection of the road centre line and the mid point ofthe rail tracks, when the driver of a road vehicle must first seea train approaching from the left in order to safely cross thetrack from a stopped position is:
S3L = + J +GS
(11)
The minimum distance of an approaching train from theintersection of the road centre line and the mid point of therail tracks, when the driver of a road vehicle must first see atrain approaching from the left in order to safely cross thetrack from a stopped position is:
S3R = J +GS (12)
Table C3: Heavy Vehicle Speed/Acceleration Performance(RTA, 1990 and QT, 1993)
Type of Distance Time Average AverageVehicle Travelled (sec) Speed Acceleration
(m) (m/sec) (m/sec)
Laden Rigid Truck 22.4 9.3 2.4 0.50(RTA 1990)Laden Semi Trailer 28.9 12.6 2.3 0.36(RTA 1990)Laden B-double 34.4 13.6 2.5 0.37(RTA 1990)Laden Road Train 46.4 21.3 2.2 0.29(RTA 1990)Laden 19m 27.5 11.3 2.4 0.43Semi-Trailer (QT Mt Cotton 8.7 3.2 0.73Facility 1993)Laden 19m 34.5 13.8 2.5 0.36Semi-Trailer(QT Mt Cotton 10.8 3.2 0.59Facility 1993)
NOTE: In addition to the data provided in Table C3, limiteddata collected by ARRB (Barton 1990:6) suggests the averagespeed of a heavy vehicle commencing from a stopped positionequals 3.3 m/sec over a typical crossing distance. The MainRoads Department (Western Australia) (1991:13) quotesvalues of acceleration obtained from American literatureranging from “45 m/sec2 for the acceleration of trucks in firstgear, to 0.54 m/sec2 over a distance of around 12m, thengradually back down to a value of 0.5 m/sec2 for a distance ofaround 50”. For the required crossing visibility at the criticalcase, they subsequently recommend the adoption of a heavyvehicle acceleration value of 0.5 m/sec2 to “be on theconservative side”, and indicate that this value has beenshown “to be acceptable by measuring the acceleration ratesof a number of fully laden trucks, which resulted in valuesbetween 0.55 m/sec2 and 0.90 m/sec2”.
RURAL ROAD DESIGN124
0.5WR
sin Z
0.5WR
sin ZVT
3.6
WR
tan ZWT
sin Z+
a2
+ 2CV + CT + L 1/2
VT
3.6
WR
tan ZWT
sin Z+
a2
+ 2CV + CT + L 1/2
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Figure C2: Approach Visibility: At Grade Railway/Road Crossings
Case 1(i)Motorist approaching crossing sights train, deceleratesand stops at the holding line.
Case 1(ii)Motorist approaching crossing sights train, proceeds andsafely clears the crossing.
Notation (units and/or general case assumptions are shown inbrackets):
S1 Minimum distance of an approaching road vehiclefrom the nearest rail when the driver of the vehicle cansee an approaching train (m);
S2 Minimum distance of an approaching train from thepoint of impact with a road vehicle, when the driver ofthe road vehicle first sees a train approaching (m);
S2L Minimum distance of an approaching train from theintersection of the road centre line and the mid pointof the rail tracks, when the driver of the road vehiclefirst sees a train approaching from the left (m).
S2R Minimum distance of an approaching train from theintersection of the road centre line and the mid pointof the rail tracks, when the driver of the road vehiclefirst sees a train approaching from the right (m).
VT The speed of the train approaching the crossing (theallowed operating speed of trains, as advised by the railauthority (km/h).
VV The 85th percentile road vehicle speed in the vicinity ofthe crossing. The road speed limit plus 10% is areasonable approximation where the 85th percentilespeed is not known.
CV Clearance from the vehicle stop or holding line to thenearest rail (general case assumption = 3.5 m).
CT Clearance or safety margin from the vehicle stop orholding line on the departure side of the crossing(general case assumption = 5 m).
L Length of road vehicle (m).Ld Distance from the driver to the front of the vehicle
(general case assumption = 1.5 m).WR Width of the travelled way (portion of the roadway
allocated for the movement of the vehicles) at thecrossing (m).
WT Width, outer rail to outer rail, of the rail tracks at thecrossing (1.1 m for single track, 5.1 m for doubletrack).
X1L Vehicle driver viewing angle measured from distance S1on the road centre line, where a driver must first see atrain approaching from the left at distance S2 from thecrossing.
X1R Vehicle driver viewing angle measured from distance S1on the road centre line, where a driver must first see atrain approaching from the right at distance S2 fromthe crossing.
Z = Angle between the road and the railway at the crossing(degrees).
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Figure C3: Crossing Visibility: At Grade Railway/Road Crossings
Case 2Motorist stopped at crossing requires adequatetime to accelerate and safely clears the crossing.
Notation (units and/or general case assumptions are shown inbrackets):
S3 Minimum distance of an approaching train from thepoint of impact with a road vehicle, when the driver ofthe road vehicle must first see an approaching train inorder to safely cross the tracks.
S3L Minimum distance of an approaching train from theintersection of the road centre line and the mid pointof the rail tracks, when the driver of a road vehiclemust first see a train approaching from the left in orderto safely cross the track from a stopped position at thestop or holding line (m).
S3R Minimum distance of an approaching train from theintersection of the road centre line and the mid pointof the rail tracks, when the driver of a road vehiclemust first see a train approaching from the right inorder to safely cross the track from a stopped positionat the stop or holding line (m).
VT The speed of the train approaching the crossing (theallowed operating speed of trains, as advised by the railauthority) (km/h).
L Length of road vehicle (m).
Ld Distance from the driver to the front of the vehicle(general case assumption = 1.5m).
CV Clearance from the vehicle stop or holding line to thenearest rail (general case assumption = 3.5m).
CT Clearance or safety margin from the vehicle stop orholding line on departure side of the crossing (generalcase assumption = 5m).
WR Width of the travelled way (portion of the roadwayallocated for the movement of the vehicles) at thecrossing (m).
WT Width, outer rail to outer rail, of the rail tracks at thecrossing (1.1m for single track, 5.1m for double track).
X2L Vehicle driver viewing angle measured from at theSTOP line to a train approaching from the left atdistance, S3 from the crossing.
X2R Vehicle driver viewing angle measured from at theSTOP line at the road centre line to a train approachingfrom the right at distance, S3 from the crossing.
Z= Angle between the road and the railway at the crossing(degrees).
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AUSTROADS
ROAD DESIGN SERIES
ISBN: 0 85588 655 2AP-G1/03
Rural Road DesignA Guide to the GeometricDesign of Rural Roads
Rura
l Ro
ad
Desig
n A
Guid
e to
the G
eom
etric D
esig
n o
f Rura
l Roads
AU
STR
OA
DSA
cces
sed
by A
US
TR
OA
DS
- G
HD
PT
Y L
TD
on
10 A
pr 2
006
