Section 6 Speed Parameters_noPW

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Section 6

Speed Parameters


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GUIDELINE 3AACRA GEOMETRIC DESIGN MANUAL

TABLE OF CONTENTS

6 DESIGN SPEED ..................................................................................................................................... 6-16.1 General ............................................................................................................................................ 6-16.2 Car Design Speeds.......................................................................................................................... 6-16.3 Truck Design Speeds....................................................................................................................... 6-26.4 Design Consistency ......................................................................................................................... 6-36.5 High Relative Speeds ...................................................................................................................... 6-36.6 References....................................................................................................................................... 6-46.7 Relationship to Other Chapters........................................................................................................ 6-46.8 Relationship to Other Manuals......................................................................................................... 6-4

TABLES

Table 6.2-A:- Minimum Design Speed for Cars and Speed Limits by Road Type......................................... 6-2Table 6.3-A :- Truck/Car Speed Relationship................................................................................................. 6-3

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6 DESIGN SPEED

6.1 General

A fundamental aim of geometric road design is to provide road geometry (curvature, sight distance,superelevation etc.) that is suitable for the speeds of the vehicles that operate on the road. The actualoperating speeds of vehicles can vary with time due to changes in traffic conditions and from point to pointdue to changes in the road environment, road characteristics, speed limits, driver behaviour and vehicletypes.

On roads of consistently high geometric standard (i.e. one where all of the geometric elements are designedfor 100 to 110 km/h or more), operating speeds on horizontal curves and straights are not significantlydifferent and typically do not exceed 110 km/h. However, on roads where the geometric elements aredesigned for speeds less than 100 km/h, operating speeds will vary over the length of the road with higherspeeds being used on straights and the larger radius horizontal curves.

Research that led to the development of the Austroads speed environment model showed that drivers inrural areas mainly react to visible cues or hazards, and the principal visible cue in respect of speed choice isthe apparent horizontal curvature of the road ahead. Drivers react much less to vertical curvature and to apotential hazard which is out of view, eg. beyond a crest vertical curve, even though attention may be drawnto the hazard by signs or other devices.

In urban areas, operating speeds are more aligned with speed limits, but tend to be above these limits.Accordingly, recommended design speeds for freeways and arterial roads are based upon the proposedspeed limit plus 10 km/h (with the exception of ramps at interchanges).

The geometric constraints usually present at interchanges usually makes the use of relatively generousdesign speeds impractical. Drivers using these ramps are expected to be at an increased level of alertnessto drivers on a through carriageway. Accordingly, design speeds equal to the proposed speed limit oradvisory speed are acceptable.

On feeder roads and residential streets, the speed environment is lowered by the urban setting and thecomplementary road layout that limits the length of streets, provides speed control with bends, intersectiontreatments, speed control devices such as slow points and speed humps. Design speeds lower than thespeed limit in these areas are acceptable.

6.2 Car Design Speeds

Table 6.2-Ashows design speeds for cars to be used for each road type.

Design speeds for through carriageways of freeways and arterial roads are based upon the proposed speedlimit plus 10 km/h.

Ramp design speeds are equal to the proposed speed limit.

Ramps connecting to arterial roads should have a posted speed the same as the arterial road (except loops).

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Freeway to freeway ramps (except loops) should have a design speed of at least 80 km/h in suburban areas,and at least 60km/h in inner urban areas.

Loop ramp design speeds as low as 40km/h may be used.

Table 6.2-A:- Minimum Design Speed for Cars and Speed Limits by Road Type

Road Type Proposed Speed Limitor

Speed Environment

Minimum DesignSpeed

for Cars

Suburban Freeway Through Carriageway 100 110

Suburban Freeway to Freeway Ramp 80 80

Suburban Freeway to Arterial Road Ramp 60/70* 60/70

Suburban Freeway Collector-Distributor Road 80 90

Inner Urban Freeway Through Carriageway 80 90Inner Urban Freeway to Freeway Ramp 60 60

Inner Urban Freeway to Arterial Road Ramp 60/70* 60/70

Inner Urban Freeway Collector-Distributor Road 70 80

Freeway Loop Ramp Truck Advisory Sign 35 40

Continuous Frontage Road 70 80

Arterial Road Controlled Access 70 80

Discontinuous Frontage Road 60** 60**

Arterial and sub arterial road with frontage access 60 70

Feeder Roads and industrial/commercial accessroads

60 60

Low Volume Residential Feeder Roads (


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Table 6.3-Ashows a typical relationship between truck speeds and car speeds. This indicates that wheretruck characteristics are to be used for design checks, the truck design speed may be chosen to beapproximately 10km/h less than the car design speed except on freeways and on uphill grades, where higherand lower speeds apply respectively.

On freeways with flat grades, truck speeds are similar to car speeds.

On uphill grades truck speeds will reduce, and the truck design speed should be estimated frommeasurements of actual conditions (if the design is an upgrade of an existing road) or from truckperformance data such as contained in AASHTO (2001).

Table 6.3-A :- Truck/Car Speed Relationship

Car speed 110 100 90 70 60 50 40

Truck

Speed

100 90 80 60 52 43 34

6.4 Design Consistency

It is highly desirable that roads are designed so that they are relatively simple and straightforward for driversto choose the required safe speed and position of their vehicles. Geometric inconsistencies that require alarge change in speed or direction violate driver expectancy and should be avoided.

A major geometric inconsistency is the provision of a low speed geometric element following a high speedgeometric element. This geometry requires drivers to make a large decrease in their speed to enable theirvehicle to remain stable on the low speed geometric element.

The result of drivers not recognising the required action for these geometric features greatly increases thechance of a single vehicle accident occurring. In comparison, the adverse consequences of a driver notrecognising the required action on a geometric element with a small decrease in speed at the start of theelement are much lower. A larger decrease in speed between successive elements will also lead to moresevere accidents.

6.5 High Relative Speeds

A major geometric inconsistency is the provision of geometry which results in a high potential relative speedof vehicles. Common situations where high potential relative speeds may be encountered include thefollowing:

at-grade intersections between high speed roads

at-grade intersections with angles between roadways considerably less than or greater than 90 degrees- design considerations to minimise the potential relative speed of vehicles at intersections are given inSection 13

on roundabouts with minimal entry curvature and/or minimal deflection design considerations tominimise the potential relative speed of vehicles at roundabouts are given in Section 14

short on-ramps or off-ramps at interchanges - design considerations to minimise the potential relativespeed of vehicles at interchanges are given in Section16

steep grades with high speed horizontal alignment (slow moving trucks and fast cars)

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Section 6 Speed Parameters DRAFT FINAL Rev. 0 February 2003Page 6-4

6.6 References

AUSTROADS (2002) Urban Road Design

NAASRA (1984) Grade Separated Interchanges A Design Guide

AASHTO (2001) A Policy on Geometric Design of Highways and StreetsJVMAH (1989) Australian Model Code for Residential Development

6.7 Relationship to Other Chapters

Provides the Speed data for Sections 9, 10, 11,12, 13, 14, 15, 16 and 21

6.8 Relationship to Other Manuals

Guideline 1 Traffic & Axle Load Study (TAL)

Guideline 2 Pavement Design Manual (PMD & MRM)

Guideline 3 Geometric Design Manual (GDM)

Guideline 4 Bridge Structures Design Manual (BCSDM)

Guideline 5 Soils & Materials Manual (SMM)

Guideline 6 Drainage Design Manual (DDM)

Guideline 7 Street Lighting Design Manual (SLDM)

Guideline 8 Technical Specifications (TS)

Guideline 9 Bidding Documents (BD) & Conditions of Contract (COC)

Section 6 Speed Parameters_noPW

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