REAM Guidelines On Geometric Design of Roads
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Transcript of REAM Guidelines On Geometric Design of Roads
F'OITEWORD
Road Engineering Association of Malaysia (REAM), througlr the cooperation and support ofvarious road authorities and engineering institutions in Malaysia, publishes a series of officialdocuments on STANDARD SPECIFICATIONS, GUIDELINES, MANUALS andTECHNICAL NOTES which are related to road engineering. The aim of such publications isto achieve quality and consistency in road and highway construction.
The cooperating bodies are:-
Public Works Department, MalaysiaMalaysian Highway AuthorityInstitution of Engineers, MalaysiaInstitution of Highways & Transportation (Malaysian Branch)
The production of such documents are carried out through several stages. The documents areinitially compiled/drafted by the relevant Technical Committee and subsequently scrutinisedby the relevant Standing Committee of REAM. They are finally endorsed by road authoritiesand practitioners of road engineering at a conference/workshop before publication. REAMwelcomes feedback and suggestions which can update and improve these documents.
This GUIDE ON GEOMETRIC DESIGN OF ROADS is the revision of the existingArahan Teknik (Jalan) 8/86 "A Guide on Geometric Design of Roads" published by JabatanKerja Raya.
The revision was undertaken by the Standing Committee on Technology and RoadManagement of the Road Engineering Association of Malaysia (REAM) with the mandategiven by the Director-General of Public Works Department, Malaysia.
The draft of this Arahan Teknik was presented and discussed at the Forum on Technologyand Road Management in November 1997 and the 4th Maiaysian Road Conference inNovember 2000.
It is hoped that this Guide will address the shortcomings of Arahan Teknik (Jalan) 8/86 andwill be accepted by the highway engineering fraternity in the country.
ROAD ENGINEERING ASSOCIATION OF MALAYSIA46-4, Jalan Bola Tarnpar 13114, Section 13,40100 Shah Alam, Selangor, Malaysia.
Tel:603-55136521 Fax:603-55136523 e-mail:[email protected]
ROAD ENGINEERING ASSOCIATION OF MALAYSIA
A GUIDE ON GEOMETRIC DESIGN OF ROADS
TABLE OF CONTENTS
INDEX
I. INTRODUCTION
2. ROAD CLASSIFICATIONS AND DESIGN STANDARDS
PAGE NO.
I
2.1
2.2
2.3
)4
J.l
J.Z
3.3
J-+
3.5
4.1
A'
+.J
4.4
5.1
5.2
5.3
5.4
5.5
5.6
5.7
11
ll12
l.+
18
20
ROAD CLASSIFICATION / HIERARCHY
DESIGN STANDARDS FOR ROADS
ACCESS CONTROL
SELECTION OF DESIGN STANDARDS
z
2
5
6
8
25
25
a1J+
44
52
DESIGN CONTROL AND CRITERIA
TOPOGRAPHY AND LAND USE
TRAFFIC
DESIGN VEHICLES CHARACTERISTICS
SPEED
CAPACITY
A ELEMENTS OF DESIGN
SIGHT DISTANCE
HORIZONTAL ALIGNMENT
VERTICAL ALIGNMENT
COMBINATION OF HORIZONTAL AND VERTICAL
ALIGNMENT
5. CROSS SECTION ELEMENTS
PAVEMENT
LANE WIDTH & MARGINAL STRIPS
SHOULDERS
KERBS
SIDEWALKS
TRAFFIC BARRIERS
MEDIANS
61
6l
62
62
66
69
6q
69
INDEX
6.
5.8
5.9
5. l0
5.t I
TABLE OF CONTENTS (Cont'd)
SERVICE ROADS
U-TURN
BRIDGE AND STRUCTURE CROSS SECTION
BUS LAYBYES
PAGE NO.
10
73
16
to
OTHER ELEMENTS AFFECTING GEOMETRIC DESIGN 80
6.I ROAD SAFETY 80
6.2 DRAINAGE 80
6.3 LIGHTING 8I
6.4 UTILITIES 81
6.5 SIGNING AND MARKINGS 8I
6.6 TRAFFIC SIGNALS 8I
6.1 EROSION CONTROL, LANDSCAPE DEVELOPMENT 82
AND ENVIRONMENTAL IMPACTS
TABLES
TABLE 2-I
TABLE2-2A
TABLE2-28
TABLE 2-3
TABLE 3.1
TABLE 3-2A
TABLE 3-2B
TABLE 3-3
TABLE 3-4
TABLE 3-5A
TABLE 3-58
TABLE 4-1
TABLE4-2
TABLE 4-3
TABLE 4-4
TABLE 4-5
TABLE 4-6
TABLE4.]A
TABLE 4-78
TABLE 4-8
TABLE 4-9A
TABLE 4-98
TABLE 4-9C
TABLE 4-9D
TABLE 4-9E
TABLE 4-9F
TABLE OF CONTENTS (Cont'd)
CHARACTERISTICS OF ROAD CATEGORIES
SELECTION OF ACCESS CONTROL (RURAL)
SELECTION OF ACCESS CONTROL (URBAN)
SELECTION OF DESIGN STANDARD
DIMENSION OF DESIGN VEHICLES
DESIGN SPEED FOR RURAL ROADS
DESIGN SPEED FOR URBAN ROADS
CONVERSION FACTORS TO P.C.U.
LEVELS OF SERVICE
DESIGN LEVEL OF SERVICE AND VOLUME/
CAPACITY RATIO (RURAL)
DESIGN LEVEL OF SERVICE AND VOLUME/
CAPACITY RATIO (URBAN)
MINIMUM STOPPING DISTANCE
EFFECTS OF GRADES IN STOPPING SIGHT
DISTANCE _ WET CONDITIONS
DECISION SIGHT DISTANCE
MINIMUM PASSING SIGHT DISTANCES
(2LANE-2WAY)
MINIMUM RADIUS
RELATIONSHIP OF DESIGN SPEED TO MAXIMUM
RELATIVE PROFILE GRADIENTS
MAXIMUM GRADES FOR R5 STANDARD ROADS
MAXIMUM GRADES FOR U5 STANDARD ROADS
MAXIMUM GRADES FOR U6 AND R6 STANDARD
ROADS
PAGE NO.
AT
8
8
9
14
19
1q
2l
22
aA
24
zo
2'7
28
30
3s
5l
DESIGN SUPERELEVATION TABLE (RURAL) 38
DESIGN SUPERELEVATION TABLE (URBAN) 39
PAVEMENT WIDENING ON OPEN ROAD CURVES 43
MAXIMUM GRADES FOR RI. R2. Ul & U2 45
STANDARD ROADS
MAXIMUM GRADES FOR R3 AND R4 STANDARD 45
ROADS
MAXIMUM GRADES FOR U3 AND U4 STANDARD 45
ROADS
A<
46
ul
TABLES
TABLE 4-1OA
TABLE 4-IOB
TABLE 5-1
TABLE 5-2
TABLE 5-3A
TABLE 5-3B
TABLE 5-4
TABLE 5-5A
TABLE 5-58
TABLE 5-6
TABLE OF CONTENTS (Cont'd)
CREST VERTICAL CURVE (K VALUES)
SAG VERTICAL CURVE (K VALUES)
TYPICAL PAVEMENT SURFACE TYPE
LANE & MARGINAL STRIP WIDTHS
USABLE SHOULDER WiDTH (RURAL)
PAVED SHOULDER WIDTH (URBAN)
PAVED SHOULDER WIDTH (RURAL)
MEDIAN WIDTH (RURAL)
MEDIAN WrDTH (URBAN)
DESIRABLE DISTANCE BETWEEN U-TURNS
PAGE NO.
50
51
6l
6l
64
65
65
70
70
74
FIGUR.ES
FIGURE 2-I
FIGURE 3-1
FIGURE 3-2
FIGURE 3-3
FIGURE 3-4
FIGUR.E 4-1
FIGURE 4-2
FIGURE 4-3
FIGURE 4-4
FIGURE 4-5
FIGURE 4-6
FIGURE 4-6
FIGURE 4-6
FIGURE 4-6
FIGURE 4-6
FIGURE 4.6
FIGURE 4-6
FIGURE 5-1
FIGURE 5-2A
FIGURE 5-28
FIGURE 5-3
FIGURE 5-4
FIGURE 5-5
FIGURE 5-6
FIGURE 5-7
TABLE OF CONTENTS (Cont'd)
PAGES NO
FLOW CHART FOR SELECION OFDESIGN STANDARDS IO
P DESIGN VEHICLE 15
SU DESIGN VEHICLE 16
WB-15 DESIGN VEHICLE I1
RELATIONSHIP OF LEVEL OF SERVICE TO OPERATING 23
SPEED AND VOLUME / CAPACITY RATIO
MEASURING SIGHT DISTANCE ON PLAN
MEASURING SIGHT DISTANCE ON PROFILE
COMPARISON OF SIDE FRICTION FACTORS ASSUMED
FOR DESIGN OF DIFFERENT TYPES OF ROADS
DIAGRAMMATIC PROFILES SHOWING METHODS OF
ATTAINiNG SUPERELEVATION
CRITICAL LENGTHS OF GRADE FOR DESIGN FOR
TYPICAL HEAVY TRUCK OF 180kg/kw.
A&BC&DE&FG,H&IJ,K&LM
N
TYPICAL CROSS SECTION OF SHOULDER AND VERGE
ROAD KERB DETAILS
ROAD KERB DETAILS
TWO-WAY SERVICE ROAD
MINIMUM DESIGN FOR DIRECTU-TURNS
TYPICAL BRIDGE CROSS SECTIONS
CLEARANCES AT UNDERPASS
LAYOUT OF BUS LAYBYES
32
JJ
35
4]
4't
<A
55
56
57
58
59
60
63
67
68
72
'75
77
18
'79
.*1i 'i{F,:i'. Ei:. "'r:' iisif-.:" ;l ::5;;r.?;
A Guide on Geometric Design of Roads
CHAPTER 1 - INTRODUCTION
This Guide is limited to the geometric features of road design as distinguished from structural
design. It is intended as a manual on the geometric design of road, inclusive both
in rural as well as in urban conditions. The geometric design of road is only applicable to
Rural or Urban areas as specifically indicated in this Guide.
This Guide is to be applied to all new construction and improvements of roads for vehicular
traffic. Comments from users will be most welcomed as this Guide will continue to be updated
from time to time.
This Guide is to be used in conjunction with other Arahan Tekniks that have been or
will be produced by Jabatan Keria Raya or other Agencies.
A Guide on Geometrtc Design of Roads
CHAPTER 2 - ROAD CLASSIFICATIONS AND DESIGN STANDARDS
2.r ROAp CLASSTFTCATTON / HrERARelgy
2.1.1 The Importance of Road Classification
The importance of defining a hierarchy of roads is that it can help clarify policiesconcerning the highway aspects of individual planning decisions on properties served bythe road concerned. Further'more specific planning criteria could be developed andapplied according to a road's designation in the hierarchy: for example design speed,width of carriageway, control over pedestrian, intersections, frontage access etc. In thisway the planning objectives would be clear for each level of road in the hierarchy andpolicies on development control and traffic management would reinforce one another.
2.1.2 Functions of Road
Each road has its function according to its role either in the National Network, RegionalNetwork, State Network or City/Town Network. The most basic function of a road istransportation. This can be further divided into two sub-functions; namely mobility andaccessibility. However, these two sub-functions are in trade off. To enhance one, theother must be limited. In rural areas, roads are divided into five categories, narnely,EXPRESSWAY, HIGHWAY, PRIMARY ROAD, SECONDARY ROAD ANdMINOR ROAD. In urban areas, roads are divided into four categories, namelyEXPRESSWAY, ARTERIAL, COLLECTOR and LOCAL STREET. They are inascending order of accessibility and thus in descending order of mobility.
2.1.3 Road Category and Their Application
Categories of Road
2 GROUPS
URBAN (4 Categories)
1. Expressway2. Arterial3. Collector4. Local Street
RURAL (5 Categories)
l. Expressway 4. Secondary Road2. Highway 5. Minor Road3. Primary Road
A Guid.e on Geometric Design af Roads
Categories of roads in Malaysia are defined by their general functions as follows:
(a) Expresswav
An Expressway is a divided highway for through traffic with full control ofaccess and always with grade separations at all intersections'
In rural areas, they apply to the interstate highways for through traffic and form
the basic framework of National road transportation for fast travelling. They
serve long trips and provide'higher speed of travelling and comfort. To maintaiu
this, they are fully access controlled and are designed to the highest standards.In
urban areas, they form the basic framework of road transportation system in
urbanised area for through traffic. They also serve relatively long trips and
smooth traffic flow and with full access control and complements the Rural
Expressway.
HighwayThey constitute the interstate national network for intermediate traffic volumes
and complements the expressway network. They usually link up directly or
indirectly the Federal Capital, State capitals, large urban centres and points ofentry/exit to the country. They serve long to intermediate trip lengths. Speed oftravel is not as important as in an Expressway but relatively high to medium
speed is necessary. Smooth traffic is provided wrth partial access control.
Primary Roads
They constitute the major roads forming the basic network of the road
transportation system within a state. They serve intermediate trip lengths and
medium travelling speeds. Smooth traffic is provided with partial access
control.They usually link the State Capitals and district Capitals or other Major
Towns.
Secondary Roads
They constitute the major roads forming the basic network of the road
transportation system within a District or Regional Development Areas. They
serve intermediate trip lengths with partial access control. They usually link up
the major towns within the District or Regional Development Area.
Minor Roads
They apply to all roads other than those described above in the rural areas. They
form the basic road network within a Land Scherne or other sparsely populated
rural area. They also include roads with special functions such as holiday resort
roads, security roads or access roads to microwave stations. They serve
mainly local traffic with short trip lengths with no access control.
(b)
(c)
(d)
(e)
A Guide on Geometric Design of Roads
(0 ArterialsAn arterial is a continuous road within partial access control for through trafficwithin urban areas. Basically it conveys traffic from residential areas to thevicinity of the central business district or from one part of a city to another whichdoes not intend to penetrate the city centre. Anerials do not penetrate identifiableneighbourhoods. Smooth traffic flow is essential since it carries larse trafficvolumes.
CollectorsA collector road is a road with partial access control designed to serve as acollector or distributor of traffic between the arterial and the local road systems.Collectors are the major roads which penetrate and serve identifiableneighbourhoods, commercial areas and industrial areas.
Local Streets
The local street system is the basic road network within a neighbourhood andserves primarily to offer direct access to abutting land. They are links to thecollector road and thus serve short trip lengths. Through traffic should bediscouraged.
The characteristics for road categories are summarised as in Table 2-r.
TABLE 2.1 : CHARACTERISTICS OF ROAD CATEGORIES
(e)
(h)
AREAROAD
CATEGORIESTrip Length Design Volume Speed NETWORK
Long Med Short Hieh Med Low High Med Low
RURAL
Expressway I
-I National network
Highway
-
I I t I I National network
Primary Road I I I I E State networkSecondary Road I I !E * E - District network
Minor Road I IT
-Supporting network
URBAN
Expressway
-I E ! r National network
Arterial T IMajor links to
Urban centres
Collector E I E E - I Major streets within
urban centres
Local Street E E EMinor streets/town
network.
A Guide on Geometric Design of Roads
2.2 DESIGN STANDARDS FOR ROADS
2.2.1 The Importance of Standardisation
Technically, the geometric design of all roads need to be standardised for the following
reasons:-
(a) to provide uniformity in the design of roads according to their performance
requirements.
to provide consistent, safe and reliable road facilities for movement of traffic.
(c) to provide a guide for less subjective decision on road design.
2.2.2 Rural and Urban Areas
Urban areas are defined as roads within a gazetted Municipality limits or
township having a population of at least 10,000 where buildings and houses are gathered
and business activity is prevalent. Any roads outside the Municipality limits is
considered rural, including roads connecting Municipalities that are more than
5 kilometres apart.
There is no fundamental difference in the principles of design for rural and urban roads.
Roads in urban areas, however, are characterised by busy pedestrian activities and
frequent stopping of vehicles owing to short intersection spacings and congested built-up
areas. Lower design speeds are usually adopted for urban roads and different cross
sectional elements are applied to take into account the nature of traffic and adjoining land
use. It is for these reasons that variations in certain aspects of geometric design are
incorporated for these two broad groups of roads.
2.2.3 Application of Design Standards for Roads
The design standard is classified into six (6) groups (R6, R5, R4, R3, R2 & R1) for rural
(R) areas and into six (6) groups (U6, U5, U4,U3,U2 & Ul) for urban (U) areas. Each
of these standards are listed below with descending order of hierarchy.
(a) Standard R6/U6: Provides the highest geometric design standards for
rural or urban areas.They usually serve long trips
with high travelling speed (90 kph or higher)of
travelling,comfort and safety. It is always designed
with dividedcarriageways and with fuli access control.
The Rural and Urban Expressway falls under this
standard.
(b)
A Guide on Geomelric Design of Roads
(b) Standard R5/U5:
(c) Standard R4N4:
(d) Standard R3/U3:
(e) Standard R2N2:
(0 Standard R1/U1:
Provides high geometric standards and usually serve longto intermediate trip lengths with high ro mediumtravelling speeds (80 kph or higher). It is usually withpartial access control. The Highway, Primary Road and
Arterial fall under this standard. It is sometimes designedas divided carriageways with partial access control.
Provides medium geometric standards and serveintermediate trip lengths with medium travelling speeds(70 kph or higher). It is also usually with partial access
control. The Primary Road, Secondary Road, MinorArterial and Major Collector fall under this standard.
Provides low geometric standard and serves mainly localtraffic. There is partial or no access control. TheSecondary Road, Collector or Major Local Streets arewithin this standard. The travelling speed is usuallv60 kph.
Provides low geometric standards for two way flow.It is applied only to local traffic with low volumes ofcommercial traffic. The Minor Roads and Local Streets
fall under this standard. The travelling speed is usually50 kph.
Provides the lowest geometric standards and is applied toroad where the volumes of commercial vehicles are verylow in comparison to passenger traffic. The travellingspeed is 40 kph or less. In cases where commercial trafficis not envisaged such as private access road in low costhousing area, the geometry standards could be loweredespecially the lane width and gradient.
2.3 ACCESS CONTROL
2.3.1 Degree of Control
Access control is the condition where the right of owners or occupants of abutting landor other persons to access, in connection with a road is fully or partially controlled by thepublic authority.
A Guide on Geometric Design of Roads
Control of access is usually classified into three types for its degree of control, namelyfull control, partial control and non-control of access.
Full Control of Access means that preference is given to through traffic by providing
access connecting with selected public roads only and by prohibiting crossings at grade
or direct private driveway connections. The access connections with public roads varies
from 2 km in the highly developed central business area to 8 km or more in the sparsely
developed urban fringes.
Partial Control of Access means that preference is given to through traffic to a degree
that in addition to access connection with selected public roads, there may be some
crossings trafficked roads. At-grade intersections should be limited and only allowed at
selected locations. The spacin g of at-grade intersections preferably signalised may vary
from 0.4 km to 1.0 km.
To compensate for the limited access to fully or partially access controlled roads,
frontage or service roads are sometimes provided along side of the main roads.
In Non-Control Access, there is basically no limitation of access.
2.3.2 Selection of Access Control
The selection of the degree of control required is imporlant so as to preserve the as-built
capacity of the road as well as improve safety to all road users. Two aspects pertaining
to the degree of control is to be noted.
(a) during the time of design in the consideration of accesses to existing
develooments.
(b) after the completion of the road in the controi of accesses to futuredevelonments.
The selection of degree of access control depends on traffic volumes, function of the road
and the road network around the areas. Tables 2-2A and2-2F_ are general guides for the
selection of desree of access control.
A Guide on Geometric Design of Roads
TABLE 2-ZA: SELECTION OF ACCESS CONTROL (RURAL)
Xt'tn Standard
_____
Road categorr---'--R6 R5 R4 R3 R2 R1
Expressway
HighwayPrimary Road
Secondary Road
Minor Road
F
P
P P
P P
N N
TABLE 2-28: SELECTION OF ACCESS CONTROL (URBAN)
-----=pesign Standard\\\__r\\_
Road Category \-r'-\U6 U5 U4 U3 U2 U1
Expressway
ArterialCollectorLocal Street
F
P
P
P
P
N
P
N N N
Note : F = Full Control of Access,P = Partial Control of Access,N - No Control of Access
2.4 SELECTION OF DESIGN STANDARDS
2.4.1 Selection of Design Standard
The selection of the required design standard should begin with the assessment of thefunction of the proposed road and the area it traverses. If there is an overlapping offunctions, the ultimate function of the road shall be used for the selection criteria. Theprojected average daily traffic (ADT) at the end of the design life (15 years aftercompletion of the road) should then be calculated and from Table2-3, the requireddesign standard can be obtained. From the capacity analysis the required nurnber oflanes can then be calculated.
A Guide on Geometric Design of Roads
The selection of design standards used for various categories of roads is as
shown in Table 2-3 while the process of selecting design standard are as shown inFigure 2-I.
TABLE 2.3: SELECTION OF DESIGN STANDARD
Area\ Projected
\ ADTRoad \Category \
AllTrafficVolume
>10,000
10,000
to
3,000
3,000
to
1,000
1,000
to
150
< 150
RURAL
Expressway
HighwayPrimary Road
Secondary Road
Minor Road
R6
R5p< R4
R4 R3D' RI
URBANExpressway
ArterialsCollectorLocal Street
U6
U5
U5
U4U4U4
U3
U3 U2 U1
A Guide on Geometric Design of Roads
FIGURE 2-1 : FLOW CHART FOR SELECTION OF DESIGN STANDARDS
Project Identification
Designate Road GroupAccording to Nature ofArea Road Traverses
Select Road
Category based on
function e.g. Arterial
Select Road
Category based on
function e.g. Primary Road
Estimate ADT at
end of desisn life
Determine design
standard e.g. R6,U3 etc.
Route Location
Survey and Design
Table 2.3
r0
3.1
A Guide on Geometric Design of Roads
CHAPTER 3 - DESIGN CONTROL AND CRITERIA
This chapter discusses those characteristics that govern the various elements in the design ofhighway. These include the topography and land use through which the route is to be located,
the traffic that will use the route, design vehicles characteristics, design speed of the route and
the capacity to be provided along the route.
TOPOGRAPHY AND LAND USE
The location of a road and its design are considerably influenced by the topography,physical features, and land use of the area traversed. Geometric design elementssuch as alignment, gradients, sight distance and cross-section are directly affectedby the topography, and must be selected so that the road designed will reasonably fitinto those natural and man-made features and economise on construction and
maintenance.
The topography through which the road passes can generally be divided into threegroups, namely, FLAT, ROLLING and MOUNTAINOUS; where
FLAT terrain means: The topographical condition where highway sightdistances, as governed by both horizontal andvertical restrictions, are generally long or could be
made to be so without construction difficulty orexpertise. The natural ground cross slopes (i.e.,
perpendicular to natural ground contours) in a flatterrain are generally below 37o.
The topographical condition where the naturalslopes consistently rise above and fall below the
road or street grade and where occasional steep
slopes offer some restrictions to normal horizontaland vertical roadway alignment. The naturalground cross slopes in a rolling terrain are
generally between 37o-25%o.
The topogaphical condition where longitudinal and
transverse changes in the elevation of the ground
with respect to the road or street are abrupt and
where benching and side hill excavation are
frequently required to obtain acceptable horizontaland vertical alignment. The natural ground cross-
slopes in a mountainous terrain are generallyabove 25Vo.
ROLLING terrain means:
MOUNTAINOUS terTaln means:
11ll
A Guide on Geometric Design of Roads
The topography of a route may affect the operation characteristic of the roadway and thismay affect the type of road to be built. For example, steep gradient will reduce thecapacity of a2-Iane road (single carriageway) and lower the operation speed of this link.However, if a dual carriageway or a wider road is used, the effects will be much less andhence the operation speed and characteristics of the road will be enhanced. Consequently,the nature of the terrain sometimes determines the type of road to be built.
Topographical conditions may also affect the cross-sectional alrangement of dividedroads. In some cases a facility of a single road formation may be appropriate; in othersit may be more fitting to locate the road within two separate road formation due toeconomic or environmental considerations.
In urban areas, land development for residential, commercial and industrial purposes willrestrict choice of road location, lower running speed, generate more turning movements,and require more frequent intersections than in open rural areas. Geologic and climaticconditions must also be considered for the location and geometric design of a road.
3.2 TRAFFIC
The design of a road should be based on traffic data, which serve to establish the "loads"for geometric design. Traffic data for existing roads or section of road are available fromthe annual "Road Traffic Volume, Malaysia" published by Highway Planning Unit(HPU) of the Ministry of Works, Malaysia.
3.2.1 Average Daily Traffic (ADT)
ADT represents the total traffic for the year divided by 365, or the average traffic volumeper day. ADT is imporlant for purposes such as determining annual usage as justificationfor proposed expenditures, or for design of structural elements of a road. The projectedADT is also used to designate the standard of road as shown in Table 2.3 - Selection ofDesign Standards. However, the direct use of ADT in geometric design is not appropriatebecause it does not indicate the significant fluctuation in the traffic volume occurringduring various months of the year, days of the week or hours of the day. A moreappropriate measurement is the hourly volume which is used to determine the capacityrequirement of the road.
3.2.2 Design Hourly Volume (DHV)
The traffic pattern on any road shows considerable fluctuation in traffic volumes duringthe different hours of the day and in hourly volumes throughout the year. It is difficultto determine which of these hourly traffic volumes should be used for design. It wouldbe wasteful to base the design on the maximum peak hour traffic of the year, yet the useof the average hourly traffic would result in an inadequate design.
TZ
A Guide on Geometric Design of Roads
To determine the hourly traffic best fitted for design, a curve showing the variation inhourly traffic volume during the year is used. The hourly traffic used in design is the
30th. highest hourly volume of the year, abbreviated as 30 HV. The design hourlyvolume, abbreviated as DHV is the 30HV of the future year chosen for the design.
The above criteria is applicable to most rural and urban roads. However, for roads onwhich there is unusual or highly seasonal fluctuation in traffic flow such as holiday resortroads, the 30HV may not be appropriate. It may be desirable, to choose an hourly volumefor design (about 50 percent of the hourly volumes) expected to occur during a very fewmaximum hours (15 to 20) of the design year whether or not it is equal to 30HV.
3.2.3 Design Hourly Volume Ratio (K)
K is the traffic ratio of DHV to the designed ADT. K's value ranges froml%o to 20Vo. Theactual value should be obtained from traffic data or census. As a guide, K=12.0Vo forurban roads and K=15%o for rural roads may be used. Where the design of the road
facility is sensitive to the value of K, the K value should be established from trafficsurvey/study.
The directional distribution of traffic (D) during the design hour should be determinedby field measurements on the facilities. Generally D value of 60Vo can be used in urbanareas and 65Va in rural areas. Traffic distribution by directions is generally consistentfrom year to year and from day to day except on some roads serving holiday resort area.
3.2.4 Traffic Composition
This refers to the percentage of various classes of vehicles in the DHV. Vehicle ofdifferent sizes and weights have different operating characteristics which must be
considered in geometric design. Commercial vehicles generally are heavier, slower and
occupy more roadway space, thus imposing greater traffic effect on the road than
passenger vehicles"
Vehicles of various sizes and weights are classified into the following six (6) groupsunder the annual National Traffic Census conducted by HPU:a. motor cyclesb. cars and taxisc. light vans and utility vehiclesd. medium lorries (2 axle)e. heavy lorries (3 or more axles)f. buses
On existing road network, the traffic composition at peak hour can be established from"Road Traffic Volume Malaysia" published by the Highway Planning Unit, Ministry ofWorks.
3.2.5 Projection of Traffic
New roads or improvements of existing roads should be based on future traffic expected
A Guide on Geometric Design of Roads
to use the facilities. Desirably, a road should be designed to accommodate the traffic thatmight occur within the Iife of the facility under reasonable maintenance. l'he projectionof traffic for use in the design should be based on a period of l5 years after completionof the road
3.3
The geometric design of roads is affected by the physical characteristics of vehicles andthe proportions of various sizes vehicles using the roads. A design vehicle is a selectedmotor vehicle, the weight, dimensions and operating characteristics of which are used toestablish highway design controls to accommodate vehicles of a designated type. Forpurposes of geometric design, the design vehicle should be one with dimensions andminimum turning radius larger than thoie of almost all vehicles in its class. Since roadsare designed for future traffic, the sizes of vehicles used in the design should bedetermined by analysing the trends in vehicle dimensions and characteristics.
3.3.1 Design Vehicles
The design vehicles to be used for geometric design follows that used by AASHTo asin chapter II of AASHTo "Design vehicle" - -A policy of Geometric Design ofHighways and streets (1994). Figure 3-1,3-2,and 3-3 show the dimensions and turningcharacteristics for the p, SU and wB-r5 design vehicres.
3.3.2 Summary of Dimension of Design Vehicles
Table 3-1 below summarises the design vehicle dimensions and characteristics.
TABLE 3-I-DIMENSION OF DESIGN VBHICLES
Design Vehicles Dimension in Metres
Turning Radius(Metres)
InnerOuter
TypeEquivalent
Type inAASHTO
Wheelbase
Overhang
OverallLength
Overallwidrh Height
Front Rear
Passenger Car P 3.4 0.90 I.5
1.8
5.8 2.1 1.3 4.2 | --)
Rigid Truck SU 6. l0 1.2 9.1 2.60 4.10 8.5 12.8
Semi-Trailer wB-15 9. 10 0.9 0.60 16.1 2.60 4"10 5.8 13.7
Note: Marimum alIowable overall f"n(i) Rigid vehicle - 12.2 m &0 ft\(ii) Articutated vehicte _ 16.0 m (52.5 ft)(iii) Semi-Trailer - I 2.5 m &tft\(iv) Traiter - 9.0 m (29.5 fi)(v) Truck Trailer - 18.0 m (59 fr)
B. Maximum allowable overail width under currenf Maraysian Legisration is 2.5 m.
14
FIGURE 3.1 : P DESIGN VEHICLE
,/4 ,1t. | |/ry'
/ -///
//// / -/:A/ / .t(g$-L'//.'Y'//.-r.
/ / -'/ /'//..t.'
//,.t-'//.t/-
/ / ,t' -.'/ /z' -'/ .rf/-/'/ //' '\\ W-2
,' / ,/' ->.\-,i,r/'-Jix
\-r1?'-- l9x.--.--\
---.
r!\. --. \\rl tt. ----._ ---=-
it., tr. --.tS-
\-
''Q'.
r5
FIGURE 3.2 : SU DESIGN VEHICLE
//At f. l /
/' 60/,/ ,7v/.2/ '/ '-'4J,' // .t' {V.'
/' / -.rr' ,'//.t-'
/ ,/ .t /./'1,/.t// / /' '//' / -t ,.'/ -E-
,/,i17,?.ol',//_._._\ __
I,II
,YK,, t). \\r--.-I '\ -.-
- A*----.-{o.\
t,,\\ t'
f,"i_
369SCALE IN MFTERS
t6
FIGURE 3.3 : WB-ls DESIGN VEHICLE
//rf1 // /.6) /r e//t - / ..'
// i ."r19 "//rt\7/
ti.t.'/////i-"'//irtz'
./ / -z/ ,'.//.r// tz- .a
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------- q;-i t '\ '\ | r ---i,/ \. \. i \. --_:/ \ \ I \!/ \ \ I \l'r\.1\
\\.'",1 l\. 'rllt.\ \\\\\ ^\r (z\\\ \,4\ \\\
\
ffi0s6912SCALE IN MTTERS
l t 1
t7
A Guide on Geometric Design of Roads
3.4 SPEED
Speed is a primary factor in all modes of trahsportation, and is an important factor in thegeometric design of roads. The speed of vehicles on a road depends, in addition tocapabilities of the drivers and their vehicles, upon general conditions such as the physicalcharacteristics of the highway, the weather, the presence of other vehicles and the legalspeed limirations.
The speed is selected to meet the needs of the road to fulfil its function. Thus roads whichare planned to provide long distance travel will be designed with a higher speed whilethose which provides shorter distance travel can be given a lower design speed.
3.4.1 Design Speed
Table 3-2A and 3-28 indicate the selection of design speeds rvith respect to rural andurban standards.
Design speed is defined as: "a speed selected to establish specific minimum geometricdesign elements for a particular section of highway". These design elements includevertical and horizontal alignment, and sight distance. Other features such as widths ofpavement and shoulders, horizontal clearances, etc., are generally indirectly related todesign speed.
The choice of design speed is influenced primarily by factors such as the terrain,economic, environmental factors, type and anticipated volume of traffic, functionalclassification of the highway, and whether the area is rural or urban. A highway in levelor rolling terrain justifies a higher design speed than one in mountainous terrain.
Where a difficult location is obvious to approaching drivers, they are more apt to accepta lower design speed than where there is no apparent reason for it. Where it is necessaryto reduce design speed, many drivers may not perceive the lower speed condition ahead,and it is important that they be warned in advance. The changing conditions should beindicated by such traffic devices as speed zone signs and appropriate warning signsdepicting the conditions ahead.
A highway carrying a large volume of traffic may justify a higher design speed than aless important faciliti' in a similar topography, pafticularly where the savings in vehicleoperation and other cos.ts are sufficient to offset the increased cost of right of way andconstruction. On the contrary, a lower design speed should not be assumed for asecondary road where the topography is such that drivers are likely to travel at highspeeds.
Taking into account the above considerations, as high a design speed as feasible shouldbe used. It is preferable that the design speed for any section of highway be a constantvalue. However, during the detailed design phase of a project, special situations may
18
A Guide on Geometric Design of Roads
DesignStandard
Catesorv of Road
Design Speed (kph)
Terrain
Flat Rolling Mountainous
R6 Expressway 110 r00 80
R5HighwayPrimary Roads
100
r009090
'7n
70
R4Primary RoadsSecondary Roads
9090
80
80
6060
R3 Secondary Roads 80 60 50
R2Minor Roads
60 50 40
RI 50 50 30
TABLE 3-2A: DESIGN SPEED FOR RURAL ROADS
TABLE 3-2B-- : DESIGN SPEED FOR URBAN ROADS
DesignStandard
Category of Road
Design Speed (kph)
Area Type
I il m
U6 Expressway 100 90 80
U5ArterialsCollectors
9080
7010
6060
U4ArterialsCollectorsLocal Streets
801070
606060
505050
U3CollectorsLocal Streets
6060
5050
4040
U2UI
Local Streets5040
4A
30
3030
Note : Type I - relatively free in road location with very little problems as regards land
acquisition, affected buildings or other socially sensitive areas.
Type II - Intermediate between I and IILType III - Very restrictive in road location with probiems as regards land acquisition,
affected buildinss and other sensitive areas.
19
A Guide on Geometric Design of Roads
arise in which engineering, economic, environmental, or other considerations make itimpractical to provide the minimum elements established by the design speed. The mostlikely examples are partial or brief horizontal sight distance restrictions, such as thoseimposed by bridge rails, bridge columns, r'etaining walls, sound walls, cut slopes, andmedian barriers.
The cost to correct such restrictions may not be justified. Technically, this will result ina reduction in the effective design speed at the location in question. Such technicalreductions in design speed shall be discussed with and upprou"d by Road Authorities.
3.4.2 Operating Speed
Operating speed is the highest overall speed at which a driver can travel on a given roadunder favourable weather conditions and prevailing traffic conditions without at any timeexceeding the design speed on a section by section basis.
3,4.3 Design Sections
In the design of relatively long road, it is desirable, although not always feasible to allowfor a constant design speed. Changes in terrain and other physical controls may dictatea change in the design speed on certain sections. If a different design speed is introduced,the change should not be abrupt but should be effected over a ruffi.i"nt distance topermit drivers to change speed gradually before reaching the section of highway withlower design speed. There should be a transition section of at least I km per every t0 kphdrop in design speed to permit drivers to change speed gradually before reaching thesection of the road with a different design speed. The transition sections are sections withintermediate design speeds. Where the magnitude of the change in the design speed arelarge, more than one transition section will be required.
The intermediate design speed shall follow the sequence as established in Table 3-2Aand 3-28, i.e. If the design speed of section A is I l0 kph and that of section B is 90kph,then the design speed of the transition section between them shall be 100knh.
3.5 CAPACITY
The term highway capacity pertains to the ability of a roadway to accommodate rraffic.It is defined as the maximum number of vehicles that can pass over a given section of alane or a roadway during a given time period under prevailing roadway and trafficconditions. Capacity considered here is applicable only to uninterrupted flow or openroadway conditions. Capacity for intemrpted flow as at intersections will be dealt withseparately.
Capacity is often stated in terms of passenger car units (p.c.u). Table 3-3 gives theconversion factors to be used in converting the various classes of vehicles to passengercar units.
20
A Gui.de on Geometric Design of Roads
TABLE 3.3.CONVERSION FACTORS TO P.C.U.
Equivalcnt Value in P.C.U
Type of Vehicles Rural Standard Urban Standard Roundabout Design Traffic SignalDesign
Passenger Car 1.00 r.00 1.00 1.00
Motor cycles r.00 0.7.5 0.75 0.33
Light Vans 2.00 2.00 2.00 2.00
Medium Lorries 2.50 2.50 2.80 1.75
Heavy Lorries 3.00 3.00 2.80 2.25
Buses 3.00 3.00 2.80 2.25
Source : 1. Road Research Laboratory "Research on Road Traffic", HMSO, London, 1965, pp201
2. Highway Planning Unit, Ministry of Works, Malaysia
3.5.1 Capacity under ideal condition
Under ideal condition, the possible capacity for uninterrupted flow are as follows:
a. For 2-lane two way (total) = 2,800 pcu/hrb. For multi lane (per lane) = 2,000 pcu/hr
Ideal conditions consist of the following for two -lane roads:
a. Design speed greater than or equal to 100 kphb. Lane widths greater than or equal to 3.65m (i2')c. Clear shoulders wider than or equal to 1.83m (6')
d. No "No passing zones" on the highwaye. All passenger cars in the traffic streamsf. A 50/50 directional split of trafficg. No impedient to through traffic due to traffic control or turning vehiclesh. Level terrain
Where one or more of these conditions are not met, the actual capacity will be reduced.
The effects of each are discussed in the Highway Capacity Manual which gives
adjustment factors for the determination of the design capacity.
3.5.2 Design Volume
Design volume is the volume of traffic estimated to use the road during the design year,
which is taken as l5 years after the completion of the road. The derivation of the design
hour volume (DHV) is as discussed in Section 3.2.2.
2l
A Cui.de on Geometric Design of Roads
Design of new highways or improvements of existing highways usually should not be
based on current traffic volume alone. Considerations should be given to the future traffic
expected to use the facilities. A highway should be designed to accommodate the trafficthat might occur within the life of the facility under reasonable maintenance.
It is difficult to define the life of a highway because major segments may have differentlengths of physical life. Furthermore, the design of highway based on life expectancy is
greatly influenced by economics.
3.5.3 Service Volume
The service volume is the maximum volume of traffic that a designed road would be able
to serve without the degree of congestion falling below a preselected level as defined by
the level of service which is the operating conditions (freedom to manoeuvre) at the time
the traffic is at the design hour volume. Table 3-4 gives an indication of the levels ofservice used while Figure 3-4 gives a schematic concept of the relationship of level ofservice to operating speed and volume/capacity ratio.
TABLE 3-4 : LEVELS OF SERVICE
Level ofService
Remarks
A Free flow with low volumes, densities and high speeds. Drivers can
maintain their desired speeds with little or no delay.
B Stable flow. Operating speeds beginning to be restricted somewhat
by traffic conditions. Some slight delay.
C Stable flow. Speeds and manoeuvrability are more closely
controlled by higher volumes. Acceptable delay.
D Approaching unstable flow. Tolerable operating speeds which are
considerably affected by operating conditions. Tolerable delay.
E Unstable Flow. Yet lower operating speeds and perhaps stoppages
of momentary duration. Volumes are at or near capacity congestion
and intolerable delay.
F Forced Flow. Speeds and volume can drop to zero. Stoppages can
occur for long periods. Queues of vehicles backing up from a
restriction downstream.
22
FIGURE 3.4 : RELATIONSHIP OF LEI'EL OF SERVICE TO
OPERATING SPEED AIYD VOLUME iCAPACITY RATIO.
!t)!.)o.aoll
'.4oL0,oo
Volume / Copocity Rotio
z5
A Guide on Geometric Design of Roads
3.5.4 Design Level of Service and Volume/Capacity Ratio
The selection of the design level of serviqe is as given in Table 3-5A and 3-58. Thislevel of service concept can be simplified into volume to capacity ration (V/C) for thepurpose of design to determine the service volume as indicated in Section 3.5.3. Table3-5A and Table 3-58 also gives the ranges of V/C ratio consistent with the levels ofservice for the purpose of design.
TABLE 3-5A : DESIGN LEVEL OF SERVICE AND VOLUME/CAPACITY RATIO (RURAL)
Road Category Desisn Level of Service Volume /CapacityRatio
Expressway C 0.70-0.80
Highway C 0.70-0.80
Primary Road D 0.80-0.90
Secondary Road D 0.80-0.90
Minor Road E 0.90- 1.00
TABLE 3-58 : DESIGN LEVEL OF SERVICE AND VOLUME/CAFACITY RATIO (URBAN)
Road Category Desien Level of Service Volume /CapacityRatio
Expressway D 0.80-0.90
Arterial D 0.80-0.90
Collector D 0.80-0.90
Local Street E 0.90-1.00
.A
A Guide on Geometric Design of Roads
4.1
CHAPTER 4 - ELEMENTS OF DESIGN
SIGHT DISTANCE
4.1.1 General
Sight distance is the length of road ahead visible to drivers. Ability of a driverto see ahead is of the utmost importance to the safe and efficient operation of aroad. The designer must provide sight distance of sufficient length in which driverscan control the speed of the vehicles so as to avoid striking an unexpected obstacleon the travelled way. Also, at frequent intervals and for a substantial portionof the length, Z-lane undivided roads should have sufficient sight distance to enabledrivers to overtake vehicles without hazard. Decision sight distance, is the distancerequired for a driver to detect an unexpected hazard and to select an appropriatespeed or path thus by completing a manoeuvre which avoid this hazard. Sightdistance thus includes stopping sight distance, passing sight distance and decisionsight distance.
4.1.2 Stopping Sight Distance
The stopping sight distance is the length required to enable a vehicle travelling at or nearthe design speed to stop before reaching an object in its path. Minimum stopping sightdistance is the sum of two distance.
(i) the distance traversed by a vehicle from the instant the driver sights an object forwhich a stop is necessary, to the instant the brakes are applied; and
(ii) the distance required to stop the vehicle after the brake application begins.
(a) Perception and Brake Reaction Time
For safety, a reaction time that is sufficient for most operators, rather thanfor the average operator, is used in the determination of minimum sightdistance. A perception time value of 1.5 seconds and a brake reactiontime of a full second are assumed. This make the total perception and
brake reaction time to be 2.5 seconds.
(b) Braking Distance
The approxirnate braking distance of a vehicle on a level roadway isdetermined by the use of standard formula:
V2
254fd=
25
A Guide on Geometric Design of Roads
where d = brake distance, (m)
V= initial speed, (kph)f = coefficieirt of friction between tires and roadway
In this formula the f factor varies considerably due to many physical
elements such as condition of tires, condition of pavement surface, the
presence of moisture etc. The value of f for design are inclusive of all
these factors.
(c) Design Values
The sum of the distance traversed during perception and brake reaction
time and the distance required to stop the vehicle is the minimum
stopping sight distance. The values to be used for minimum stopping
sight distances are as shown in Table 4-1.
TABLE 4.1 : MINIMUM STOPPING DISTANCE
Effect of Grades on Stopping Sight Distance
When a road is on a grade the standard formula for braking distance is
V2D_ zsatl,e)
in which g is the percentage of grade divided by 100.
The effect of grade on stopping sight distance is as shown in Table 4-2.
(d)
Min Stopping Sight Dist. (m)Design Speed (kph)
110
r009080t060504030
2s0205r70t40110
85
65A<AJ
30
1A
A Guide on Geometric Design of Roads
(e)
(f)
TABLE 4.2 : EFFECTS OF GRADES IN STOPPING SIGHTDISTANCE _ WET CONDITIONS
Every effort should be made to provide stopping distances greater than
the minimum design values shown in Table 4-1 especially at locationswhere there are restrictions to sight in the horizontal plane.
Decision Sight Distance
Stopping sight distances are usually sufficient to allow reasonablycompetent and alert drivers to come to a hurried stop under ordinarycircumstances. However, these distances are often inadequate when
drivers must make complex or instantaneous decisions, when informationis difficult to perceive, or when unexpected or unusual manoeuvres are
required. Limiting sight distances to those provided for stopping may
also prelude drivers from performing evasive manoeuvres, which are
often less hazardous and otherwise preferable to stopping. Even with an
appropriate complement of standard traffic control devices, stopping sightdistances may not provide sufficient visibility distances for drivers tocorroborate advance warning and to perform the necessary manoeuvres.It is evident that there are many locations where it would be prudent toprovide longer sight distances. In these circumstances, decision sightdistance provides the greater length that drivers need.
Decision sight distance is the distance required for a driver to detect an
uriexpected or otherwise difficult-perceive information source or hazard
in a roadway environment that may be visually cluttered, recognise the
hazard or its potential threat, select an appropriate speed and path, and
initiate and complete the required safety manoeuvre safely and efficiently.Because decision sight distance gives drivers additional margin for errorand affords thern sufficient length to manoeuvre their vehicles at the same
or reduced speed rather than to just stop. its values are substantiallygrcater than stupping sight distance.
DesignSpeed (kph)
Stopping SightDistance (m) for
Downgrades
AssumedSpeed forCondition
(koh)
Stopping SightDistance (m) for
Upgrades37a 6Vo 97o 3Vo 6Vo 9Vo
3040506070
8090100
110
30466689118
149181
221
267
3l4869
94126161
t95241293
)L50'73
101
136
t76714261327
30404755
637071
85
91
29+J5671
90t071aAlLa
148
r68
2942546986102119140159
)9,
41
5Z6183
981taI lJ
t34l5i
21
A Guide on Geometric Design of Roads
Drivers need decision sight distance whenever there is a likelihood forerror in either information reception, decision-making, or control actions.The following are examples of critical locations where these kinds oferrors are likely to occur, and where it is desirable to provide decisionsight distance: interchange and intersection locations where unusual orunexpected manoeuvres are required; and changes in cross section such
as toll plaza and lane drops.
The decision sight distances in Table 4-3 provide values to be used bydesigners for appropriate sight distances at critical locations and serve as
criteria in evaluating the suitability of the sight lengths at these locations.Because of the additional safety and manoeuvrability these lengths yield,it is recommended that decision sight distances be provided at criticalIocations or that these points be relocated to locations where decisionsight distances are available. If it is not possible to provide thesedistances because of horizontal or veftical curvature or if relocation is notpossible, special attention should be given to the use of suitable trafficcontrol devices for providing advance warning of the conditions that arelikely to be encountered.
TABLE 4-3 : DECISION SIGHT DISTANCE
DesignSpeed (kph)
Decision Sight Distance for AvoidanceManoeuvre (meters)
B C D
5060708090r00ll0
15
95125155
i85225
26s
160
20s250300360415,,t<<
145
115200230215
3r5335
200235215315
360405
42s
Decision sight distance values that will be applicable to most situationshave been developed from empirical data. The decision sight distancesvary depending on whether the location is on a rural or urban road, andon the type of manoeuvre required to avoid hazards and negotiate thelocation properly. Table 4-3 shows decision sight distance values forvarious situations rounded for design. As can be seen in Table 4-3,generally shorter distances are required for rural roads and when a stopis the manoeuvre involved.
28
A Gui.de on Geometric Design of Roads
The following avoidance manoeuvres are covered in Table 4-3:. Avoidance Manoeuvre A : Stop on rural road.. Avoidance Manoeuvre B : Stop on urban road.. Avoidance Manoeuvre C : Speed/path/direction change on rural
road.. Avoidance Manoeuvre D : Speed/path/direction change on urban
road.
In computing and measuring decision sight distances, the 1070mm eyeheight and 150mm object height criteria used for stopping sight distancehave been adopted. Although drivers may have to be able to see the
entire roadway situation, including the road surface, the rationale for the
150mm object height is as applicable for decision as it is for stopping
sight distance.
4.1.3 Passing Sight Distance
Most roads in rural areas are two-lane two way on which vehicles frequently overtakeslower moving vehicles, the passing of which must be accomplished on a lane regularlyused by the opposing traffic. Passing sight distance for use in design should be
determined on the basis of the length needed to safely complete a normal passing
manoeuvre.
The minimum passing sight distance for two-lane highways is determined as the sum offour distances:-
(i) distance traversed during the perception and reaction time and during the initialacceleration to the point of encroachment on the passing lane.
(ii) distance travelled while the passing vehicle occupies the passing lane. ,
(iii) distance between the passing vehicle at the end of its manoeuvre and the oppositevehicles.
(iv) distance traversed by an opposing vehicle for two-thirds of the time the passing
vehicle occupies the passing lane.
(a) Design Values
The total passing sight distance is determined by the sum of the above fourelement" Table 4-4 gives the minimum values to be used for each design speed.
In designing a road, these distances should be exceeded as much as practicableand such sections provided as often many as can be done with reasonable costs
so as to present as many passing opportunities as possible exceeded.
29
A Guide on Geometric Design of Roads
I
I
III
I
i
TABLE 4-4 : MINIMUM PASSING SIGHT DISTANCES(2-LANE - 2 WAY)
Design Speed (kph) Minimum Passing Sight Dist. (m)
110
100
90807060504030
730610610550490410350290230
Effect of Grade on Passing Sight Distance
Specific adjustment for design use is not available. The effect of grade is notconsidered in design as the effect is compensated in upgrade or downgrade.However, it should be realised that greater distances are needed for passing ongrade as compared to level conditions. The designer should recognise thedesirability of increasing the minimum as shown in Table 4.4.
Frequency and length of Passing Sections
Sight distance adequate for passing should be provided frequently on 2-laneroads. Designs with only minimum sight distance will not assure that safepassing can always be made. Even on low volume roads a driver desiring to pass
may, upon reaching the section, find vehicles in the opposing lane and thus beunable to utilise the section or at least not be able to begin to pass at once.
The percentage of length of road or section of road with sight distance Ereaterthan the passing minimum should be computed. The adequacy of sight distanceis determined by analysis of capacity related to this percentage, and this wouldindicate or not alignment and profile adjustments are necessary to accommodatethe design hour volumes. Generally this percentage should be about 60Vo for flatterrain, 40Vo for rolling terrain and 20Vo for mountainous terrain. The avaiiablepassing sight distances should not be concentrated in one section but be evenlydistributed throughout the entire road. It is inrportant to note that in order to caterfor high volume of traffic at high level of service, it requires that frequent andnearly continuous passing sight distances be provided.
(b)
(c)
30
T
A Guide on Geometic Design of Roads
4.1.4
It is not necessary to consider passing sight distance on roads that have
two or more traffic lanes in each direction of travel. Passing manoeuvres
on multilane roads are expected to occur within the limits of each
carriageway allocated for that direction of travel. Thus passing manoeuvres
that require crossing the centerline of four-lane undividual road are reckless
and should be prohibited.
Criteria for Measuring Sight Distance
(a) Height of Driver's Eye
The eyes of the average driver in a passenger vehicle are considered to be
l070mm above the road surface.
(b) Height of Object
A height of object of 150mm is assumed for measuring stopping sight distance
and the height of object for passing sight distance is 1300mm both measured fromthe road surface.
Sight distance should be considered in the preliminary stages of design when both
the horizontal and vertical alignment are still subject to adjustment. The sightdistance should be determined graphically on the alignment plans and recorded
at frequent intervals, for both direction of travel. This will enable the designer
to appraise the overall layout and effect a more balanced design by minoradjustments in the plan or profile.
Horizontal sight distance should be measured in the inside of a curve at the center
of the inside lane. Vertical sight distance should be measured along the
longitudinal profile of the central line, using the height of driver's eye (1070mm)
and the object height (150mm). Figure 4-l and 4-2 provrde examples ofmeasuring sight distance in both plan and profile respectively. For two lane two-way roads passing sight distance in addition to stopping sight distance and
decision sight distance should be measured and recorded. Sight distances, in this
case, are to be checked from the centerline of the road.
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33
A Guide on Geometric Design of Roads
4.2 HORIZONTAL ALIGNMENT
4.2.1 General
In the design of horizontal curves, it is necessary to establish the proper relation between thedesign speed and curvature and also their joint relations with superelevation and sidefriction. From research and experience, limiting values have been established for thesuperelevation (e), and the coefficient of friction (fl.
4.2.2 Superelevation Rates
The maximum rates of superelevation usable are controlled by several factors such asclimatic conditions, terrain conditions and frequency of very slow moving vehicles thatwould be subjected to uncertain operation. While it is acknowledged that a range of valuesshould be used, for practical purposes in establishing the design criteria for horizontalalignment, a maximum superelevation rate of 0.10 is used for roads in rural areas and 0.06for roads in urban areas.
4.2.3 Minimum Radius
The minimum radius is a limiting value of curvature for a given speed and is determinedfrom the maximum rate of superelevation and the maximum allowable side friction factor.
The minimum safe radius (Rmin) can be calculated from the standard curve formula.
Rmin = 127 (e+f)
WhereRmin = minimum radius of circular curve (m)
V = Design Speed (kph)
e = maximum superelevation rate
f = maximum allowable side friction factor
v2
34
4 Guile on Geometric Design of Roads
Figure 4-3 illustrates the range of side friction factors (0 that has been derived using
imperial methods for the various types of roads.
ion between t
,ation and si
blished for r,
FIGURE 4-3 : COMPARISON OF SIDE FRICTIONFACTORS ASSUMED FOR DESIGN OF
DIFFERENT TYPES OF ROADS
0.5
Maximum side friclion ./lactor tor new tyres on /wet concrete pavement
]-f 1l
Assum ed for designof turning roadways
concrete p
I
I
I
lyres on slvement
el
r..Assumed for design of
1/ tow speed urban roadsff
f--t ruraland high speedurban highways
A 0.4
actors SUCh r ;g vehicles th H'ange of valu, E u'r
for horizont 5areas and 0.( E
! O)
q)
(a
0.1
is determinekiction facto
formula.
U 30 40 50 60 70 80 90 100 I l0
Speed (kph)
Table 4-5 lists the minimum radius to be used for the designated speed and maximum
superelevation rates.
TABLE 4.5: MINIMUM RADIUS
Design Speed (kph) Minimum Radius (m)
e = 0.06 e = 0.10
110
100
90807060504A
30
s60465335284195
150
100
6035
s00315305230n512585
5030
35
A Guide on Geometric Design of Roads
While the values in Table 4-5 lists the minimum radius that can be used, it should be noted,that the above minimum radii will not provide the required stopping sight distance andpassing site distance within typical cross-sections. Hence, all elforti should be made todesign the horizontal curves with radii larger than the minimum values shown for greatercomfort and safety.
4.2.4 Transition (Spirat) Curves
Vehicles follow a transition path as it enters or leave a circular horizontal curye. To designa road with builrin safety, the alignment should be such that a driver travelling at the designspeed will not only find it possible to confine his vehicle to the occupied lane but will beencouraged to do so. Spiral transition curves are used for this purpose. Generally, theEulers spiral, also known as the clothoid is used. The degree of curve varies from zero atthe tangent end of the spiral to the degree of the circular arc at the circular curve end.
a) Length of Spiral
The length of spirals to be used are normally calculated from the Spiral formula orfrom the empirical superelevation runoff lengths. The following formula is used bysome for calculating the minimum length of a spiral:
, 0.0702v'L= RC
.,'where L = minimum length of spiral;
V - speed (kph);
R = curvo radius; and
C = rate of increase of centripetal accelerating (m/sec3)
The factor C is an imperial value and a range of lto 3 have been used for highways.Highways design does not normally require the level of precision achieved using thisformula. A more practical control for determining the length of a spiral is that *fri.ttequal the length required for superelevation runoff.
Superelevation runoff is the general term denoting the length of highway neededto accomplish the change in cross slope from a section with adverse crownremoved to a fully superelevated sectjon or vice versa. Current practice indicatesthat the appearance aspect of superelevation runoff largely governs the length.The length of superelevation runoff should not exceed a longitudinal slopeas indicated in Table 4-6.
36
A Guide on Geometric Design of Roads
TABLE 4.6: RELATIONSHIP OF DESIGN SPEED TO MAXIMUMRELATIVE PROFILE GRADIENTS
Table 4-74 and 4-78 gives for the various design speeds and degree of curvature,the minimum lengths of spiral, the superelevation rates and the limiting curvaturefor which superelevation is not required.
For pavements with more than 2 lanes and standard lane width of 3.5m, the
superelevation runoff lengths should be as follows.
(i) 3 lane pavements - 1.2 times the length for 2-lane roads.
(ii) 4?aneundivided pavements - 1.5 times the length for 2 lane roads.
(iii) 6 lane undivided pavements * 2.0 times the length for 2 lane roads.
Design Speedv (kph)
Maximum Relative Gradients (and EquivalentMaximum Relative Slopes)
for Profiles between the Edge of Two-LaneTravelled Way and the Centerline (Vo)
30405060708090100
110
0.75 (l:133)0.70 (l:143)0.65 (l:150)0.60 (1:167)0.55 (l:182)0.50 (1:200)0.48 (l:210)0.45 (I:222)0.42 (l:238)
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4.2.5 Methods of Attaining Superelevation
Three specific methods of profile design in attaining superelevation are (a) revolving the
pavement about the centreline profile (b) revolving the pavement about the inside edge
profile, and (c) revolving the pavement about the outside edge profile. Figure 4-4 illustrates
these three methods diagrammatically. The rate of cross slope is proportional to the distance
from start of superelevation runoff.
(a) Figure 4-4A illustrates the method where the pavement section is revolved about the
centreline profile. This general method is the most widely used in design since the
required .hung" in elevation of edge of pavement is rnade with less distortion then
by the other methods. The centreline profile is the base line and one-half of the
required elevation change is made at each edge'
(b) Figure 4-48 illustrates the method where the pavement section is revolved about the
inside edge profile. The inside edge profile is determined as the line parallel to the
calculateJ centreline profile, One half of the required change in cross slope is made
by raising the centre line profile with respect to the inside pavement edge and the
other half by raising the respect to the centreline profile.
(c) Figure 4-4C illustrates the method where the pavement section is revolved about
outside edge profile involves similar geometrics as (b), except that the change is
affected below the upper control profile.
Except when site condition specifically requires, method (a) shall be adopted for undivided
roads.
4.2.6 Superelevation Runoff with Medians
In the design of divided roads, the inclusion of a median in the cross section alters somewhat
the superelevation runoff treatment, The three general cases for superelevation runoff
design are as follows:
(a) The whole of the travelled way, including the median, is superelevated as a plane
section. This case is limited to namow medians and moderate superelevation rates
to avoid substantial differences in elevation of the extreme pavement edges because
of the median tilt. Diagrammatic profile controls is sirnilar to Figure 4'4a^ except' that the two rnedian edges will appear as profiles only slightly removed from
centreline.
(b) The medial is lield in a horizontal plane and the two pavements separately are
rotated around the median edges. This case has most application with medians of
intermediate width. Runoff design uses the median edge profile as the control' One
pavement is rotated about its lower edge and the other about its higher edge. The
diagrammatic profile control is similar to Figure 4-48 and 4'4C with the centreline
grade control the same for the two pavements.
40
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(c) The two pavements are separately treated for run-off with a resultant variable
difference in elevation at the median edges. The differences in elevation of the
extreme pavement edges are minimised by a compensating slope across tlie median'
A fairly wide section is necessary to develop right shoulder areas and desired gentle
slope between. Thus this case is more applicable to wide median of 9m or more'
The pavement rotation can be made by any methods in Figure 4-4.
4.2.7 Pavement Widening on Curves
Pavements on curves are widened to make operating conditions on curve comparable to
those on tangents. Pavement widening on curves is the difference in pavement width
required on a curve and that use in a tangent. Table 4-8 gives the widths of pavement
widening that are required on open road curves.
Widening should be attained gradually on the approaches to the curve to ensure a reasonable
smooth alignment on the edge of pavement and to fit the paths of vehicles entering or
leaving the curve. Preferably, widening should be attained over the superelevation runoff
length with most of all of the widening attained at the start of circular curve poin
4,2.8 Sight Distance on Horizontal Curves
Another element of horizontal alignment is the sight distance across the inside of curves.
Where there are sight obstructions (such as walls, cut slopes, buildings and guardrails), a
design to provide adequate sight distance may require adjustment if the obstruction cannot
be removed. Using the design speed and a selected sight distance as a control, the designer
should check the actual condition and make the necessary adjustment in the manner most
fitting to provide adequate sight distance.
4,2.9 General Controls for Horizontal Alignment
In addition to the specific design elements for horizontal alignment, a number of general
controls ur".".ognised and should be used. These controls are not subject to empirical or
formula derivation but are important for the attainment of safe and smooth-flowing roads.
These are:-
(a) The horizontal alignment should be consistent with the topography and width
preserving developed properties and community values. Winding alignment
Lo*posed of short curves should be avoided. On the other hand very long straights
should also be avoided. The maximum length of straight section of the highway
should be limited, where possible, to 2 minutes travelling time.
ft) The use of the minimum radius for the particular design speed should be avoided
wherever possible. Generally flat curves should be used, retaining the maximum
curvature for the most critical conditions.
(c) Consistent alignment should always be sought. Sharp curves should not be
introduced at thd end of long tangents. Where sharp curves must be introduced, it
should be approached, where possible, by successively sharper curves from the
generally flat curvature.
42
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For small deflection angles, curves shcluld be su{'f iciently long to avoid the
appearance o1'a kink. Curves should be at least l-50m long for a central angle of5o anci the length should be increased 30m foreach ludecrease in the centreline
angle. The n-rinin-rurn length clf horizontal curve on main roads, should be about
3 times the <iesign speed, L.,,ri,, = 3y (iri n-retef). on high speed controlled access
f acilities the desirable minirrum length of horizontal cun,e should be double the
minimum length i.e. [-.,j", = 6V.
(e) Any abrupt reversal in alignment should be avoided. The distance between
reverse curves shoulcl be the sum o1-the superelevation runofl'lengths and the
tangent runout lengths.
(f) The 'broken back' an'angentent of curves should be avoided. Use of spiral
transitions or a compound curve alignment is prelerable for such conditions if itis unavoidable.
(g) other than tangent or flat curvature should be avoided <-in high, long embankment
fills. In the absence of'cut slopes, shrubs, and tree above the roadway, it is
difficult fbr drivers to perceive the extent of curvature and adjust their operation
to the conditions.
(h) Caution should be exercised in the use of compound circular curves. While the
use of compound curves affords flexibility in fitting the highway to the terrain
and other ground controls, the simplicity with which such curves can be used
clften ternpts the <iesigner to use them without restraint. Preferably their use
should be avoided where curves are sharp. Cornpound curves with large
differences in curvature introduce the same problems that arise at a tangent
approach to a circular curve. Where topography or right-of-way restriction make
their use necessary, the radius of the flatter circular arc, R1, should not be more
than 50 percent greater than the radius of the sharper circuiar arc, R2, i.e., R1
should not exceed i,5 Rz. A several-step compound curve on this basis is
suitable as a fornt of transiticln to sharp curves. A spiral transitiott between flatcurves and sharp curves is even inore desirable. On one-rvay roads such as
rarnps, the dil rence in radii of compound curves is not so ituporiant if the
second curve is flatter than the first. However, the use of compound curves on
ramps. which results in a flat curve between two sharper curves, is not good
practice.
(i) The designer should ensure thaL the required sight distance (i.e. decision sight
ciistance) is providecl when approaching interchanges and intersections'
4"3 \/ERTICAL AI,IGNMF'NT
4.3.1 Maximum Grades
The vertical profile of road affect tlie perfomrance of vehicles. The effect of grades on
trucks which have weight power ratio of about 18Okg/kw, is considered. The maximuln
grade controls in tenns of design speed is summarised in T'able 4-9A to 4'9F.
(d)
44
7
A Guide on Geometric
TABLE 4-9A: MAXIMUM GRADES FOR Rt. R2. Ul & U2STANDARD ROADS
TABLE 4-9B : MAXIMUM GRADES FOR R3 AND R4 STANDARD R0ADS
TABLE 4.9C : MAXIMUI\{ GRADES FoR u3 AND u4 STANDARD ROADS
TABLE 4.9D z MAXIMUM GRADES FOR R5 STANDARD ROADS
TABLE 4-98 : MAXIMUM GRADES FOR U5 STANDARD ROADS
Type of Terrain/Area Design speed (kph)
30 40 50 60 70 80Flat & Type IRolling & Type IIMountainous & Tvpe III
8
lll6
7
lll5
7
l0l4
lt0t3
1
9
t2
6
8
l0
Type of Terrain Design speed (kph)
50 60 70 80 90 r00FlatRollingMountainous
1
9
r0
7
8
IO
7
8
l0
6
7
9
6
7
9
5
6
8
Area Type Design speed (kph)
40 50 60 70 80Type IType IIType III
9
t2t3
9
t1
I2
9
10
12
8
9
1l
7
8
10
Type of Terrain Design speed (kph)
60 -7n80 90 r00 110
FlatRollingMountainous
5
6
8
5
61
i+
5
7
45
6
a
A-6
aJ
+
5
Area Type of TerrainDesign speed (kph)
s0 60 t0 80 90 r00Type IType IIType III
6q
II
7
8
10
6
7
9
6
7
9
5
6
8
5
6
8
TABLE 4-9F : MAXIMUM GRADES FOR U6 AI{D R6 STAI'{DARD ROADS
Design speed (kType of Terrain/Area
Flat & Type IRolling & Type IIMountainous & TVpe III
The total upgrade for any section of road should not exceed 3000 m unless the grade is
less than 4 Vo.
4.3.2 Minimum Grades
A desirable minimum grade or 0.5 percent should be used. A grade of 0.35 percent may
be allowable where a high type pavement accurately crowned is used' On straight
stretches traversing across wide areas of low lying swamps use of even flatter grades may
be allowabte witn lrior approval. However, the design of the stonn water drainage outlet
should be considered carefully at these location to ensure that flooding of the travelled
lanes is avoided.
Where less than 0.5Volongrtudinal gradient is necessary, the superelevation runofflength
used should be the minimum valul permitted. This will minimise flat area along the
carriageway where the cross falls are less than \Vo. The use of open textule wearing
course at this locations should be considered; this will heip in preventing hydropianing'
4.3.3 Critical Grade Length
The term "critical grade length" indicates the maximum length of a designated upgrade
upon which a loaded truck can operate without an unreasonable reduction in speed' To
establish the design values for ciitical grade lengths, for which gradeability of trucks is
the determining factor, the following assumptions are made:-
(i) The weighr-power rario of a loaded truck is about 180 kglkw.
(ii) The average running speed as related to design speed is used to approximate the
speed of vehicles beginning an uphill climb'
(iii) The common basis for determining the critical grade length is a reduction in
speed of trucks below the averageiunning speed. Studies show that accident
involvement rate increases siglificantly when truck speed reduction exceeds
l5kph. For example accideniinvolvement rate is 2.4 times greater for 25kph
reduction than for a l5kph reduction.
The length of any given grade that will cause the speed of a representative truck
(lS0kglkw) enterin! the grade at 90k1r/h to reduce by various amounts below the
average running rp""O in shown graphically in Figure 4-5. The 25kph speed reduction
curve should be used as the general design guide for determining the critical iength of
srade.
AA
50
300 400 500 600
LEIIGTII OF GRADE (METERS)
FIGT]RE 4-5 : CRITICAL LENGTIIS OF GRADE FOR DESIGNFOR TYFICAL ffiAVY TRUCK OF ltOkslkW'
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+t
A Cuide on Geometric Design of Roads
Where a particular section is made up of a combination of upgrade, the length of critical
grade, measured from tangent point to tangent point, should take into consideration, the
entire section of the combination.
Where the length of critical grade is exceededand especially where grade exceed5Vo,
consideration should be given to providing an added uphill lane, that is, the climbing
lane, for slow moving vehicles, particulariy where the volume is at or near capacity and
the truck volume is hish.
4.3.4 Climbing Lanes for Two Laned Roads
The following conditions and criteria, which reflect economic consideration, should be
satisfied to justify a clirnbing lane:
(i) Upgrade peak traffic flow rate in excess of 2AA vehicies per hour
(ii) Upgrade truck peak flow rate in excess of 20 vehicles per hour
(iii) One of the following conditions exist:
. A 25kph or greater speed reduction is expected for typical heavy truck.
. Level of service E or F exist on the grade
. A reduction of two or more levels of services is experienced approaching the
start of the grade.
Where climbing lanes are provided, the following requirement are to be followed:-
(a) the climbing lanes should begin near the foot of the grade and should be
proceeded by a tapered section of at least 50m iong'
(b) the width of the clirnbing lane should be the same as lhe main carriageway lane
width and in any case should not be less than 3.25m.
(c) the section of the road must be separated by a New Jersey Type Concrete Median
with adequate signing and markings and the opposing lane widened also to 2
lanes to make it a four lane divided carriageway.
(d) the climbing lane should end at least 60m beyond the crest and in particular
should be at a point where the sight distance is sufficient to permit passing with
safety. In addition, a coffesponding taper length as in (a) of l00m should be
provided.
(c) the shoulder on the outer edge of the climbing lane should be as wide as the
shoulder on the normal two laned section. Where conditions dictate otherwise,
a useable shoulder width of 1.25m is acceptable.
48
7
A Guide on Geontetic Design of Roads
4.3.5 Passing Lane Sections on Two-Lane Roads
Wliere a sufficient number and iength of safe passing sectiolt canuot be obtaincd in the
design of horizontal and vertical alignment alone, an occasional section of four lanes
may be introduced to provide more sections and length safe for passing. Such section
are particularly advantageous in rolling terrain, especially where the alignment is
winding or where the vertical profile includes critical lengths of grade. Four lane
sections should be sufficiently long to permit its effective usage.
The sections of four lanes introduced need not be divided. The use of a median, however
is advantageous and should be considered on roads carrying 500 vehicles per hour orlnore.
The transitions between the two lane and four lane pavements should be located where
of change in width is in full view of the driver. Sections of four-lane road, particularly
divided sections should not be longer than 3 km so as to ensure that the driver does not
lose his awareness that the road is basicaliy a two lane facility.
Where four lane sections are not practical, passing one direction lane sections shall be
introduced at regular intervals. The passing lane must be at least 125m long and of fulllane width and preceded by a taper of 50m at the beginning and 100m at the end.
4.3.6 Climbing Lanes on Multilane Roads
Multilane roads more frequently have sufficient capacity to handle their traffic load,
including the normal percentage or slow-moving vehicles without becoming congested.
However where the volume is at or near capacity ( 1700 vehicle per hour per lane) and
the truck volume is high (2107o of total flow) so as to interfere with the normal flow oftraffic then addition of climbine lanes should be considered.
4.3.7 Vertical Curves
Vertical curves are used to effect a gradual change between tangent grades. They should
be simple in application and should result in a design that is safe, comfortable inoperation, pleasing in appearance and adequate for drainage. For simplicity, the
parabolic curve with an equivalent vertical axis centered on the vertical point ofintersection is used.
The rate of change of grade to successive points on the curve is a constant amount for
equal increments of horizontal distance, and equals the algebraic difference between the
intersecting tangent grades divided by the length of curve or A,/I- in percent per meter.
The reciprocal L/A is the horizontal distance in metre required to effect a 1 percent
change in gradient and is a measure of curvature. This quantity (L/A), termed k, is used
in determining the horizontal distance from the beginning of the vertical curve to the
apex or low point of the curve. The k value is also useful in determining the minimumlengths of vertical curves for the various design speeds.
The lengths of vertical curves used should be as long as possible and above the minimumvalues for the design speeds where economically feasibie.
49
A,9uide on Geometic De
(al Crest Vertical Curves
Minimum lengths of crest vertical curves are determined by the sight distancerequirements. The stopping sight distance is the ma.ior controi for the safeoperation at the design speed chosen. Passing sight disiances are not used as irprovides for an uneconomical design. An exception may be at decision areassuch as sight distance to ramp exit gores where iong.r rengths are necessary.
The basis formula for length of a parabolic vertical curve in terms of algebraicdifference in grade and sight distince (using an eye height of l.Orn and objectheight of 0.15m) are as follows:
Where S is less than L, L= AS2404
Where S is greater than L, L= 25 _ 404--'-
Where L = length of vertical curve(m )
S = sight distance (m)
A = algebraic difference in grades (percent)
Table 4-t0A indicates the required K values that areto be used in design for thevarious design speeds.
TABLE 4-r0A: CREST VERTTCAL CURVE (K VALUES)
Sag Vertical Curves
At least four different criteria for establishing lengths of sag verticai curves arerecognised. These are (r) headlight sight distan "",\zlriaer c'omforr, (3) drainagecontrol and (4') a rule of thumb for general use and this criterion is used toestablish the design vaiues for a range of lengths of sag vertical curves. It isagain convenient to express the design control in terms of the K value.
Table 4-108 indicates the minirnum K varues that are fo be used.
Longer curves are desired wherever feasible and should be used, but where Kvalues in excess of 55 are used, special attention to drainage must be exercised.shorter sag verticar curves may bliustified for economic ,Juronr, in cases wherean existing element such as a structure which is not ready for replacementcontrols the vertical profile.
(b)
MinimumK value
50
A Guide on Geometic Design of Raads
Drainage of curbed pavements are especially important on sag vertical curves
where a gradeline of not less than 0.3 percent within 15m of the level point must
be maintained.
TABLE 4-108 : SAG VERTICAL CURVE (K VALUES)
DesignSpeed(km/h)
110 100 90 80 10 60 50 40 30
MinimumK value
62 51 40 a^3L 25 l8 T2 8 4
4.3.8 General Controls for Vertical Alignment
In addition to the specific controls, there are several general controls that should be
considered.
(a) A smooth gradeline with gradual changes should be strived for in preference to
a line with numerous breaks and short lengths of grade. While the maximum
grade and the critical length are controls, the manner in which they are applied
and fitted to the terrain on a continuous line determines the suitability and
appearance of the finished product.
(b) The 'roller coaster' or the 'hidden-dip' type of profile should be avoided. They
are avoided by use of horizontal curves or by more gradual grades.
(c) A broken back gradeline should be avoided, particularly in sags where the full
view of both vertical curves is not pleasing. This effect is very noticeable on
divided roadways with open median sections.
(d) On long grades, it is preferable to place the steepest grades at the bottom and
lighten the grades near the top of the ascent or to break the sustained grade by
short intervals of higher grade instead of a uniformed sustained grade that might
be only slightly below the allowable minimum-
(e) Where intersections at grade occur on sections with moderate to steep grades, itis desirable to reduce the gradient through the intersection.
5l
4"4
Horizontal and vertical alignment should not be designed independent.ly. Theycomplement each other. Excellence in their design and in the desrgn of theircombination increase ufility and safety, encourage uniforrn speecl, and improveappearance, armost arways without acr<iitionar cost.
Proper combination of horizontal alignment. and profire is obtained by engineenng studyand consideration of the following general controls.
(a) Curvature and grades should be in proper balance. Tangent alignment or flatcurvature at the expense ofsteep or long grades, and excessive curvature with flatgrades, are both poor design. A logicaio-esign is a compromise between the two,which offers the most in safety, capacity, ea-se and uniiormity of operation, andpleasing appearance within the pracrical limits of terrain and area traversed.
(b) vertical curvature superimposed upon ho.izontal curvature, or vice versa,generally results in a more pleasing iacility but it should be analysed for effectupon traffic' Successive changes in profile not in combination with horizontalcurvature may result in a series of humps visible to the driver for some distance,ahazardous condition.
(c) Sharp horizontal curvature should not be introduced at or near the top of apronounced crest vertical curve. Their condition is hazardous in that the drivercannot perceive the horizontal change in alignment, especially at night when theheadlight beams go straight ahead into ,pu.!. Thehaiardor tnis ariangement isavoided if the horizontal curve is made longer than the vertical curve. Also,suitable design can be by using design valuesiboue the minimum for the designspeed.
(d) Somewhat allied to the above, sharp hori zontal curvature should not beintroduced at or near the low point of u p.onoun.ed sag verlical curve. Becausethe road ahead is foreshortened any but flat horizontal curvature assumes anundesirable distorted appearance, fufther, vehicular speeds, particularly of trucks,often are high at the bottom of grades and erratic operation may result, especiallyat night.
(e) on 2-lane roads the need for safe passing sections at frequent intervals and foran appreciable percentage ofthe length olthe road often supersedes the generaldesirability for combination of horizontal and vertical alignment. In these cases' it is necessary to work toward long tangent sections to secure sufficient passi'gsight distance in design.
(0 Horizontal curvature and profile should be made as flat as feasible at intersectionswhere sight distance along both roads is important and vehicles may have to slowdown or stop.
52
(e) On divided roads, variation in the width of median and the use of separate
profiles and horizontal alignments should be considered to derive design and
operational advantages of one-way roadways. Where traffic justifies provision
of 4 lanes, a superior design without additional cost generally results from the
concept and logical design basis of one-way roadways.
In residential areas the alignment should be designed to minimise nuisance
factors to the neighbourhood. Generally, a depressed road makes it less visible
and less noisy to adjacent residents. Minor horizontal alignment adjustments can
sometime be made to increase the buffer zone between the road and the
residential areas.
Examples of poor and good practices are illustrated in Figure 4-6 z A to 4-6 : N
below:
(h)
(i)
TANGENT AIIGNMENT
PREFERRED--\
(A) PRONLE WITII TA}IGENT ALIGNMENT
NorE :- AVoID DESIGNING LITTLE LocAt DIps IN AN orrIERwIsE LoNc.UNIFORMGRADE,
PREFERRED
(B) PROFILE WITH CUR.VED ALIGNMENT
NOTE :- sHoRT HUMps IN TIIE GRADE SHOULD BE AVOIDED.
F'IGURE4-6:A&B
LrNE oF srcHrnslTsgEN BoTToMLAND
PREFERRED
PROFILE
(C) DISTANT VIEW SHOWING BUMPS IN PROFILE GRADE LINE
NOTE :- A DISTANT SIDE VIEW OF A LONG GRADE TANGENT WILLREVEAIEVERYBUMP ONIT.
(D) SHORTTAI\IGENT ON A CREST BETWEEN TWOHORIZONTAL CUR\rES
NoTE :- THIS COI.ItsINATION IS DEFICIENT FOR TWO REASONS'
THE TANGENT BEfi[ryEN T}iE CUR\ES IS TOO SHORT, AND TIIE
REVERSE OCCI.'RS ON A CREST.
FIGURE4-6zC&D
ALIGNMENT
55
AlIGMvTENT
PROFILE
(E) SEARP ANGLE APPEARANCE
NOTE:- TT{IS COMBINAfiON PRESENTS A poORA?PEARANCE. T}tEHORZONTAL CLJRVE LOOKS LIKE A SHAR-P ANGLE,
(F") DTSJOTNTED EFFECTNorE :- A DISTOINTED EFFECT occ{tRs WHEN Tr{E BEGINNI}JG oF A HoRIzoNTAL
CTIRVE IS HIDDEN FROM TI{E DRTVER BY A INTERVENING CREST WHILETIIE CONTINUATION OF TI{E CTIRVE IS VISIBLE IN THE DISTANCE BEYONDTHE INTERVENING CREST..
FIGURE4-6:E&F
56
V
SLrt\{MIT
PROFILE
(G) COINCIDTNG CURVES IN HORTZONTAL ANDVERTICAL DIMENSIONS
NOTE :- WHEN HORIZONTAL AND VERTICAL CttRVEs coINcIDE A VERY
SATISFACTORY APPEARANCE RESULTS.
Gr) oPPosrNG cuRvES rN HORTZONTAL ANDVERTICAL DIMENSIONS
NOTE :- WHEN HORIZONTAI AND VERTICAT CURVES OPPOSE' A VERY
SATISFACTORY APPEARANCE RESTILTS.
-MIMMUM CTIRVES FOR TFM
/ DESIGNSPEED
ALIGNMENT
\ ./-\/l-\
L DESIRABLE CURVE FOR APPeeRaNce
-]TI) FLAT C{IRVES APPROPRIATE FOR HORIZONT,AL WITTI. ,
SMALL CENTRAL A}IGLES REGARDLESS OF PROFILE
NOTE:- VERY LONG FLAT CLJRVES, EVEN WHERENOT REQUIRED BY TI{E DESICN
SPEED, AIJO HAVE A PLEASING APPEARANCE WHEN TTIE CE}]TRALANGLE IS VERY SMALL.
FIGURE4-6:G,H&I
ATIGNMENT
57
-]_---\ t\_
t-I
HORTZONTAL AIIGNMENT-*{-
tt
- -
L
-
\ Il-
PROFILE
(o COINCIDING CURVES VERTTCES rN HORTZONTALAND VERTICAL DIMENSIONS
NOTE :- THE CLASSIC CASE OF COORDINATION BETWEEN HORIZONTALAND VERTICAI ALIGNMENT IN WHICH TT{E VERTICES OFHORIZONTAL AND VERTICAL CURVES COINCIDE. CREATINGA zuCH EFFECT OF THREE-DIMENSIONAL S.CLIRVES,COMPOSED OF CO}.IVEX AND CONCAVE IIELD(ES,
l-I
I
I 1-I
j
-
_-
-
HORIZONTAI AI,IGNMENT
'\ -/-\-_-I \
PROFILE
(K) COINCIDING VERTICES WITH SINGLE.PHASE SKIPNOTE :- A LENGITIMATE CASE OF COORDINATION : ONE pHASE IS SKIPPED IN
TI{E HORTZONTAL PLANE, BUT VERTICES STILL COINCIDE. TI{E LONGTANGENT INPI.ANE IS SOF-|ENED BY VERTICAI CT,IRVATURE,
{
I
I
/_----l -..-*--/
--i_I
1 HORLZONTATAUGN},IE{I Itlll__r_
.-----l--"\ \-!--'FROFILE
(L) WEAK COORDINATTON OF EORTZONTAL ANDVERTICAL ALIGNMENTS
NOTE :- A CASE WTIH WEAK COORDINATION WHERE TI{E VERTTCAT ATTGNMENT ISSHIFTED HALF A PTIASE WTTH RESPECT TO HORIZONTAI ALIGNMENT SOT}IE VERTICES COINCIDE WTru POINTS OF IhIFLECTION. TIIE SUPERELEVATIONIN T}IIS CASE OCCURS ON GRADE, WHILE CRESTS AND SAGS TTAVE NORMALCROSS SECTIONS IN TIIE FIRST CASE, SIIPERSLEVATION OCCI]RS ON CRESTSSAGS, WHILE GRADES I{A!E NORh,T,AL CROSS SECTIONS.
FIGURE 4-6 z J,K&L
58
:\o
t$rddF(
-/A\Jl-(-
UtZri fin2 EfiH3 iFr3 EfiH
H 3Fii BsE/ fitr.1A Yi.,r\l v)vHiE XbH? Q?zF Y.6tt)Z xr?rd hFH
Z H;=
3 iHa
i i'a"l"F Z{rltHZ ixFlE FT;HD TEFEF HAfi5l+l rvH*l{AtllE6vz
59
z\o
IvF4tv.-)rlr\JF{r-.-{
mFz-!{azrhv-l.]FI
U-,xo\H7\ > FLI -ZI A <l!, ZAE4 ea-
^>'lr xHts !?<F-4 e -;'ar-l x 42. EEOii Q c.<N fr,92l.l - .^& ''l^2-&A :<pv V r(Jtr i:?Hfa' Z 2 r':i r-(,IJ ENXz eE:o 6 i,E.F-( ,,;
zzA,4F(AvA\,Ql-.1Av
^lvriv
zvk7xe<E>a\aXXqfi<E(JmF>
it
>lI1
\,,1v
Fz€Xfi<x>1stfiFd
t*T>
60
A Cuide on Geometric Design o.f Roads
CHAPTER 5 - CROSS SECTION ELEMENTS
5.1 PAVEMENT
5.f .1 Surface Type
The selection of the pavement type is determined by the volume and composition oftraffic, soil characteristics, weather, availability of materials, the initial cost and the
overall annual maintenance and service life cost.
Pavements may be considered as three general types according to the volume of trafficthey carry - high, intermediate and low. High traffic volume pavements have smoothriding qualities and good non-skid properties in all weather conditions. Intermediatetraffic volume pavements vary from high traffic volume pavements only slightly, are less
costly and with less strength than high traffic volurne pavements. Low traffic volumepavements range from surface treated earth roads and stabilized matertals to loose
surface such as earth and gravel.
The important characteristics of surface type in relation to geometric design are the
ability of a surface to retain the shape and dimensions, the ability to drain, and the effect
on driver's behavior. Table 5-l gives the general selection of the pavement surface
types for the various road standards.
For minor roads of grades exceeding 87o, the surface and shoulder should be sealed to
prevent erosion.
The structural design of flexible pavement should be in accordance to Arahan Teknik(Jalan) - 5/85 "Manual On Pavement Design".
TABLE 5-1 : TYPICAL PAVEMENT SURFACE TYPE
DesignStandard
Description
R6ru6R5ru5R4N4R3ru3RzN2Rlrul
Asphaltic Concrete/ConcreteAsphaltic Concrete/ConcreteDense Bituminous Macadam/Asphaltic Concrete/ConcreteB ituminous Macadam/ConcreteSurface Treatment/Semi sroutGravel/Semisrout
5.1.2 Normal Cross Slope
Cross slopes are an important element in the cross-section design and a reasonably steep
lateral slope is desirable to minimise water ponding on flat sections of uncurbed
pavements due to pavement imperfections or unequal settlements and to control the flowof water adjacent to the curb on curbed pavements. The range of cross slopes for various
pavement types varies from 2.5Vo to 6.0Vo.
6l
A Guide on Geometic Design
5.3 SHOULDERS
The capacity of a llqnwav primarily depends on rhe number of traffic lanes and rheirwidths' The lane width is determin"d uy the size of vehicle, averagedaily traffic volumeof cornmercial vehicies and the requi.emenfs for overtaking and passing.
Marginal strip is a nalrow pavement strip attached to both edges of a cariageway. It ispaved to the same standard as the traffic lanes. For divided roads, the marginal strips areprovided on both sides of the carriageways. The marginal strip is included as part of theshoulder width' Edge line ma.kingire applied to demarcate the through lane and themarginal strip and is to be located within the marginal strip.
Table 5'2 indicates the lane and marginal strip width that are to be used for the variousroad standards.
Note : x denotes the total two_way width
5"3.1 Definition
A shoulder is defined as the portion of the roadway continuous with the traveled way foraccommodation of stopped vehicles, for emergen.f ur. and for laterar support to thepavement.
A typical cross section showing shoulder and verge is as shown in Figure 5.r.
TABLE 5.2 : LANE & MARGINAL STRIP wIDTHs
Design Standard Lane Width (m) Marginal Strip Width (m)
Interchange RamplSingle LaneMulti Lanes
Single Lane
4.503.504.50
Lt 1.50 Rt 0.50Lt 0.50 Rt 0.50Lr 1.50 Rt 0.50
re*ia':r%
az
ou0Lq)
A
-Icq
hc)
aI
-tl-t4
-aq.(
I-.FI+JIq)00rn
Lr\\J-c!I.-gt-t-ltnq)LFI
bo.-r-.H
a)
c)l-!)o-B
II
- --+I{Jl
.61
'H
T-I
I
I r..
-l c)
[)I;>l t
l-rlt)
a
rh
Fbo
'fF
(J
63
:
::
A Guide on Geometric Design of Roads
5.3.2 Functions
The main functions of shoulders are :_
(a) space is provided for emergency stoppin g freeo{ the traffic iane.(b) space is provided for the occasirnal motorist who desires to stop for various
reasons.
(c) space is provided to escape pofential accidents or reduce their severity.(d) the sense of openness created by shoulders of adequate width contributes to
driving ease and comfort.
(e) sight distance is improved in cut sections, thereby improving safety.(0 highway capacity is improved and uniform speed is encouraged.(g) lateral clearance is provided for signs and guardrairs.(h) structural support is given to the pavement.
5.3.3 Width of Shoulders
The normal usable shoulder width thatshould be provided along high type facilities is3m' However' in difficult terrain and on low volume roads, usable shoulders of this widthmay not be feasible.
Table 5-3A and 5-38 gives the widths of shoulders for the various road standards inrural and urban areas.
The shoulder widths on structures are defined in Section 5.10. The width of shouldersshould also take into consideration car parks.
TABLE 5-3A : USABLE SHOULDER WIDTH (RURAL)
Usable Shoulder Widrh (m)
Rollin Mountainous
R6R5R4R3R2R1
3.003.003.00)\n2.401.50
3.003.003.002.502.0a1.50
2.5A
2.s02.002.AA
r.50r.50
64
A Guide on Geometric Design of Roads
TABLE 5-38 : PAVED SHOULDER WIDTH ruRBAN)
Paved Shoulder Width (m)
Area Type *
I II**. III**
U6U5U4U3U2U1
3.003.003.002.502.001.50
3.003.002.502.001.501.50
2.502.502.041.50
1.50
1.50
* For Area Type definition, see Table 3-28x* For Area Type II & III, U1 to U4, shoulder may be replaced by sidewalk
5.3.4 Type of Shoulders
There are basically two types of shoulders namely paved and unpaved.
Paved shoulders can be either bituminous or concrete surfaced while unpaved shoulders
can be either crushed rock. earth or turf shoulders.
The paved shoulder widths are as shown in Table 5.4. The figures given are in addition
to the width required for parking.
TABLE 5.4 : PAVED SHOULDER WIDTH (RURAL)
DesignStandard
Paved Shoulder Width (m)
R6R5R4R3
Min 2.5Min 2.0
1.5
1.5
The structure and the cross slope of these shoulders are different and they are described
below. .
5.3.5 Shoulder Cross Slope
Ali shoulders should be sloped sufficiently to rapidly drain surface water but not to the
extend that vehicular use would be hazardous. Because the type of shoulder constructionhas a bearing on the cross-slope, the two should be determined jointly.
Bituminous and concrete surface shoulders should be sloped from 2.5 to 6 percent, gravel
or crushed rock shoulders from 4 to 6 percent and turf shoulders 6 percent. Where kerbs
are used on the outside of the shoulders, the minimum cross-slope should not be less than
4 percent to prevent ponding on the roadway.
65
4 9!id" on Geometric De
At superelevated areas, the shoulder cross-slopes should be adjusted to ensure that themaximum ,.roll over,'does not exceed 6 percent.
5.3.6 ShoulderStructure
For shoulders to function effectively, they must be sufficiently stable to supportoccasional vehicle loads, in all kinds of weather without rutting. paved or stabilisedshoulders offer manY advantages in this aspect. The structu.e oi the paved shouldersshould be similar to that of the carriageway for ati design standards. H"*;;;, ;;;;gradients exceed 8vo the shoulders snouti also be close turfed or sealed to preventerosion for R2 and Rl roads.
5.4 KERBS
5.4.1 General Consideration
The type and location of kerbs appreciably affect driver's behaviour and in turn thesafety and usage of a road. Kerbs are uied for drainage control, pavement edgedelineation, aesthetics, delineation of pedestrian walkways and to assist in the orderlydevelopment of the roadside.
Kerbs are needed mostly on roads in urban areas. In rurar areas, the use of kerbs shouldbe avoided as far as possible except in localised areas which has predominant aspects ofan urban condition.
5.4.2 Types of Kerbs
The two general classes of kerbs are barrier kerbs and mountable kerbs and each hasnumerous types and detail design lach may be designed as a separate unit or integrallywith the pavement. They may also be designed wltn I gutter to form a combination kerband gutter sectjon.
Barrier kerbs are relatively high and steep faced and are designed to inhibit or discouragevehicles from leaving the roadway. They should not be used on expressways. Theyshould also not be used where the design speeds """""a 70 kph or in combination withtraffic barriers' They are recommended foi use in builrup areas adjacent to footpathswith considerable pedestrian traffic, where pedestrian traffic is light, a semi-barrier typemay be used.
Mountable kerbs are used to define pavement edges of through carriageways. Forchannelisation islands, medians, outer separators or any other required delineation withinthe roadway, a semi-mountable type may be used.
Figures 5'2A and 5'28 show the various standard types of kerbs that are to be used.
66
t7
FIGURE 5.2A : ROAD KERB DETAILS
TYPE OF K-ERB USF
I,)BA.RRHffiS
|sOF@
B1
t-il
Euiatsts w ucd to inhr'bitvcbicla ftom leving thcrudrry. Tbcycmmadcdf6uc ia boilt-rp rru Edi&@tto fmPatis or o partr withoridablo pctsiu trafficod thc daigl spccd shotdd bclcs rh^n 70 lpL
Typc Bl kab ir garally uscd at
outcr wc of thc supclcvarodscdiou-
Typc 82 tctb shoulda bc ucCfc aorosl ms stimsGcb ud thc aupaclov8tedscd.ioB al i.Mmclo@lleisuf&cmE
EOP@
'N@o?ls
Nomalfy wd fo &l.inariouud drsiDrgc m dlint rs6tioil. lt i6 sitsblc fqall rcads including expnxs*ays.It ir alrc fc uc o bridga whicb
b offsst &on hridgc nililg.
NOTES:-
I. CONCRETE FOR CAST INSMU OR PRECAST CONCRETE KSRB SHALL BE GRADE 30/20.
2. CONCRTTE BEDDING SHALL BE GRADE 15,20.
3. THE FIMSHED KERB SHALL BE TRUE TO LINE, GRADE AND LEVEL TO WITHIN 15M AND
SIIALL PRESENT A SMOOTH A?PEAXANCE FREE FROM KINKS AND DISTORTION.
TYPE OF KERB USE
3.)MOWSUtrS
tgl
"r=L
I
t"l"
pWot@l
Moutrblc Lcrbs t12e Ml udM2 rc dcigncd to discowcwt trEfrc fioE moutBg a
t'affic irlud Roudebou'l s@adis! qc?t for lo.ng qovd-dllmtold volcl6.
TypB M2 kdb6 sc d6igred to
colldt BEf8cc mo{f ud ballow vchicld to @s ovdud to psrk cltr of tbccmirgmy duing magmcY.
1.) GAISIEL I€RJS
Cbrucl kabouldbcwdalonr thc pavod sboulda wbctbc irfad reoffis colidmbtYluee. It is nomrllY wcd oo
d-bshot wbdc dniD hs !o bcphccd dmg tbo pavod sbouldsi--cdiatly i! Asnt of b$iq(GudreilN* JaneY Briq)It ws dairncd to minimiz thcavce cffi(' of thc cbruclon tbc stairg md vcbiclcsboulds bc sblo to drivc ovathis chancl nfcly.
61
20 i00 50 100 (mi!.)
--mnm$
SECTION A-A
l00m CONCRETEFOI'NDATION
FTGURE 5.28: ROAD KERB DETAILS
TE PIPE
WITH RECTA IEULAR OPENING
()2
ya
FRONTVIEW OF KERB
1300r
68
.7
A Guide on Geomelric Design d Roads
The width of kerbs are considered as cross section elements entirely outside the trafficlane width. However, for drainage kerbs, the gutter section may be considered as pan ofthe marginal strip. Where roadways do not have any marginal strip, kerbs should be
offsetted by a minimum of 0.25 m.
5.4.3 Channel Kerbs
Channel kerbs should be considered for roads built on hieh embankment and for roads
with wide roadway.
SIDEWALKS
Sidewalks are accepted as integral parts of urban roads and should be provided except
on Urban Expressways or Major Arterials where the presence of pedestrians are minimal.
However, the need for sidewalks in many rural areas is great because of the high speed
and general lack of adequate lighting and due consideration must be given for itespecially at points of community development such as schools, local business and
industrial plants that results in high pedestrian concentrations. While there are no
numerical warrants, the justification for a sidewalk depends on the vehicle-pedestrian
hazard which is governed by the volumes of pedestrian and vehicular traffic, theirrelative timing and the speed of the vehicular traffic.
In urban areas, sidewalks can be placed adjacent to the kerbs and raised above the
pavement. In the absence of kerbs, a strip of a minimum width of l.0m must be provided
between the sidewaik and the travelled way.
In rural areas, sidewalks must be placed well away from the travelled way and separated
from the shoulder by at least 1.0m.
A desirable width of 2.0m is to be provided for all sidewalks. Where there are restrictions
on right of way, a minimum of clear width of 1.5m can be considered. When provided,
sidewalks must have all weather surfaces. There must be no obstructions on sidewalks.
TRAFFIC BARRIERS
Traffic barriers are used to minimise the severity of potential accident involving vehicles
leaving the traveled way. Because bariers are ahazard in themselves, emphasis should
be on minimising the number of such installations. Arahan Teknik (Jalan) l/85 "Manual
On Design Guidelines of Longitudinal Traffic Barrier" (May, 1984) should be used for
the design of longitudinal traffic bariers.
MEDIANS
5.7.1 General
A median is a highly desirable element on all roads canying four or more lanes and
should be provided wherever possible. The principal functions of a median are to provide
the desired freedom from the interference of opposing traffic, to provide a recovery area
for out-of-control vehicles, to provide for speed change and storage of right-tunringvehicles and to orovide for future lanes.
5.6
5./
69
A Guide on Geometric
For maximum efficiency, a median should be highly visible both night and day and indefinite contrast to the through traffic lanes.
5.7.2 Median Width
Medians may be depressed, raised or flush with the pavement surface. They should beas wide as feasible but of a dimension in balanceo wltn other components of the cross-section. The general range of median width varies from a minimum of l.5m in a AreaType III urban situation to a desirable width of l8 m on a rural expressway. on widemedians, it is essential to have a depressed centre or swale to provide for drainage.
Table 5-5A and 5-58 gives the minimum and desirable widrhs and types of medians rharare to be applied to the various road standards. The median widths u,
""prrrr.d are the
dimensions between the through lane edges and includes the right shouiders if any.
5.8.1 General
Service roads are generally found in urban areas and they can have numerous functions,depending on the type ofroad they serve and the character of the sunounding area. Theymay be used to control access or function as a street facility serving adjourning property.
They segregate local traffic from the higher speed through traffic and intercept drivewaysof residences and commercial establishments along the road. Service roads also not onlyprovide more favourable access for commercial ind residential development than thefaster moving afterials but also helps to preserve the safety and capacity of the latter.
TABLE 5-5A : MEDIAN WIDTH (RURAL)
Median Width (mTerrain
Rollin MountainousMinimum Minimum Desirable Minimum
TABLE 5.58 : MEDIAN WIDTH ruRBAN)
Median Width (m)
Minimum Minimum
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A Guide on Geometic Design of Roads
5.8.2 Design Requirements
From an operational and safety standpoint, one way service roads are much preferred totwo-way and should be considered. One-way operation inconveniences local traffic tosome degree, but the advantages in reduction in Vehicular and pedestrian conflicts atintersecting streets often fully compensate for this inconvenience.
Two-way service roads may be considered for partially developed urban areas where theadjoining road system is so irregular and disconnected that one-way operation wouldintroduce considerable added travel distance and cause undue inconvenience. Two-wayservice roads may also be necessary for suburban or rural areas where points of accessto the through facility are infrequent; where the service roads are widely spaced or wherethere is no parallel street within reasonable distance of the service roads in urban areasthat are developed or likely to be developed.
The design of a service road is affected by the type of service it is intended to provide.When provided, they should always cater for at least one side on-street parking. Theyshould be at least I .25 m wide for one-way operation and9.25 m for two-way operation.Figure 5.3 gives the recommended layout for a two-way operation service road frontingan urban arterial where the distance between junctions is greater than 0.5 km. Theminimum reserve width for a service road is 12 m.
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5.9 U.TURN
5.9.1 General
U-turns are categorised into two (2) types:
i) Dedicated U-turn - These U-turns are grade separated from the through traffic.This form of U-tum should be considered for high speed and high volumehighway as the diverging and merging movements are on the slow lanes.
ii) Median Opening U-turn.
5.9.2 Location of U-Turns
U-turns may be required in the following circumstances:
a) Beyond intersections to accommodate minor turning movements not otherwiseprovided in the intersection or interchange area.
b) Just ahead of an intersection to accommodate U-turn movements that wouldinterfere with through and other turning movements at the intersection. Wherea fairly wide median on the approach roadway has few openings, U-turning is
necessary to reach roadside areas. Advance separate openings for U-turn outsidethe intersection proper will reduce interference.
c) In conjunction with minor crossroads where traffic is not permitted to cross the
major road but instead is required to turn left, enter the through traffic stream,
weave to the right, U-turn, then retum. On high-speed or high-volume roads, the
difficulty and long lengths required for weaving with safety usually make this
design pattern undesirable unless the volumes intercepted are light and the
median is of adequate width. This condition may occur where a crossroad withhigh volume traffic, a shopping area, or other traffic generator that requires a
median opening nearby.
d) Locations occurring where regularly spaced openings facilitate maintenanceoperations, policing, repair service of stalled vehicles, or other highway-relatedactivities. Openings for this purpose may be needed on controlled-access roads
and on divided roads through undeveloped areas.
e) Locations occuming on roads without control of access where median openings
at optimum spacing are provided to serve existing frontage developments and at
the sanre time minimize pressure for future ntedian openings.
The locations of U-turns are very important and traffic safety should be given dueconsideration. As a general guide, Table 5.6 can be used to detennine the desirabledistances between U-turns.
13
A Guide on Geometric Design
TABLE 5.6 : DESIRABLE DISTANCE BETWEEN U.TURNS
5.9.3 DesignConsideration
u-turning vehicles interfere with through traffic by encroaching on part or all of thethrough traffic lanes. They are n11de uilo* speeds and the required speed change isnormally mad^e 91
the through traffic Ianes with weaving to and riom ttre outer lanes. Assuch, U-turn facilities are potentialhazard areas. Careful consideration should be madeto design and locate the u-turn that are safe for ail the road users.
For direct u-turns, the width of the highway, including the median should be sufficientto permit the turn to be made without encroachment beyond the outer edges of thepavements' Figure 5'4 gives the minimum width of the median required for the varioustypes of maneuvers for direct U_turns.
The U-turn should be designed such that the through traffic approaching the U-turn aretravelling at much lower than the highway desigipeed. Traffic signage and calmingdevices, etc. should be considered.
Distance BetweenU-Turns
Design j Distance BetweenStandard I U-Turns
R6R5R4
No U-turns allowed3km2km
U6U5U4U3
No U-turns allowed2kmlkmlkm
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A Guide on Geometric Design of Roads
5.10
5.10.1 Width of Shoulders
The width of the usable shoulders should follow that as indicated in Table 5-3A and5'38. The width of the shoulder of bridges and structures should be the same as that ofthe carriageway.
Figures 5.5 show the typical bridge cross section.
5.10.2 Required Clearance
The vertical height clearance of all structures above the roadway shall be aminimum of 5.3 meters over the entire width of traffic lanes, auxiliary Ianes andshoulders. It is recommended that an additional 0.1 meter be allowed for futurepavement resurfacing. Whenever resurfacing will reduce the clearance to lessthan 5.3 meters, milling will be required to maintain the minimum clearance.
For ciearance requirements over railways and waterways, reference should bemade to the relevant authorities.
Figure 5.6 indicates the clearance requirements for undemasses of various crosssections.
(a)
(b)
(cJ
5.11 BUS LAYBYES
Bus laybyes serve to remove the bus from the through traffic ianes. Its location anddesign should therefore provide ready access in the safest and most efficrent mannerpossible. The basic requirement is that the deceleration, standing and acceleration of thebuses be effected on pavement areas clear of and separated fromihe through traffic lanes.
The locations of bus laybyes are impoftant so as not to impede the normal flow of traffic.In this respect, bus laybyes must not be located on any interchange ramps or srmctures,slip roads or within 60 m of any junction or intersection. The distance between laybyestoo should not be less than 150 m. Liaison will need to be made also with the relevantbus companies and the respective Local Authority.
For school areas, provision of bus laybyes and parkings areas should be specificallystudied so as to ensure the unirnpederj flow of triffic.
Figure 5.7 shows the typical layout and dimensions of bus laybyes that are to be usedin both rural and urban areas. If possible, a dividing area between the outer edge of theroadway shoulder and the edge of the bus laybye lane should be provided. This dividingarea should be as wide as possible but not less than 0.6 m.
The pavement areas of the laybyes should be of concrete or other material fbr contrastin coiour and texture with the through traffic Ianes to discourage through traffic fromencroaching on or entering the bus stop.
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FIGURE 5.6 : CLEARANCES
LEFT CLEARANCES
AT UNDERPASS
RIGHT CLEARANCES
3.0 m (Desirable)1.5 m (Minimum)
TRAfFIC LANES
CENTRE PIER OR ABU"TMENT-A-
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WITH SIDEWALK-E-
2.0 m QDesirable)1.25 m (Minimnm)
5.3 m
WITH AIIXTLIARY LANE-c-
WITH AIIXILIARY LANE-F-
2.0 m (Desirable)1.25 n (Minimum)
5.3 m
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6.1
CHAPTER 6 . OTHER ELEMENTS AFFECTING GEOMETRIC DESIGN
ROAD SAFETY
Road safety has been a much neglected area in the design of roads, With an increasingnumber of accidents each year, attention to road safety should be emphasised, While theroad element is oniy one of the three groups of influences causing accidents, it isnonetheless the responsibility of the designer to provide as safe a road environment aspossible,
In this aspect, the road design should be one with uniformly high standards appliedconsistently over a section. It should avoid discontinuities such as abrupt major changesin design speeds, transitions in roadway cross-section, the introduction of a short-radiuscurve in a series of longer radius curves, a change from full to partial control of access,constrictions in roadway width by narrow bridges or other structures, intersectionswithout adequate sight distances, or other failures to maintain consistency in the roadwaydesign. The highway should offer no surprises to the driver, in terms of either geometricsor traffic controls.
The tendency to base- road designs upon minimum standards rather than to adopt anoptimum design is unfortunate and must be avoided. A more liberal and optimum designmust always be considered, and although it would cost a little more in terms of capiLlcost of the project, it would increase the road's level of safety substantially.
All too often' minimum design standards attempt to rely, on the use of warning signs orother added roadside appurtenances for the safe operat;on that could have been built inby the use of higher design standards. Often tle safety deficiencies generated byminimum design are impossible to correct by any known tiaffic device or ippurtenance.A warning sign is a poor substitute for adequate geometric design.
]n the light to improve road safety, Road Safety Auditing (RSA) has been introduced inMalaysia since 1994-_It is a formal process to identify deficiencies in road design atdifferent stages as welr as to identify hazardous features on existing roads for erimrnationso as to make the road safer for all road users.
RSA is carried out in five stages. The feasibility and planning stage (Stage r),preliminary design stage (stage2), detailed design itage (stage 3), pre-opening stage(Stage 4) and operationaVaudit of existing roads lStage S;.
Rgference on RSA should be made to the "Guidelines For The Safety Audit Of RoadsIn Malaysia" published by the Roads Branch, pubiic works Department Malaysia.
D&AINAGE
Road drainage facilities provide for carrying water across the right of way and forremoval of storm water from the road itself. These facilities include bridges, culverts,channels, gutters and various types of drains. Drainage design considerations are anintegral part of geometric design and flood plain encriachments fiequently affect thehighway alignmenr and profile.
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A Guide on Geometric Design of Roads
6.3
The cost of drainage is neither incidental nor minor on most roads. Careful attention to
requirements for adequate drainage and protection of the highway from floods in allphase of location and design will prove to be effective in reducing costs in both
construction ant maintenance.
Reference should be made to the relevant JKR manual on this subiect.
LIGHTING
Lighting may improve the safety of a road and the ease and comfort of on operationthereon. Lighting of rural highways may be desirable but the need is much less than on
roads in urban areas. They are seldom justified except on critical portions such as
interchanges, intersections, railroad grade crossings, narrow or long bridges, tunnels and
areas where roadside interference is a factor.
Where lighting is being considered for future installation, considerable savings can be
effected through design and installation of necessary conduits under the pavements and
kerbs as part of the initial construction and should be considered.
Reference should be made to the relevant JKR manual on this subiect.
UTILITIES
All road improvements, whether upgraded within the existing right of way or entirely on
new right of way, generally involves the shifting of utility facilities. Although utilitiesgenerally have little effect on the geometric design of the road, full consideration should
be given to measures necessary to preserve and protect the integrity and visual qualityof the road, its maintenance efficiency and the safety of traffic.
Utilities relocation shouid form part of design and the designer should liaise closely withthe relevant service authorities in determining the existing utilities and their proposed
relocation.
SIGNING AND MARKINGS
Signing and marking are directly related to the design of the road and are features oftraffic control and operation that the engineer must consider in the geometric layout ofsuchfacility. The signing and marking should be designed concurrently with the
geometrics as an integral part, and this will reduce significantly the possibility of future
operational problems. The signing and marking should follow the standards that have
been established. Reference should be made to Arahan Teknik (Jalan) 2A,2F.,2C and
2D on the design, usage and application of signs and markings.
TRAFFIC SIGNALS
Traffic control signals are devices that control vehicular and pedestrian traffic by
assigning the right of way to various movements for certain pretimed or traffic actuated
intervals of time. They are one of the key elements in the function of many urban roads
and should be integrated with the geometric design so as to achieve optimum operational
efficiency.
6.s
6.6
81
ENVIRONMENTAL IMPACTS6.7
Erosion prevention is one of the major factors in design, construction anclof highway. It should be considered early in the location and design stages.of erosion control can be incorporated into geometric design, particularlysection elements.
matntenanceSome degreein the cross-
Landscape development should be in keeping with the character of the road and itsenvironment. The general are of improvement include the followins:
(a) preservation of existing vegetation(b) transplanting of existing vegetation where feasible(c) planting of new vegetation(d) selective clearing and thinning(e) regeneration of natural plant slecies and material.
Landscaping of urban roads assume an additional importance in mitigating the manynuisances associated with urban traffic. Reference stroulO be made to the relevant JKRmanual on this subject
The highway can and should be located and designed to complement its environment andserve as a catalyst to environmental improvement. The area surrounding a proposedhighway is an interrelated system of natural, manmade and sociologic variables. Changesin one variable within this system cannot be made without some effect on other variable.Some of these consequences may be negligible, but others may have strong and lastingimpact on the environment, including the sustenance and quality of human lif.. B".uur"highway location and design decisions have an effect on udju""n t areadevelopment, itis important that environmental variables be given full consideration. Environmentalimpacts include social, economic and physical impacts. In geometric design, onlyphysical impact- are aasessedl while the social and economic impacts would have beentaken care of during the planning stage.
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A Guide on Geometric Design of Roads
REFERENCES
l. ARAHAN TEKNIK JALAN 8/86: A Guide on Geometric Design of Roads, 1986
2. ISMAIL, MOHD. A: Thesis on Review of Highway Geometry Design Standards inMalaysia, 1996
3. AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATIONOFFICIALS (AASHTO): A Policy on Design of Urban Highways And Arterial Streets,
r913
4. THE INSTITUTION OF HIGHWAYS AND TRANSPORTATION WITHDEPARTMENT OF TRANSPORT: Roads and Traffic in Urban Areas, 1987
5. TRANSPORT ROAD RESEARCH LABORATORY AND OVERSEAS
DEVELOPMENT ASSISTANCE. UNITED KINGDOM : TowaTds SafeT ROAdS iN
Developing Country, 199 I
6. JABATAN PERANGKAAN MALAYSIA: Banci Penduduk Malaysia, 1991
7. JABATAN PERANCANG BANDAR DAN DESA: Manual Piawaian Perancangan,
JPBD (IP) S.273 FN/HHffeb. 1998
8. AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATIONOFFICIALS (AASHTO): A Policy on Design of Highways And Streets, 1994
9. TRANSPORTATION AND RESEARCH BOARD: Highway Capacity Manual, Special
Report 209,1985
10. HIGFIWAY PLANNING UNIT, MINISTRY OF WORKS MALAYSIA: Trip Generation
Manual 1997.
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A Guide on Geometric Design of Roads
ACKNOWLEDGEMENTS
PUBLIC WORKS DEPARTMENT, MALAYSIA
Dato'Ir. Hj. Zainibin Omar
STANDING COMMITTEE ON TECHNOLOGY AND ROAD MANAGEMENT
Dato' Ir. Chew Swee Hock (Chairman)
TECHNICAL COMMITTEE ON GEOMETRICS ffC3)
Ir. Han Joke Kwang (Chairman)Ir. Koid Teng Hye (Secrerary)Ir. Aznam Abdul Rahim (WG Leader)Ir. Tai Meu Choi (WG Leader)Ir. Sam El-Ashwawi (WG Leader)
84