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Transport and Road Research Laboratory Overseas Unit
Department of Transport
Overseas Development Administration
Overseas Road Note 6
A guide to
geometric design
Overseas Unit
Transport and Road Research Laboratory
Crowthorne Berkshire United Kingdom
1988 ACKNOWLEDGEMENTS
This
note has been produced for the Overseas Unit of the
Transport Research Laboratory by Roughton and
Partners, Consulting Engineers. The Project Manager for
TRL was Dr R Robinson of the Overseas Unit
First Published 1988; Reprinted 1998
TRL is committed to optimising energy efficiency,
reducing waste and promoting re-cycling and re-use. In
support of these environmental goals, this note has been
printed on recycled paper, comprising 1OO% post-
consumer waste, manufactured using a TCF (totally
chlorine free) process.
OVERSEAS ROAD NOTES
Overseas Road Notes are prepared principally for road
and road transport authorities in countries receiving
technical assistance from the British Government. A
1imited number of copies is available to other
organisations and to individuals with an interest in roads
overseas, and may be obtained from
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United Kingdom
© Crown Copyright 1988
Limited extracts from the text may be produced
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ISSN 0951-8797 CONTENTS
page
1. INTRODUCTION1
Purpose of this Note 1
Approach to design 1
Selection of design standard 1
Cross sectional considerations 1
Design speed 1
Safety1
Economic design 1
Road function2
Traffic flow3
Design flow3
Composition 4
Capacity4
Terrain4
Curvature standards 4
The design process 5
Basic parameters 5
Select Design Class 5
Determine trial alignment 5
Design Class standards 5
Approach speed estimation 6
Economic consequences 6
Relaxation of standards 6
Economic return 6
2. CROSS-SECTION7
Basic considerations for determining widths 7
Carriageways and shoulders 7
Passing places8
Curve widening8
Lateral and vertical clearances 9
Crossfall9
Carriageway markings 10
Provision for non-motorised travellers 10
Rights-of-way10
3. HORIZONTAL ALIGNMENT 11
Circular curves11
Adverse crossfall 11
Superelevation12
Transition curves 12
Other considerations 13
4. VERTICAL ALIGNMENT 14
Components of the vertical alignment 14
Crest curves14
Sag curves17
Gradient19
Climbing Lanes20
5. ECONOMICS AND SAFETY 21
Economic Assessment 21
Safety21
Non-motorised traffic 21
Driver safety 21 page
REFERENCES23
APPENDIX A : GLOSSARY OF TERMS 23
APPENDIX B ESTIMATION OF VEHICLE SPEED 25
APPENDIX C : PHASING OF THE VERTICAL AND HORIZONTAL ALIGNMENT 28
Defects in the alignment due to misphasing 28
Types of misphasing and corresponding corrective action 28
Insufficient separation between curves 28
The vertical curve overlaps one end of the horizontal curve 28
Both ends of the vertical curve lie on the horizontal curve 28
The vertical curve overlaps both ends of the horizontal curve 28
The economic penalty due to phasing 29 INTRODUCTION
PURPOSE OF THIS NOTE
1.1 This Note gives guidance on geometric design and
the setting of geometric design standards for single
carriageway rural (inter-urban) roads in developing
countries. It is aimed at government officials who are
responsible for formulating policy on geometric design
and at engineers who are responsible for preparing road
designs. It will also be of interest to personnel in aid
agencies and consultancies who are responsible for the
preparation and design of road projects. Many countries
will have existing standards different from those
described in this guide. This should not preclude the use
of the standards in this guide, although where good local
cost and benefit information is available, some aspects
may need to be reviewed.
1.2Geometric design is the process whereby the
layout of the road in the terrain is designed to meet the
needs of the road users. The principal geometric features
are the road cross-section and horizontal and vertical
alignment. The use of geometric design standards fulfills
three inter-related objectives. Firstly, standards are
intended to provide minimum levels of safety and
comfort for drivers by the provision of adequate sight
distances, coefficients of friction and road space for
vehicle manoeuvres; secondly, they provide the
framework for economic design; and, thirdly, they ensure
a consistency of alignment. The design standards adopted
must take into account the environmental road
conditions, traffic characteristics, and driver behaviour.
1.3The derivation of the standards recommended in
this Note, and summarised in Tables 1.1 and 1.2, is
described in TRRL Contractor Report 94 (Boyce et al
1988).
1.4A glossary of terms in this guide is given as
Appendix A.
APPROACH TO DESIGN
Selection of design standards
1.5The section of design standards is related to road
function, volume of traffic and terrain, with additional
procedures for the recognition and appropriate treatment
of potential hazards (Tables 1.1 and 1.2). Opportunities
for the relaxation of standards have also been identified.
1.6A basic assumption in the approach is that drivers
receive clues about the standard of the road from local
surrounding features such as the terrain, levels andtypes of flow, as well as geometric elements. Additional
design consideration or special signing will only be necessary
where the information available to the driver may lead to
incorrect interpretation and consequent danger.
Cross-sectional considerations
1.7Cross-section parameters are related to traffic flows of
all types, and will vary with the requirements of vehicular
traffic and with the needs of pedestrians and non-motorised
vehicles. In many developing country situations, it will be
necessary to consider cost effective ways of segregating non-
motorised traffic at the earliest stage in the design process.
Design speed
1.8Design speed is used as an index which links road
function, traffic flow and terrain to the design parameters of
sight distance and curvature to ensure that a driver is
presented with a reasonably consistent speed environment. In
practice, most roads will only be constrained to minimum
parameter values over short sections or on specific geometric
elements.
Safety
1.9There is very little information from developing
countries on the effects of changes in standards on accident
rates. Indeed. equivalent information from developed
countries is also limited. Highway engineering safety is
usually assumed to be optimised by linking geometric
elements to a design or operating speed, so that the resulting
geometry has a consistency which reduces the likelihood of a
driver being presented with an unexpected situation. This
concept of driver expectation forms the basis of this set of
design standards.
Economic design
1.10Designs should be justified economically, and the
optimum choice will vary with both construction and road
user costs. Construction costs will be related to terrain type
and choice of pavement construction, whereas road user costs
will be related to level and composition of traffic, journey
time, vehicle operation and road accident costs. Methods of
determining these costs are given in Overseas Road Note 5
(TRRL Overseas Unit 1988).
1.11The most economic designs will often not involve the
use of minimum standards, as levels of traffic may be such
that the additional vehicle operating cost, accident, and travel
time saving benefits from wider, straighter and shorter roads
may more than offset the extra construction costs needed.
1 1.12As flows increase, vehicle-to-vehicle interactions
become more important and congestion may result in
increases in journey times and accident risk if additional
lanes are not added. The scope of this Note has been
limited to single carriageway roads, and consideration of
the possible introduction of dual carriageways should be
made when flows approach 15,000 vehicles per day.
ROAD FUNCTION
1.13 Each inter-urban road may be classified as being
arterial, collector or access in nature as shown in Figure
1.1.
Fig.1.1 Road hierarchy and function
TABLE 1.1: ROAD STANDARDS
1.14Arterial roads are the main routes connecting national
and international centres. Traffic on them is derived from that
generated at the urban centres and from the inter-urban areas
through the Collector and Access road systems. Trip lengths
are likely to be relatively long and levels of traffic flow and
speeds relatively high. Geometric standards need to be
adequate to enable efficient traffic operation under these
conditions, in which vehicle-to-vehicle interactions may be
high.
1.15 Collector roads have the function of linking traffic to
and front rural areas, either direct to adjacent urban centres, or
to the Arterial road network. Traffic flows and trip lengths
will be of an intermediate level and the need for high
geometric standards is therefore less important.
1.16 Access roads are the lowest level in the network
hierarchy. Vehicular flows will be very light and will be
aggregated in the Collector road network. Geometric
standards may be low and need only be sufficient to provide
appropriate access to the rural agricultural, commercial and
population centres served. Substantial proportions of the total
movements are likely to be by non-motorised traffic.
1.17Whilst this hierarchy is shown simplistically in Figure
1.1, in practice there will be many overlaps of function and
clear distinctions will not always be apparent on functional
terms alone. This hierarchy should not be confused with the
division of administrative responsibilities which may be based
on historic conditions.
* The two way traffic flow is recommended to be not more than one Design Class step in excess of first year ADT.
+ For unpaved roads where the carriageway is gravelled, the shoulders would not normally be gravelled; however,
for Design Class D roads, consideration should be given to gravelling the shoulders if shoulder damage occurs.
2 TABLE 1.2 : SPEED RELATED DESIGN PARAMETER
1.18For the lowest Design Class of road, it is
inappropriate to design on the basis of geometric
standards, and the sole criterion of acceptability will be
the achievement of an appropriate level of access.
Design, in these situations, should be based on minimum
values of radii, width and gradient for the passage of a
suitable design vehicle.
TRAFFIC FLOW
Design flow
1.19The functional hierarchy is such that traffic is
aggregated as it moves from Access to Collector to
Arterial road, and levels of flow will normally be
correlated to road type. However, flow levels will vary
between countries and regions and it is important that the
designation of a road by functional type should not give
rise to overdesign for the levels of traffic actually
encountered. Uneconomic designs reduce the likelihood
of roads being built and result in wastage of often scarce
national resources.1.20Design Classes A to F have associated bands of traffic
flow as shown in Table 1.1. The range of flows extends from
less than 20 to 15,000 motorised vehicles per day, excluding
motorcycles, and covers the design conditions for all single
carriageway roads.
1.21Although the levels of flow at which design standards
change are based on the best evidence available, the
somewhat subjective boundaries should be treated as
approximate in the light of the uncertainties inherent in traffic
estimation and economic variability
. Therefore, design flows
should normally be constrained to be no more than one
Design Class step higher than the annual average daily traffic
(ADT) in the first year of trafficking. Thus, a road with a first
year traffic flow of 390 vehicles per day rising to 1,100
vehicles per day should be constructed to Design Class C
rather than Design Class B geometry (see Table 1.1). The
design flow band in this case is therefore 400-1000 vehicles
per day. Design to the higher Design Class would result in an
overdesigned facility during
3 almost the whole of the life of the road and may provide
a solution that was less than the economic optimum. If
the initial flow were 410 vehicles per day, design would
still be to Design Class C. It is particularly important that
roads are not overdesigned on the basis of high traffic
growth rates which normally incorporate considerable
uncertainty.
Composition
1.22Although, in some situations, heavy vehicles
have a greater effect on congestion than light vehicles, no
attempt has been made to use passenger car unit (pcu)
equivalent values. The relative effects of heavier vehicles
vary with level of flow, geometry, and vehicle
performance and consistent values that are well
researched are not available for the range of flows
covered in this design guide. All flows are therefore
presented as ADT values. However, where there are very
high percentages of heavy vehicles in a traffic stream,
consideration may be given to the enhancement of
standards, and particularly of carriageway width.
Capacity
1.23Congestion increases with increased traffic
flow when there is a lack of overtaking opportunity. The
result is high journey times and vehicle operating costs,
often accompanied by more accidents as frustrated
drivers take risks.
1.24Practical capacity is usually estimated to have
been reached when the level of congestion becomes
"unacceptable". Capacity reduces with increased
proportions of heavy vehicles, greater unevenness in
directional flows, reduced overtaking opportunities,
animal drawn vehicles and pedestrian activity. Normally
acceptable practical capacity will be about 1500 to 2000
vehicles per hour, but may be increased substantially by
the provision of short sections of climbing and
overtaking lanes.
1.25Capacity is only likely to be approached for
road Design Class A, or at the higher flow levels, road
Design Class B, particularly in the more rugged terrain if
adequate overtaking opportunities are unavailable.
TERRAIN
1.26A simple classification of "level", "rolling" and
"mountainous" has been adopted and is defined by both
subjective description and by the average ground slope.
The average ground slope is measured as the number of 5
metre contour lines crossed per kilometre on a straight
line linking the two ends of the road section. (The slope
may be interpolated using other contour intervals on a
proportional basis).
41.27
Level (0-10 five metre ground contours per
kilometre). Level or gently rolling terrain with largely
unrestricted horizontal and vertical alignment. Minimum
values of alignment will rarely be necessary. Roads will,
for the most part, follow the ground contours and
amounts of cut and fill will be very small.
1.28 Rolling (11-25 five metre ground contours per
kilometre). Rolling terrain with low hills introducing
moderate levels of rise and fall with some restrictions on
vertical alignment. Whilst low standard roads will be
able to follow the ground contours with small amounts of
cut and fill, the higher standards will require more
substantial amounts.
1.29 Mountainous (Greater than 25 five metre
ground contours per kilometre). Rugged, hilly and
mountainous with substantial restrictions in both
horizontal and vertical alignment. Higher standard roads
will generally require large amounts of cut and fill.
1.30In general, construction costs will be greater as
the terrain becomes more difficult and higher standards
will become less justifiable or achievable in such
situations than for roads in either flat or rolling terrain.
Drivers should also expect lower standards in such
conditions and therefore adjust their driving accordingly,
so minimising accident risk. Design speed will therefore
vary with terrain.
CURVATURE STANDARDS
1.31 Minimum horizontal and vertical curvatures
are governed by maximum acceptable levels of lateral
and vertical acceleration and minimum sight distances
required for safe stopping and passing manoeuvres.
These design parameters are, in turn, related to the
vehicle speeds assumed in the design. Curvature
standards are thus either explicitly or implicitly
dependent on an assumed design speed.
1.32Within this guide, the adopted design speeds
are explicitly stated and, as shown in Tables 1.1 and 1.2,
have been taken to vary with both terrain and level of
traffic flow. However, it must be emphasised that these
speeds are intended to provide an appropriate consistency
between geometric elements rather than as indicators of
actual vehicle speeds at any particular location on the
road section.
1.33The use of lower design speeds in the more
difficult terrain is intended to incorporate an element of
reduced driver expectation and performance as well as
the need to keep construction costs to acceptable levels.
As flows increase, the level of benefits from reduced
road length also increase and generally support higher
standards with more direct and shorter routes. THE DESIGN PROCESS
1.34The design process is shown in Figure 1.2 with
the main features detailed below. The emphasis
throughout is on the need to obtain best value for money.
Basic parameters
1.35Initially, the basic parameters of road function.
traffic flow and terrain type are defined.
Select Design Class
1.36On the basis of the above estimates, a Design
Class is selected from Table 1.1. Values of Design
Class boundaries are for guidance only, and the lower
Design Class should be chosen in borderline cases.
Determine trial alignment
1.37A road consists of a series of discrete
geometric elements of horizontal and vertical curvature.
Contiguous groups of these elements combine to form
sections. In this guide, the minimum length of a road
section is considered to be about one kilometre.
FIG. 1.2 : The design process
1.38The initial stage in selecting an alignment for a
new road is to sketch a route on a contoured map or
aerial photograph. A similar process can be carried out
when investigating the upgrading of an existing road. By
reference to the standards, the designer will have some
knowledge of appropriate minimum radii for thescale of the map or photograph. Consideration will be
given to gradient by reference to the contours of a map,
or by relief when using stereo photographs. Several
alternative alignments should be tried. The design
process should be carried out in conjunction with on-site
inspections and surveys. One or two of the alignments
should be chosen for additional studies in more detail and
be subject to further design and assessment prior to
possible construction.
1.39On two lane roads, the horizontal alignments
should be designed to maximise overtaking opportunities
by avoiding long, continuous curves. Instead, relatively
short curves at, or approaching, the minimum radius for
the design speed should be used in conjunction with
straights or gentle, very large radius curves. Conversely,
an alignment of flowing curves may reduce real
overtaking opportunities, thus encouraging injudicious
driver behaviour. On two lane single carriageway roads
in developing countries, the provision of adequate
overtaking opportunities may be particularly important
because of the large proportions of slow moving
vehicles.
1.40Often a new road will be built to replace an
existing facility. The structural features of the existing
road, including bridges, embankments and cuttings may
have substantial residual value and influence alignment
choice.
1.41The geometric standard of individual elements
of the road will vary with the terrain. It is necessary that
elements of lower geometric standard are identified to
ensure that they will not result in unacceptable hazards to
approaching vehicles. These elements will be readily
identifiable from the preliminary horizontal and vertical
curvature profiles. The tests for the necessary
consistency are simple, as described below, and should
be carried out if there is any doubt as to the acceptability
of an element.
Design Class standards
1.42It is recommended that, where the standard of a
geometric element falls substantially below that on the
approach section, its adequacy should be checked by
estimating approach speed from the relationships given
in Appendix B. Geometric elements should not normally
be designed to a Design Class more than a one Design
Class step lower than the approach speed to that element.
However, two Design Class steps may be achieved by
successive reductions from a design speed of, for
example, 85 km/h in rolling terrain to 70 km/h and then
60 km/h (see paras 1.48-51). If this is not possible,
consideration must be given to redesign of the element or
alterations to the geometry of the approach section to
obtain this speed reduction.
5 Approach speed estimation
1.43The speeds of freely moving vehicles on an inter-
urban road usually conform to a normal distribution
within which percentile values of speed are
approximately related as follows:
• 1.2 x 15th percentile speed = 50th percentile speed
• 1.2 x 50th percentile speed = 85th percentile speed
• 1.2 x 85th percentile speed = 99th percentile speed.
1.44The 85th percentile value of speed has been
used as the basis of design in this guide. Thus, 15 per
cent of the vehicles could be considered to be exceeding
the design speed on any section of road. It also follows
from the above that, for example, with a design speed of
100 km/h: I per cent would be exceeding 120 km/h; 50
per cent would be exceeding 85 km/h; and 85 per cent
would be exceeding 70 km/h. Each such speed change
has been taken to represent a consistent design step in
Table 1.1. in which rounded values have been used.
1.45A driver's ability to negotiate a geometric
element safely will depend on his approach speed
relative to a safe speed on the element. As it is not
possible to predict speed profiles accurately, it is
recommended that estimates of approach speed are made
using the relationships described in Appendix B. These
relationships produce estimates of 85th percentile speed.
Speeds are modified by geometric characteristics and
estimates of approach speed will be based on the
geometry of about one kilometre on both approaches to
the geometric element under consideration. These
approach sections should include complete design
elements ie complete horizontal or vertical curves and
gradient lengths. There are considerable uncertainties in
the accuracy of speed estimation relationships and the
results should therefore be treated as approximate.
Economic consequences
1.46If a geometric element fails to achieve the
standard chosen for design, the economic consequences
of upgrading to the standard must be considered. The
economic consequences should generally be measured as
additional cost of construction either in absolute terms or
as a proportion of the overall cost. If this cost is small,
the road alignment should normally be redesigned. If the
cost is large, consideration should be given to further
relaxation of standard as described in paras 1.48-51.
1.47In general, the higher the class of road, and hence
volume of traffic, the more likely will benefits from
vehicle operating and time cost savings lead to the
justification of a shorter, straighter route.
6
Relaxation of standards
1.48The standards summarised in Tables 1.1 and 1.2
are intended to provide guidance for designers rather
than to be considered as rigid minima. The justification
for construction of a particular road will almost always
be based on a detailed economic appraisal, and
relaxations of standards may be essential in order to
achieve an acceptable level of return on investment. In
other circumstances, an already acceptable rate of return
may be increased substantially by the inclusion of a short
section of substandard road where achievement of the
design standard would be expensive, although the safety
implications of this would need serious consideration.
1.49Relaxation of one Design Class step implies design
to the 50th percentile rather than the 85th percentile
speed. Relaxation of two Design Class steps reduces the
design to the 15th percentile speed. Experience in the UK
has shown that reduction of design parameters by one
step, equivalent to a 17 per cent reduction, is likely to
have little effect on safety. Normally, a relaxation of two
steps, equivalent to a 30 per cent speed reduction, should
not significantly increase risk where appropriate signing
or other warning measures, such as bend marker posts,
are provided. On low flow roads where most of the
drivers will be regular users, the increased risk will be
less significant and the resultant number of accidents
should be negligible. Greater care and consideration
should be given to relaxations on high flow/high speed
alignments.
1.50In special circumstances, where standards have
been reduced on successive design elements, further
relaxations may be made based on those reduced
approach speeds. Sight distances, and the potential
accident risk as a result of driver error, would need to be
considered on a site-specific basis.
1.51Reductions in standards should only apply to
stopping distances and curvature, and suitable values
have been included in Table 1.2. Widths should not be
reduced as they are particularly flow related, and
additional widening may be required on curves with the
tighter radii.
Economic return
1.52All road design projects should be subject to an
economic appraisal as recommended in Overseas Road
Note 5 (TRRL Overseas Unit 1988). It is essential that
those responsible for design should investigate whether
amendments to an alignment will produce significant
increases in economic rates of return. 2. CROSS-SECTION
BASIC CONSIDERATIONS FOR
DETERMINING WIDTHS
2.1Road width should be minimised so as to
reduce the costs of construction and maintenance whilst
being sufficient to carry the traffic loading efficiently and
safely. Recommended values are given in Table 1.1.
2.2For Access roads with low volumes of traffic
(<100 ADT), single lane operation is adequate as there
will be only a small probability of vehicles meeting, and
the few passing manoeuvres can be undertaken at very
reduced speeds using either passing places or shoulders.
Provided sight distances are adequate for safe stopping,
these manoeuvres can be performed without hazard, and
the overall loss in efficiency brought about by the
reduced speeds will be small as only a few such
manoeuvres will be involved. It is not cost-effective to
widen the running surface in such circumstances and a
basic width of 3.0 metres will normally suffice. In some
situations, 2.5 metres will allow effective passage.
Fig.2.1 TypicaI cross-section
(Dimension in mm)
2.3On roads with medium volumes of traffic (100-
1000 ADT), the numbers of passing manoeuvres will
increase and pavement widening will become worthwhile
operationally and economically. However, in view of the
generally high cost of capital for construction in
developing countries and the relatively low cost of travel
time, reductions in speed when approaching vehicles
pass will remain acceptable for such flow levels and
running surface widths of 5.0 and 5.5 metres are
recommended. For Arterial roads with higher flows (> 1000 ADT), a running surface 6.5 metres wide will allow
vehicles in opposing directions of travel to pass safely
without the need to move laterally in their lanes or to
slow down.
2.4Typical cross-sections are shown in Figure 2.1.
CARRIAGEWAYS AND SHOULDERS
2.5Shoulders are recommended for all but the lowest
Design Class and will normally be paved when the
carriageway is paved (Figure 2.1.). They are intended to
perform three main traffic functions:
•To provide additional manoeuvring space on roads of
lower classification and traffic flows
•To provide parking space at least partly off the
carriageway for vehicles which are broken down
•To enable non-motorised traffic to travel with
minimum encroachment on the carriageway.
2.6Additionally, it may be desirable to provide
sufficient width for two way movement during
roadworks.
2.7Clearly, these functions are not wholly compatible
and detailed design recommendations have been based
on the following logic.
2.8 Design Class F This class of road provides basic
access only and the motorised traffic flows are so low
that shoulders are not required. All road users will share
the 3.0 (2.5) metre carriageway and passing places will
be provided as appropriate. The width should be just
sufficient to allow the occasional vehicles to traverse the
road and design to specific geometric standards will be
inappropriate.
2.9 Design Class E 1.5 metre shoulders have been
recommended for this class of Access road as they will
allow a total road width of 6.0 metres, sufficient for two
trucks to pass with 1.0 metre clearance. Shoulders may
also be used by non-motorised traffic and pedestrians,
and potential conflicts will be acceptably low with the
few motorised vehicles on the road. In difficult terrain,
and elsewhere where construction costs are high, 1.0
metre shoulders may be acceptable, particularly when
the carriageway and shoulders are paved or where the
flow of non-motorised traffic is small.
7 2.10 Design Class D On paved roads, 1.0 metre paved
shoulders are recommended to provide a total paved
width of 7.0 metres. this will allow approaching vehicles
some lateral movement where necessary, albeit at a
reduced speed. If the shoulders are unsurfaced, a high
level of maintenance will be necessary to avoid damage
and the resulting break up of the edge of the pavement. A
minimum of 1.0 metre of surfaced shoulder will also
encourage pedestrians and non-motorised users to use the
shoulders, rather than the carriageway. Shoulder
delineation is particularly important. It is most unlikely
that non-motorised traffic will justify the construction of
additional width with the levels of motorised vehicles on
this class of road. However, the justification for surfaced
shoulders or special provision will become greater as
flows of traffic of all kinds rise (paras 2.37-42).
Conversely, full shoulders may not be necessary in
mountainous areas when construction costs are high and
non-motorised vehicle flows are low. Where this is the
case, the minimum paved width should be 5.5 metres,
and side drains may need special consideration for safety
reasons (para 5.11).
2.11 Design Class C Roads in this category will
normally be paved. Recommendations for 1.0 metre
surfaced shoulders are similar to those for Design Class
D, but the extra 0.5 metre carriageway width to give a
total paved width of 7.5 metres will allow easier passing.
Full shoulders may be omitted in mountainous or
difficult terrain where the costs of achieving desired
cross-sections are very high.
2.12 Design Class B The carriageway of 6.5 metres will
allow vehicles to pass with sufficient clearance for there
to be little speed reduction or lateral movement. The
minimum 1.0 metre shoulder will allow easier overtaking
of stopped vehicles as well as the movement of some
non-motorised traffic. Shoulders should be paved to
provide a total paved width of 8.5
metres. At high levels
of flow, where there are substantial traffic movements of
wide non-motorised vehicles, such as bullock carts, it
may be advisable to increase shoulder width in some
circumstances up to a maximum of 2.5 metres, or provide
special segregated facilities.
2.13 Design Class A Levels of traffic flow will be such
that stopped vehicles blocking any part of the
carriageway will be likely to cause a significant hazard.
Hence, normal practice will be to provide a 2.5 metre
shoulder, at least one metre of which should be paved.
However, shoulder width may be reduced to 1.0 metre in
difficult terrain where construction costs are high. The
shoulder would also be available for non-motorised
traffic and should be paved. However, in view of the
potentially high levels of service and
8associated speeds on this class of road, it is
recommended that non-motorised traffic be discouraged
and alternative segregated facilities provided where
possible.
2.14 Dual carriageway construction should be
considered where design flows approach about 15,000
vehicles per day. Design of dual carriageways is outside
the scope of this guide and reference should be made to
the Australian (NAASRA 1980) and British (Department
of Transport 1981) standards. The flow value of 15,000
is arbitrary and, in industrialised countries, wider single
carriageway roads have been found to carry up to 20,000
to 30,000 vehicles per day, albeit with some reduction in
speeds.
PASSING PLACES
2.15 The lowest Design Class with a width of 3.0 (2.5)
metres will not allow passing and overtaking to occur
and passing places must he provided. The increased
width at passing places should be such as to allow two
trucks to pass, ie a minimum of 5.0 metres total width,
and vehicles would be expected to stop or slow to a very
low speed.
2.16
Normally, passing places should be located every
300 to 500 metres depending on the terrain and
geometric conditions. Account should be taken of sight
distances, the likelihood of vehicles meeting between
passing places and the potential difficulty of reversing. In
general, passing places should be constructed at the most
economic locations as determined by terrain and ground
condition, such as at transitions from cut to fill, rather
than at precise intervals.
2.17 The length of individual passing places will vary
with local conditions and the sizes of vehicles in
common use but, generally, a length of 20 metres
including tapers will cater for most commercial vehicles
on roads of this type.
2.18 A clear distinction should be drawn between,
passing places and lay-bys. Lay-bys may be provided for
specific purposes, such as parking or bus stops, and allow
vehicles to stop safety without impeding through traffic.
CURVE WIDENING
2.19 Widening of the carriageway on low radius curves
will be essential to allow for the swept paths of larger
vehicles, and the necessary tolerances in lateral location
as vehicles follow a curved path.
2.20 Widths should be increased on horizontal curves to
allow for the swept paths of trucks and to allow drivers to manoeuvre when approaching other vehicles. The
required amount of widening is dependent on the
characteristics of the vehicles using the road, the radius
and length of the curve, and lateral clearances.
Carriageway widening is also necessary to present a
consistent level of driving task to the road users, to
enable them to remain centred in lane and reduce the
likelihood of either colliding with an oncoming vehicle
or driving onto the shoulder.
2.21 The following levels of widening are
recommended.
2.22 Single lane roads (3.0m basic width)
Curve radius (m)
20 30 40 60
Increase in width (m) 1.50 1.00 0.75 0.50
These values for widening on tight low speed bends have
been based on a typical two-axle truck with an overall
width of 2.5 metres, wheel base of 6.5 metres and overall
length of 11.0 metres. This type of truck is typical of the
two-axle vehicles to be found in most developing
countries. Articulated vehicles have not been considered
explicitly as they are not common on Access roads.
2.23 Two lane roads
Curve radius (m) <50 50-149 150-299300-
400
Increase in width (m) 1.50 1.00 0.75 0.50
2.24 The above values are guidelines only and there
will be many situations in which widening is neither
necessary nor cost-effective.
2.25 Widening should be applied on the inside of a
curve and be gradually introduced over the length of the
transition.
2.26 On the narrower two lane roads of Design Class C
and D, particularly if there are high flows of trucks, it
may be desirable to widen the roads on crest vertical
curves. Widening of 0.5 metre should be considered
where K values are within one Design Class step of the
minimum for safe stopping sight distance.2.27 On lower Design Class roads, E and F, which have
substantial curvature requiring local widening, it may be
practical to increase width over a complete section to
offer a more consistent aspect to the driver. This
enhancement of the standards should be undertaken
where other advantages such as easier construction or
maintenance can be identified and where the additional
costs are acceptably small. This argument may also be
appropriate for sections of lower curvature on roads of
Design Classes C and D.
LATERAL AND VERTICAL CLEARANCES
2.28 Typical maximum truck heights are 4.2 metres
and, to allow adequate vertical clearance and the
transport of abnormal loads, a 5.0 metres vertical
clearance should generally be allowed for in the design.
2.29 Lateral clearances between roadside objects and
the edge of the shoulder should normally be 1.5 metres.
This may be reduced to 1.0 metre where the cost of
providing the full 1.5 metres is high.
2.30 Much smaller clearances will sometimes be
necessary at specific locations such as on bridges,
although a minimum of 1.0 metre will remain desirable.
Minimum overall widths in such circumstances should be
sufficient to allow the passage of traffic without an
unacceptable reduction in speed, which will depend on
the length of the reduced width section and levels of
motorised and non-motorised traffic flow. Separate
facilities should be provided for pedestrians where
possible.
CROSSFALL
2.31 Crossfall should be sufficient to provide adequate
surface drainage whilst not being so great as to be
hazardous by making steering difficult. The ability of a
surface to shed water varies with its smoothness and
integrity. On unpaved roads, the minimum acceptable
value of crossfall should be related to the need to carry
surface water away from the pavement structure
effectively, with a maximum value above which erosion
of material starts to become a problem.
2.32 The normal crossfall should be 3 per cent on paved
roads and 4 to 6 per cent on unpaved roads. Shoulders
having the same surface as the carriageway should have
the same cross slope. Unpaved shoulders on a paved road
should be 2 per cent steeper than the crossfall of the
carriageway. The precise choice of crossfall on unpaved
roads will vary with construction type and material rather
than any geometric design requirement. In most
circumstances, crossfalls of 5 to 6 per cent should be
used, although the value will change throughout the
maintenance cycle.
9 CARRIAGEWAY MARKINGS
2.33Carriageway marking should be provided on
all two-way paved roads.
2.34The edge of the carriageway should be
delineated by continuous lines and may be supported by
surfacing road studs. or other features. The lines should
be situated on the shoulder immediately adjacent to the
running surface and should be at least lOOmm in width.
Alternatively or additionally, delineation can be provided
more permanently by sealing the shoulder with a
different coloured aggregate to the running surface. (If,
contrary to these recommendations, an unsealed shoulder
is adopted, the first 150mm should be sealed for marking
purposes).
2.35Centre line markings are also recommended on
roads of at least 5 metres width designed for two lane
operation in order that a driver may correctly locate his
lateral position. These markings should be 100mm wide
and normally be discontinuous. except where overtaking
is restricted. and may be supported by the use of road
studs.
2.36Within the requirements for centre line and
edge markings, local standards and manuals should be
used or developed to provide uniformity of marking
throughout a national road network. All road markings
should conform to international standards.
PROVISION FOR NON-MOTORISED
TRAVELLERS
2.37Consideration needs to be given to the
movement of pedestrians. cyclists and animal drawn
vehicles either along or across the road. Measurements or
estimates of such movements should be made, where
possible, to give a firmer basis for making decisions on
the design.
2.38At very low flows of motorised traffic, the
problem of interaction is likely to be small. However,
care must be taken to ensure that adequate sight distances
and/or warnings are given to a driver as he approaches
any area of high activity such as a village.
2.39 As flows become greater, the conflicts between
slow and fast moving traffic will increase and additional
widths of both shoulder and running surface may be
necessary. The increase in width will vary with the
relative amounts of traffic, their characteristics and the
terrain, and should be related to the needs of individual
countries and regions as well as individual sections of
road. In view of the relatively high costs normally
involved in widening, care should be taken to ensure that
only those sections of shoulder are widened which are
justified by local demand.
2.40Recommendations for shoulder widths in these
situations are given in paras 2.5-2.13.
1
0
102.41There may be substantial movements of
pedestrians and non-motorised vehicles which will
generally be attracted by the surface quality and all
weather properties of roads. Special provisions should be
made in situations where such flows are significant with
respect to the level of motorised vehicle movements.
Some localised shoulder improvements may be
appropriate as non-motorised traffic generally increases
near towns and villages. Two features which are
recommended where large numbers of non-motorised
users travel on the shoulders are:
• The shoulders should be sealed
•
They should be clearly segregated by the use of
edge of carriageway surface markings or other
measures.
Special crossing facilities should be provided where
possible and necessary.
2.42On high speed roads with substantial flows of
motorised vehicles, non-motorised traffic should be
given a separate segregated by a physical barrier such as
a kerb. Crossing movements should also be concentrated
at specific locations and special crossing facilities
provided. Traffic approaching these facilities should be
given adequate warning and stopping sight distances
which are greater than minimum values should be
provided where possible.
RIGHTS-OF-WAY
2.43 It is recommended that the rights-of-way
should extend to a minimum of three metres from the
edge of the road works. This right-of-way should
normally be marked by a fence for road Design Classes
A and B, and as appropriate for the lower Design
Classes.
2.44The right-of-way must include the acquisition
of land necessary for the provision of special facilities for
pedestrians and other non-motorised road users.
Consideration should also be given to the acquisition of
land for short cuts and paths for pedestrians where they
exist away from the road.
2.45Rights-of-way may be reserved for future
upgrading of the alignment, although this would not be
normal practice. 3. HORIZONTAL ALIGNMENT
CIRCULAR CURVES
3.1 When vehicles negotiate a curve, a sideways
frictional force is developed between the tyres and road
surface. This friction must be less than the maximum
available friction if the bend is to be traversed safely. For
any given curve and speed, superelevation may be
introduced to enable a component of the vehicle's weight
to reduce the frictional need. The general relationship for
this effect is:
2
V
R =
127 (e+f)
where: R = Radius of curve (metres)
V = Speed of vehicles (km/h)
e = Crossfall of road (metres per metre)
f = Coefficient of side friction force developed
between the vehicles tyres and road
pavement.
The value of e may represent the simple removal of
adverse crossfall or include superelevation.
3.2 The side friction factor may be considered to
be the lateral force developed by the driver on a level
road. The technical evidence indicates that lateral
accelerations, and hence side friction factors, increase
with reduced radii of curvature and increased speed. The
range is considerable and values of "f' found from public
road measurements have varied from just over 0.1 for
high speed roads to over 0.5 on lower speed roads. The
results of empirical studies have indicated 0.22 as a value
of "f' above which passengers experience some
discomfort. The much higher values found on low radius
curves indicate that drivers and passengers have a much
higher tolerance in these situations. The values of "f'
chosen to calculate minimum radii requirements in this
guide range from 0.15 to 0.33. A substantial reserve
exists between these comfort and control related values,
and those at which the vehicle would start to slide
sideways.
3.3In this guide, it is recommended that curves are
designed such that it is necessary for vehicles travelling
at the design speed to steer into a bend.
3.4The minimum radii values shown in Table 1.2
were derived on the basis of sideways friction factors and
superelevation. In some situations with minimal lateral
clearances, sight distance will be the factor controlling
minimum radii. Sight distances may be improved by
increasing curve radius or sight distance across the inside
of the curve.3.5Where only small numbers of specialist
vehicles are involved and the costs of improving the
alignment are high, not all vehicles can expect to traverse
a curve on a single lane road in a single manoeuvre and
reversing may be necessary.
ADVERSE CROSSFALL
3.6The normal crossfall on a road will result in a
vehicles on the outside lane of a horizontal curve needing
to develop high levels of frictional force to resist sliding;
the amount of increase being dependent on speed, curve
radius and crossfall. In order to achieve the necessary
cornering stability, it is recommended that adverse
crossfall is removed. The identification of speed and
radius combinations at which this should occur is rather
subjective as there is no evidence linking adverse
crossfall to accident risk. A side friction factor of 0.07
has been taken as giving suitable minimum radii below
which adverse crossfall should be removed. With a
normal crossfall of 3 per cent, this value results in a
minimum radii shown in Table 3.1. Values for unpaved
roads are based on a 4 per cent crossfall which is the
minimum crossfall that should be allowed before
maintenance is carried out if effective cross-drainage is
still to be provided.
TABLE 3.1: MINIMUM RADII OF CURVES
BELOW WHICH ADVERSE CROSSFALL
SHOULD BE REMOVED
* Values in the brackets are the design speeds in
km/h with zero lateral accelerations for 3 per cent
crossfall ie the speeds at which curve can be negotiated
with “hands off” (approximately one speed design step
lower).
3.7 The values shown in the table are approximate
and cut-off levels should be varied to offer consistency to
the driver. For example, two adjacent horizontal curves
on a road link, one of which is marginally above the cut-
off whilst the other is marginally below the minimum
radii given, should be treated in a similar manner in the
design.
11 F ig.3. 1 Superevelation design curves
3.8 Removal and restoration of adverse crossfall should
take place over similar distances to superelevation as
described in the following Section.
SUPERELEVATION
3.9 For small radius curves and at higher speeds, the
removal of adverse crossfall alone will be insufficient to
reduce frictional needs to an acceptable level, and
crossfall should be increased by the application of
superelevation. A minimum radius is reached when the
maximum acceptable frictional and superelevation
derived forces have been developed. These minimum
radii values are identified in Figure 3.1 for maximum
levels of superelevation of 10 per cent. These relate to
paved roads only. Although this percentage is rather
arbitrary, it is widely considered to be a value above
which drivers may find it difficult to remain centred in
lane as they negotiate a bend.
3.10 On unpaved roads, the crossfall is designed to
remove rainwater quickly and effectively, and will be
dependent on local conditions and materials. Values of
superelevation lower than the value of the crossfall will
fail to drain the surface, whist higher values will be likely
to result in erosion. On unpaved roads, the maximum
superelevation will therefore be the elimination of
adverse crossfall (see Table 3.1).
12
3.11 Where transition curves are used (paras 3.14-19),
superelevation should be applied over the length of the
transition curves. Otherwise it should be introduced such
that two thirds are applied prior to the start of the circular
curve.
3.12 For curves with radii above the minimum values,
but below the values at which adverse crossfall should be
eliminated, it is advisable to improve passenger comfort
by introducing superelevation and reducing the sideways
force. Intermediate values of superelevation are given in
Figure 3.1.
3.13 On paved roads with unsealed shoulders, the
shoulders should drain away from the paved area to
avoid loose material being washed across the road.
TRANSITION CURVES
3.14 The characteristic of a transition curve is that it
has a constantly changing radius. Transition curves may
be inserted between tangents and circular curves to
reduce the abrupt introduction of the lateral acceleration.
They may also be used to link straights or two circular
curves.
3.15 In practice, drivers employ their own transition on
entry to a circular curve and transition curves contribute
to the comfort of the driver in only a limited number of situations. However, they also provide convenient
sections over which superelevation or pavement
widening may be applied, and can improve the
appearance of the road by avoiding sharp discontinuities
in alignment at the beginning and end of circular curves.
For large radius curves, the rate of change of lateral
acceleration is small and transition curves are not
normally required.
3.16Several methods exist for the calculation of
transition curves and any may be used in most situations.
The rate of pavement rotation method has been adopted
here. The rate of pavement rotation is defined as the
change in crossfall divided by the time taken to travel
along the length of transition at the design speed. The
length of transition curve is derived from the formula:
e.V
L
s =
3.6n
where L
s = Length of transition curve (metres)
e = Superelevation of the curve (metres per metre)
V= Design speed (km/h)
n = Rate of pavement rotation (metres per metre
per second)
3.17The same values of rate of change of pavement
rotation should be used to calculate the minimum length
(L
c) over which adverse camber should be removed on
a tangent section prior to the transition:
e
n
.V
Lc =
3.6n
Where L
c =Length of section over which adverse
camber is removed
e
n =Normal crossfall of the pavement (metres
per metre).
3.18 The length of transition curve (L
s) is used to
apply the superelevation, with the adverse camber
removed on the preceding section of tangent (L
c). The
change from normal cross-section to full superelevation
at the start of the circular curve is achieved over the
superelevation run-off distance which is the sum of L
s
and L c.
3.19Several relationships are available to calculate
the coordinates of a transition curve. The shift, ie the
offset of the start of the circular curve from the line of
the tangent, should be at least 0.25 metres for appearance
purposes. The transition should be omitted if the shift is
less than this value.
OTHER CONSIDERATIONS
3.20For small changes of direction, it is often
desirable to use large radius curves. This improves the
appearance of the road by removing rapid changes in
edge profile. It also reduces the tendency for drivers to
cut the comers of small radius curves. Providing the
curve radii are sufficiently large, visibility should not be
restricted enough to prevent safe overtaking.
3.21The use of long curves of tight radii should be
avoided where possible, as drivers at speeds other than
the design speed will find it difficult to remain in lane.
Curve widening reduces such problems. In such
situations, it will usually be more important to provide
adequate overtaking opportunities with longer straights
and tighter curves, and to overcome terrain constraints,
than to allow for detailed operational problems.
3.22Abrupt changes in direction from successive
curves should be avoided where possible by the inclusion
of a tangent section in between. This will allow
appropriate changes to be made in crossfall and
superelevation.
3.23Successive curves in the same direction should
also be separated by an appropriate tangent, as drivers
are unlikely to anticipate what may be an abrupt change
in radial acceleration.
13 4. VERTICAL ALIGNMENT
COMPONENTS OF THE VERTICAL ALIGNMENT
4.1The two major aspects of vertical alignment are
vertical curvature, which is governed by sight distance
and comfort criteria, and gradient which is related to
vehicle performance and level of service
4.2Vertical curves are required to provide smooth
transitions between consecutive gradients and the simple
parabola is recommended for these. The parabola
provides a constant rate of change of curvature, and
hence visibility, along its length and has the form:
G.L x2
y =
200 L
where y = vertical distance from the tangent to the
curve (metres)
x = horizontal distance from the start of the
vertical curve (metres)
G = algebraic difference in gradients (%)
L = length of vertical curve (metres)
CREST CURVES
4.3The minimum lengths of crest curves have
been designed to provide sufficient sight distance during
daylight conditions. Longer lengths would be needed to
meet the same visibility requirements at night on unlit
roads. Even on a level road, low meeting beam headlight
illumination may not even show up small objects at the
design stopping sight distances. However, it is
considered that these longer lengths of curve are not
justified as high objects and vehicle tail lights will be
illuminated at the required stopping sight distances on
crest curves. Vehicles will be identified by the
approaching illumination and drivers should be more
alert at night and/or be travelling at reduced speed.
4.4The greater sight distances required to provide
safe overtaking opportunities are not easily provided on
crest curves. If full overtaking sight distance cannot be
obtained, the design should aim to reduce the length of
crest curves to provide the minimum stopping sight
distance, thus increasing overtaking opportunities on the
gradients on either side of the curve.
4.5Two conditions exist when considering
minimum sight distance criteria on vertical curves. The
first is
14where sight distance is less than the length of the vertical
curve, and the second is where sight distance extends
beyond the vertical curve. Consideration of the properties
of the parabola results in the following relationships for
minimum curve length to achieve the required sight
distances:
G.S
2
For S