DEAS  1206 : 2024  
ICS 93.080.01  
 
© EAC 2024                                                                                                                               First Edition 2024   
                DRAFT EAST AFRICAN STANDARD  
Geometrical design of  roads  — Code of practice  
EAST AFRICAN COMMUNITY  DEAS 1206 : 2024  
ii © EAC  2024  – All rights reserved  
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Violators may be pr osecuted . DEAS  1206 : 2024  
© EAC 2024  – All rights reserved  iii 
 Contents  Page 
Foreword  ................................ ................................ ................................ ................................ ............................  iv 
1 Scope  ................................ ................................ ................................ .......... Error! Bookmark not defined.  
2 Conformance  ................................ ................................ ............................. Error! Bookmark not defined.  
3 Normative references  ................................ ................................ ................ Error! Bookmark not defined.  
4 Terms and definitions  ................................ ................................ ............... Error! Bookmark not defined.  
5 Symbols (and abbreviated terms)  ................................ ............................ Error! Bookmark not defined.  
6 Clause  ................................ ................................ ................................ ......... Error! Bookmark not defined.  
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8 Special  ................................ ................................ ................................ ........ Error! Bookmark not defined.  
Annex  A (normative)  Annex title  ................................ ................................ .......... Error! Bookmark not defined.  
A.1 General  ................................ ................................ ................................ .......Error! Bookmark not defined.  
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Annex  B (informative)  Which styles correspond to which element  — Quick reference g uideError! Bookmark not defined.  
Bibliography  ................................ ................................ ................................ ............ Error! Bookmark not defined.  
 DEAS 1206 : 2024  
iv © EAC  2024  – All rights reserved  
 Foreword  
Development of the East African Standards has been necessitated by the need for harmonizing requirements 
governing quality of products and services in the East African Community. It is envisaged that through 
harmonized standardization, trade barriers that are encountered when goods and services are exchanged 
within the Community will be removed.  
The Community has established an East African Standards Committee  (EASC)  mandated to develop and 
issue East African Standards  (EAS) . The Committee is composed of representatives of the National 
Standards Bodies in Partner States, together with the representatives from the public and private sector 
organizations  in the community .  
East African Standards are developed through  Technical Committees that are representative of key 
stakeholders including government, academia, consumer groups, private sector and other interested parties. 
Draft East African Standards are circulated to stakeholders through the National Standards Bodies in the 
Partner States. The comments received are discussed and incorporated before finalization of standards, in 
accordance with the Principles and procedures for development of East African Standards . 
East African Standards are subject to review, to keep pace with technological advances. Users of the East 
African Standards are therefore expected to ensure that they always have the latest versions of the standards 
they are implementing .  
The committee responsible  for this document is Technical Committee EASC/TC 028, Construction of Roads, 
Rail, Air and Water Transport Infrastructure.  
Attention is drawn to the possibility that some of the elements of this document may be subject of patent 
rights. EAC shall not be held responsible for identifying any or all such patent rights.  DEAS  1206 : 2024  
© EAC 2024  – All rights reserved  v 
 Introduction  
The geometric design of roadways deals with the dimensions and layout of visible features of a roadway with 
the objective of creating the roadway facility to the characteristic and behaviour of drivers, vehicles, traffic and 
terrain. Geometric design is based on specified design standards and controls which depend on a wide range 
of roadway system factors such as roadway functional classification, terrain of the area that the roadway 
traverses, levels of service, road user characteristics, vehicle characteristics (length, width, height, wheelbase, 
weight, including acceleration and deceleration characteristics and maximum speed) and design hourly traffic 
volume, sight distances and choice of horizontal and vertical alignment, road cross section, intersection type, 
available funds, safety, and social and environmental factors.  
These factors are often interrelated. For instance, design speed depends on the functional classification which 
is usually related to expected traffic volume. The design speed may also depend on the terrain, particularly in 
cases where limited funds are available. In most cases, the principal factors used to determine the standards 
to which a particular roadway will be designed are the level of service to be provided, expected traffic volume, 
design speed, and the design vehicle. These factors, coupled with the basic characteristics of the driver, 
vehicle, and road, are used to determine standards for the geometric characteristics of the roadway, such as 
cross sections and horizontal and vertical alignments. Appropriate geometric design standards should be 
selected to maintain a desired level of service for a known proportional of different types of vehicles.  
The use of geometric design standards fulfils three inter -related objectives. Firstly, standards are intended to 
provide minimum levels of safety and comfort for drivers; secondly, they provide the framework for economic 
design; and thirdly, they ensure a consistency of alignment. As noted above, the design standards adopted 
must take also into account the environmental road conditions, traffic characteristics, and driver behaviour.  
It is expected that these guidelines of practice for or manual on roadway geometric design should include the 
following important aspects for proper roadway geometric design: Roadway functional classification, Design 
controls and criteria: these cover the characteristics of vehicles, road users, and traffic that act as criteria for 
the optimisation or improvement in design of the various road classes, Design elements: these are principal 
design elements which include sight distance, superelevation, horizontal and vertical alignments, and other 
elements of geometric design, They are joined together to create a facility that serves the traffic in a safe and 
efficient manner, consistent with the facility’s intended function. Thus, each alignment element should 
complement others to produce a consistent, safe, and efficient design, Cross section elements, Intersections, 
Road furniture and other facilities.  
The implementation of these guidelines will have the following impacts among others:  
a) It will ensure uniform roadway geometric standards and hence the optimum balance between road 
infrastructure construction cost and road user cost is obtained, considering road safety issues and natural and 
human environmental aspects in the EAC Region.  
b) It will improve efficiency of the road transport system by minimisation of road crashes and energy 
consumption.  
c) These guidelines recommend the width, height, wheelbase and minimum turning radius dimensions of 
vehicles such as the 22 m Interlinks, which have recently been permitted to traverse EAC road network, 
conform to the respective dimensions of the design vehicles documented in the existing design manuals for 
EAC Partner States and therefore will be accommodated by the existing intersections on the EAC road 
network. On the other side, all EAC Partner States will need to check whether headroom under bridge 
structures is appropriate ( that is, complies with the recommended value of 5.5 m) and make improvements, if 
necessary.  
d) Furthermore, it is proposed that the recommended standards be implemented in all new road designs, 
and road rehabilitation, reconstruction and widening projects.  
 WORKING DRAFT EAST AFRICAN STANDARD  DEA S 1206 : 2024  
 
© EAC 2024  – All rights reserved  1 
 Geometrical design of roads — Code of practice  
1 Scope  
This Draft East Africa n Standard provides general guidelines for geometrical design of roads.   
It is applicable to all mobility and access roads of Class 1, Class 2, Class 3, Class 4 and Class 5 as explained 
in 5.1.  
2 Normative references  
The following documents are referred to in the text in such a way that some or all of their content constitutes 
requirements of this document. For dated references, only the edition cited applies. For undated references, 
the latest edition of the referenced document (including any amendments) applies.  
There are no normative references in this document.  
3 Terms and definitions  
For the purposes of this document, the following terms and definitions apply.  
ISO and IEC maintain terminological databases for use in standardization at the following addresses:  
— IEC Electropedia: available at http://www.electropedia.org/  
— ISO Online browsing platform: available at http://www.iso.org/obp  
3.1 
lane  
portion of carriageway designed to accommodate a single line of moving road vehicles  
3.2  
traffic lane  
part of a carriageway intended to accommodate a single stream of traffic in one direction  
Note 1 to entry: A traffic lane can be bi -directional, such as a single lane road, a two -way turn/overtaking lane, or a 
reversible flow lane  
Note 2 to entry: Some jurisdictions allow supplemental uses of a traffic lane, such as multiple motorcycles sharing the 
width of a traffic lane or allowing a bicycle to use the edge of a lane.  
3.3 
design speed  
a selected speed used to determine the various geometric design features of the roadway to ensure a safe operation of 
vehicles.   
2 © EAC  2024  – All rights reserved  
 Note 1 to entry: The deign speed is the highest continuous speed at which individual vehicles can travel with safety on the 
road when weather conditions are favourable, traffic volumes are low and the design features of the road are the 
governing condition for safety.  
3.4 
road reserve  
strip of land legally awarded to the roads agency, in which the road is or will be situated and where no other 
work or construction may take place without permission from the roads agency. The width is measured  at right 
angle from the centreline of median to both sides of the roadway.  
Note 1 to entry  The diagrams illustrating road reserve are given in Figure 1 a and Figure 1b  
3.5 
specific road  
road leading to a specific facility and which branches off from:  
a) a national road; and  
b) a district and city road and a location where specific development activities are carried out.  
3.6 
design vehicle  
vehicle with representative characteristics such as weight, dimensions and operating characteristics used to 
stablish highway design controls for accommodating vehicles of the designated classes  
3.7 
access control  
regulation of public access rights to and from properties abutting the highway facilities  
3.8 
Levels of Service (LOS)  
qualitative measure that express the effectiveness of the road in terms of operating conditions. It is a measure 
of the effect of traffic flow factors, such as speed and travel time, interruptions, freedom of manoeuvre, driver 
comfort and convenience, and indirectly, safety and operation costs.  
Note 1 to entry: Levels of Service (LOS) is a measure used to quantify traffic flow and congestion by assigning quality 
levels of traffic based on performance measure such as speed, density, etc.  
 
3.9 
climbing lane  
auxiliary lane added outside the continuous lanes and has the effect of reducing congestion in the through 
lanes by removing slower -moving vehicles from the traffic stream, thereby maintaining capacity and freedom 
of operation on the carriageway  
Note 1 to entry: The climbing lane is also referred to as a crawler lane, truck lane  
3.10 
passing lane  
auxiliary lane that is added outside the continuous lanes and is typically provided on level sections of the 
route.  
3.11 
clear zone  
unobstructed, relatively flat area provided beyond the edge of the roadway for the recovery of errant vehicles.  
3.12  
median  
portion of a divided road separating the travelled ways for traffic in opposite directions including inside 
shoulders   
© EAC 2024  – All rights reserved  3 
 3.13  
headroom  
required height to allow traffic to pass safely under objects restricting the height  
3.14  
intersection  
as an area, shared by two or more roads, whose main function is to provide for the change of route directions.  
3.15  
grade separation  
represents a crossing of two highways (or a highway and a railroad) at different levels  
3.16  
interchange  
represents a system of interconnecting roadways, in conjunction with one or more grade separations, to 
provide for the movement of traffic between two or more roadways on different levels.  
3.17 
access road  
lowest level of road in the network hierarchy with the function of linking traffic to and from community/public 
areas, either direct to adjacent urban centres or to the nearby road network;  
3.18  
unclassified road  
roads that are not assigned to classes or categories  
3.19  
Average Daily Traffic (ADT)  
total traffic volume during a given time period in whole days greater than one day and less than one year 
divided by the number of days in that time period  
3.20  
Average Annual Daily Traffic (AADT)  
total yearly traffic volume in both directions divided by the number of days in the year  
3.21 
back slope  
area proceeding from ditch bottom to the limit of the earthworks            
3.22 
bridge  
structure erected for carrying a road over a river or any other gap with a single span -length or sum of span 
lengths of 4.0 m or more. Where the clear span is less than four metres, the structure is a culvert  
3.23 
carriageway  
part of the road constructed for use by moving traffic, including auxiliary lanes, climbing lanes, and passing 
places but excluding shoulders  
3.24 
travelled way  
That part of the carriageway used for the movement of vehicles, exclusive of auxiliary lanes, bus -bays and 
shoulders.  
3.25 
circular curve  
usual curve configuration used for horizontal curves   
4 © EAC  2024  – All rights reserved  
 3.26 
clear zone  
unencumbered roadside recovery area  
3.27 
compound curve  
curve consisting of two or more arcs of different radii curving in the same direction and having a common 
tangent or transition curve where they meet  
3.28 
cross -fall/ chamber   
tilt or transverse inclination of the cross -section of a carriageway, expressed as a percentage  
3.29  
cross -section  
vertical section of the road at right angles to the centre line  
3.30 
deflection angle  
successive angles from a tangent subtending a chord and used in setting out curves  
3.31  
design traffic volume  
number of vehicles or persons that pass over a given section of a lane or carriageway during a time period of 
one hour or more  
 
3.32 
design volume  
volume determined for use in design, representing traffic expected to use the road  
3.33  
embankment  
portion of the road prism composed of approved fill material, which lies above the original ground and is 
bounded by the side slopes, extending downwards and outwards from the outer shoulder breakpoints and on 
which the pavement is constructed  
3.34  
road function  
objective of providing a particular road link in terms of being "national", "districts or "access"  
3.35 
horizontal curve/curvature  
curve/succession of curves, normally circular, in plan  
3.36 
transition curve  
curve in which the radius changes continuously along its length, used for the purpose of connecting a straight 
with a circular curve, or two circular curves of different radii  
3.37 
vertical curve  
curve on the longitudinal profile of a road,                                 
3.38 
vertical alignment  
direction and course of the centre line in profile   
© EAC 2024  – All rights reserved  5 
 3.39 
gradient  
rate of rise or fall on any length or road, with respect to the horizontal. It is usually expressed as a percentage 
of vertical rise or fall in meters, 100 m of horizontal distance  
3.40  
horizontal alignment  
direction and course of the road centreline in plan  
3.41 
horizontal curve  
curve in plan  
3.42  
object height  
assumed height of a notional object on the surface of the roadway used for the purpose of determining sight 
distance  
3.43 
Passenger Car Equivalent Unit (PCU)  
unit of road traffic, equivalent for capacity purposes to one normal private car which is thus the unit and other 
vehicles are converted to the same unit by a factor depending on their type and circumstances  
3.44 
passing sight distance  
minimum sight distance on two -way single roadway roads that shall be available to enable the driver of one 
vehicle to pass another vehicle safely and comfortably without interfering with the speed of an oncoming 
vehicle travelling at the design speed, should it come into view after the overtaking manoeuvre is started  
3.45  
pavement  
part of a road designed to withstand the weight or loading by traffic  
3.46  
reaction time  
time taken by the driver to perceive the hazard ahead plus the time taken to activate the brake  
3.47 
shoulder  
That part of the verge adjacent to the carriageway designed to provide a safe stopping area in an emergency, 
a travel path for pedestrians and cyclists (where there is no other facility for them) and lateral support for the 
road pavement.  
3.48  
side slope  
area between the outer edge of shoulder or hinge point and the ditch bottom  
3.49  
sight distance  
distance visible to the driver of a passenger car measured along the normal travel path of a roadway to the 
roadway surface or to a specified height above the roadway surface, when the view is unobstructed by traffic  
3.50 
single lane road  
road consisting of a single traffic lane serving both directions, with passing bays   
6 © EAC  2024  – All rights reserved  
 3.51 
speed  
rate of movement of vehicular traffic or of specified components of traffic, expressed in kilometres per hour 
(km/h)  
3.52  
road hump  
physical obstruction, placed transversely on the surface of the carriageway for the purpose of reducing traffic 
speed  
3.53 
super elevation  
inward tilt or transverse inclination given to the cross section of a roadway throughout the length of a 
horizontal curve to reduce the effects of centrifugal force on a moving vehicle; expressed as a percentage     
3.54                
functional classification   
grouping of streets and roads according to the character of the service they are intended to provide  
3.55  
design hourly volume   
projected hourly volume that is used in the design   
3.54  
roadway  
portion of a road, including shoulders, intended for vehicular use  
3.57  
stopping sight distance  
distance required by a driver of a vehicle travelling at a given speed, to bring his vehicle to a stop after an 
object on the roadway becomes visible. It includes the distance travelled during the perception and reaction 
times and the vehicle braking distance   
3.58 
street  
road which has become partly or wholly defined by buildings established along one or both frontages   
3.59  
tangent  
portion of a horizontal alignment of straight geometrics   
3.60  
traffic  
vehicles, pedestrians and animals travelling along a route  
3.61  
traffic flow  
number of vehicles or persons that pass a specific point in a stated time, in both directions unless otherwise 
stated  
3.62  
traffic volume  
number of vehicles or persons that pass over a given section of a lane or a roadway during a time period of 
one hour or more. Volume is usually expressed in one of the terms: average annual daily traffic (AADT), 
Average Daily Traffic (ADT) and hourly volume  
3.63  
road  
way for vehicles and for other types of traffic which may or may not be lawfully usable by all traffic   
© EAC 2024  – All rights reserved  7 
 3.64 
street  
road which has become partly or wholly defined by buildings established along one or both frontages  
3.65 
highway  
a main road, especially one connecting major towns or cities  
3.66 
Decision Sight Distance (DSD)  
The decision sight distance is the distance needed for a driver to detect an unexpected or otherwise difficult 
to-perceive information source or condition in a roadway environment that may be cluttered, recognize the 
condition or its potential threat, select an appropriate speed and path and initiate a complex manoeuvre  
4 Symbols and/or abbreviated terms  
DHV  Design Hourly Volume  
ADT  Annual Daily Traffic  
NMT  Non-Motorised Traffic  
AADT  Average Annual Daily Traffic  
SSD Stopping Sight Distance  
PSD Passing Sight Distance  
PCU  Passenger Car Units  
AASHTO  American Association of State Highway and Transportation Officials  
CF     Cross Fall  
DSD    Decision Sight Distance  
DTV    Design Traffic Volume  
ESALs   Equivalent Single Axle Loads  
LCVC    Length of the Crest Vertical Curve  
NCF    Normal Cross Fall  
PCU    Passenger Car Equivalent Unit  
ROW    Right -Of-Way  
SS     Side Slopes  
SW    shoulder Width  
TEF    Traffic load Equivalent Factor  
VPD    Vehicle Per Day  
LOS Level Of Service   
8 © EAC  2024  – All rights reserved  
 f  side friction  
e  super elevation  
 
5 Road classification, access control and road reserve  
5.1 Road classification and access control  
5.1.1  For the purpose of design, roads design classes shall be Class 1, Class 2, Class 3, Class 4 and 
Class 5 (see Table 1), which are further classified into functional categories according to the character of the 
services they intend to provide  
5.1.2  The major considerations in classifying highways and streets functionally are mobility and access. 
The extent and degree of access control is thus a significant factor in defining the functional class of highway 
and streets. Based on function, roads are broadly classified as:  
a) mobility road:  that are intended to provide the highest level of mobility and the highest speeds over the 
longest uninterrupted distance. For mobility, high or continued speeds are desirable and variable or low 
speeds undesirable.  
b) access roads:  that are intended to provide limited mobility and are the primary access to residential 
areas, businesses, farms, and other local areas. For access roads, low speeds are desirable and high 
speeds undesirable.  
5.1.3 Roads may be designed to provide both mobility and accessibility and this leads to further 
classification of roads based on function as:  
a) arterial roads:  with full or partial access control. which are intended to provide for high levels of safety 
and efficiency in the movement of large volumes of traffic at high speeds and therefore require a regulated 
limitation of access to enhance their primary function of mobility. Access to and egress from these 
facilities is permitted only at controlled locations such as an entrance and exit ramps. The access or 
egress may also be permitted directly from or to abutting land or via a limited number of at -grade 
intersections  
b) collector roads:  that provide less mobility and therefore balance both functions of mobility and 
accessibility, combines aspects of both arterials and local streets. They collect traffic for movement 
between arterial streets and local roads and provide access to abutting land, and;  
c) access roads : that are intended to provide a limited mobility and are the primary access to residential 
areas, businesses, farms, or other abutting properties with unrestricted accessibility.  
The road functional and design class, their conventional names and access control shall be as per Table 1.  
Table  1 — Road classification and access control  
Function  Design Class  Name conventions  Minimum 
Intersection Spacing  Access 
control  
Mobility roads  Class 1  International trunk roads  
National trunk road  Principal 
Arterial/  
Freeway/  
Expressway  2500 m  Full  
© EAC 2024  – All rights reserved  9 
 Class 2  National roads  
Provincial roads  
Regional roads  
District roads  Major 
arterial/Highway  800 spacing  Full or 
partial  
Class 3  Regional roads  
District roads  
Secondary roads  Minor 
arterial/Collector  600 m  Full or 
partial  
Access roads  Class 4  District roads  
Secondary roads  Collector roads  50 m  Partial  
Class 5  Minor roads  
Local streets  Local streets  15 m  Unrestricted  
 
5.2 Road reserve dimensions and demarcation  
5.2.1 The width of road reserve for each functional design class shall be as per Table 2  and shall be 
measured at right angle to the centreline of the median as illustrated in Figure 1 a and Figure 1b .  
5.2.2  The road reserve shall be demarcated by two parallel lines at required distance from the centreline of 
the median  as illustrated in Figure 1a . 
5.2.3  The road reserve for city roads in the cities and for specific roads may be determined in accordance 
with the applicable city master plan or depending on the int ended use of the specific roads.  
Table 2 — Width of road reserve  
Design Class  Name conventions  Minimum r oad 
reserve (m)  
Class 1  International trunk roads  
National trunk road  Principal Arterial/  
Freeway/  
Expressway  50 -  
Class 2  National roads  
Provincial roads  
Regional roads  
District roads  Major 
arterial/Highway  44 
Class 3  Regional roads  
District roads  
Secondary roads  Minor 
arterial/Collector  24 
Class 4  District roads  
Secondary roads  Collector roads  24 
Class 5  Minor roads  
Local streets  Local streets  10 
  
10 © EAC  2024  – All rights reserved  
 
 
Figure 1a — Plan view of roadway illustrating road reserve   
 
 
 
 
 
 
 
 
  
© EAC 2024  – All rights reserved  11 
 
 
Figure 1b — Typical section illustrating the road reserve   
 
6 Design control and criteria  
6.1 Design Vehicle and Vehicle Characteristics  
The physical characteristics and proportions of vehicle of various sizes using highway are key control in 
geometrical design of highway. The design vehicle representative characteristics and operating characteristics 
that are used to establish highway design controls for accommodating vehicles of the designated classes.  
Three general classes of design vehicles covered by this standard are:  
1. Passenger cars class , which include passenger cars of all sizes, sport or utility vehicles, minivans, vans 
and pickup trucks.  
2. Buses class;  which includes intercity, city transit, school and articulated buses.  
3. Trucks class ; includes single -unit trucks, truck tractor -semitrailer combinations and trucks with 
semitrailers in combination with full trailers.  
Dimensions for design vehicles representing vehicles within these three general classes are given in Table 3.  
NOTE  1 for the purpose of geometrical design, each design vehicle has larger physical dimensions and larger 
minimum turning radius than most vehicle in its class. The larger design vehicles are usually accommodated in highway 
design.   
12 © EAC  2024  – All rights reserved  
 NOTE  2 The dimensions used to define design vehicles are not averages or maxima, nor are they legal limiting 
dimensions. They are, in fact, typically the 85th percentile or 15th percentile value of any given dimension. The design 
vehicles are therefore hypothetical vehicles, selected to represent a particular vehicle class.  
Table 3 — Design vehicle dimensions  
Design vehicle 
type Symbol    Dimension (m)   
Length  width  Height  Minimum design  
turning radius  Wheelbase  
Passenger cars   
Passenger car  DV1 5.8 2.1 1.3 7.31 3.35 
Buses   
Single unit bus  DV2 12.1 2.6 4.1 12.8 7.6 
Articulated bus  DV3 18.3 2.6 3.4 12.1 6.7 + 5.9  
Trucks     
Single unit truck  DV4 9.1 2.6 4.6 12.8 6.1 
Semi -trailer  DV5 18.2 2.6 4.6 12.65  5.5+10.0  
Rigid truck and 
drawbar trailer  
combination  DV6 22.0 2.6 4.6 12.0 4.6+8.2  
Interlink (with a 
short truck 
tractor)  DV7 22.0 2.6 4.6 12.0 4.1+6.52a+7.94  
Semi -trailer with 
long trailer  DV8 21.0 2.6 4.6 13.7 6.1 & 12.8  
       
a   distance between SU rear wheels and trailer front wheels  
 
6.2 Terrain  
6.2.1  The terrain category is an important factor that affects roadway geometric design. It affects the 
general constructional requirements, cost of construction driving safety and design speed.  
6.2.2  For the purpose of design, the terrain to be considered for geometrical design of highways and 
streets shall be flat, rolling, mountainous and steep, depending on the range of prevailing cross 
slope/transversal Slope as given in Table 4.  
Table 4 — Design terrain Category  
Terrain Category  Cross slope/Transversal Slope  
Flat  0% - 10% 
Rolling  10% to 25%  
Mountainous  25% to 60%   
© EAC 2024  – All rights reserved  13 
 Steep  More than 60%  
 
6.3 Driver Performance  
6.3.1  The diver’s performance such as perception -reaction time, height of driver’s eye, and object height are 
important controls to be considered for the geometrical design of highways.  
6.3.2  The Perception -Reaction  time which is the time required for the driver to process information after 
detecting and recognising a given situation shall be taken as 2.5 seconds.  
6.3.3  The recommended value for the height of Driver’s Eye shall be 1.05 m above the roadway surface.  
6.3.4  The height of the object that might appear as a threat to the driver is considered for computation of the 
required sight distance. The recommended values for vertical height for stopping sight distance, decision sight 
distance, passing sight distance shall be as per Table 5.  
Table 5 — Object height values  
 Object height (m)   
Stopping sight distance  Decision sight distance  Passing sight distance  
0.60 0.60 1.08 
 
6.4 Pedestrians and Cyclists Performance  
6.4.1  For the design of Non -Motorised Transport (NMT) facilities, the pedestrian and cyclists performance 
considered are pedestrian speed and age, bicycle speed, coefficient of friction for breaking, minimum height, 
and width for bicycle operation.  
6.4.2  The designer shall consider the speed of 1.2 m/s. Where complex elements such as channelization 
and separate turning lanes are featured, it is recommended that the designer should assess alternatives that 
will assist older pedestrians. Pedestrian safety can be enhanced by the provision of median refuge islands of 
sufficient width at wide intersections, and lighting at locations that demand multiple information gathering and 
processing.  
6.4.3  The typical bicycle performance criterial and operation characteristics shall be as per Table 6.  
Table 6 — Bicycle operation characteristics  
Bicycle  speed  
(km/h)  Minimum height (m)  Width (m)  coefficient of friction 
for braking  
 13 - 24  1.0  1.5 0.32 
 
6.5  Traffic Characteristics  
6.5.1  Traffic Composition  
6.5.1.1  It is not practical to design for a heterogeneous traffic stream and, for this reason; trucks and 
other types of vehicles are converted to equivalent Passenger Car Units (PCUs). The number of PCU's 
associated with a single vehicle type is a measure of the impedance that it offers to the passenger cars in the 
traffic stream. The number of equivalent passenger cars equalling the effect of one truck is dependent on the 
roadway gradient and, for two -lane highways, on the available passing sight distance.   
14 © EAC  2024  – All rights reserved  
 Table 7 gives the recommended Passenger Car Units (PCUs) varying with the terrain category.  
Table 7 — Passenger car equivalent factors  
Vehicle type   Terrain   
Flat Rolling  Mountainous  
 Pcu  
Passenger cars  1.0 1.0 1.5 
Light goods vehicles  1.0 1.5 3.0 
Medium goods vehicles  2.5 5.0 10.0 
Heavy goods vehicles  3.5 8.0 20.0 
Buses  2.0 4.0 6.0 
Motor cycles, scooters  0.5 1.0 1.5 
Pedal cycles  0.5 0.5 NA 
 
6.5.2  Projection of Future Demands  
The projection of future traffic demand should be based on a period of 20 years while designing for new 
roadway  and for reconstruction , 10 years for rehabilitation  and upgrade . 
6.5.2  Design Hourly Volume (DHV)  
6.5.2.1  The design hourly volume is estimated a follow:  
DHV = AADT x K or DHV = ADT x K  
Where,  
DHV   is Design Hourly Volume  
AADT   is Average Annual Daily Traffic  
ADT  is Annual Daily Traffic  
K  is a factor estimated from the ratio of the 30th HV to the AADT from a similar site  
6.5.2.2  The shall  be used to estimate K factors for estimation of DHV and in the absence of such factors 
the 30th -highest DHV can be estimated by applying 0.15 and 0.10 to ADT for rural highways and urban roads, 
respectively.  
6.6  Design Speed  
 
 
Design speed is a selected speed used to determine the various geometric design features of the roadway 
such as curvature, super elevation, and sight distance. The assumed design speed is usually expected to be a 
logical one with respect to the terrain, anticipated operating speed, the adjacent land use, and the functional 
classification of highway.  
Table 8 gives the values of design speed based on the classes of highway and the terrain category.   
© EAC 2024  – All rights reserved  15 
 Table 8 — Design speed  
Road class    Design Speed (km/h)   
Flat terrain  Rolling terrain  Mountainous  Steep  
Mobility roads  Class 1  120 100 60 60 
Class 2  110 80 50 50 
Class 3  100 80 50 40 
Access roads  Class 4  80 60 40 40 
Class 5  60 40 30 30 
 
NOTE  Except for local streets where speed controls are frequently included intentionally, it is advised to use 
as high a design speed as practical to attain a desired degree of safety, mobility, and efficient within the 
constraints of environmentally quality, economics, aesthetics, and social or political impacts. Also, above -
minimum design values should be used, where practical.  
6.7 Levels of Service  
6.7.1  The quality of traffic service provided by specific road facility under specific traffic demands shall be 
defined by means of Levels Of Service (LOS). There are six level of service ranging from level A which is the 
free flow condition, where drivers can maintain their desired speed (low volume and high speed); to level of 
service F which is the forced flow condition (most congested) where the traffic density is the maximum with 
drivers subjected to frequent stop -go and queues. There should be six LOS:  
A. Free flow:  traffic flows at or above the posted speed limit and motorists have complete mobility between 
lanes. The average spacing between vehicles should be about twenty -seven -car lengths. Motorists have a 
high level of physical and psychological comfort. The effects of incidents or point breakdowns are easily 
absorbed.  
B. Reasonably free flow:  LOS A speeds shall be maintained, manoeuvrability within the traffic stream 
should be slightly restricted. The lowest average vehicle spacing should be about sixteen -car lengths. 
Motorists still have a high level of physical and psychological comfort.  
C. Stable flow at or near free flow:  Ability to manoeuvre through lanes should be noticeably restricted and 
lane changes shall require more driver awareness. Minimum vehicle spacing should be about eleven -car 
lengths. Minor accidents may still have no effect but localized service may have noticeable effects and 
traffic delays may form behind the accident. This should be the target LOS for some urban and most rural 
roads.  
D. Approaching unstable flow:  Speeds slightly decrease as traffic volume slightly increases. Freedom to 
manoeuvre within the traffic stream shall be limited and driver comfort levels decrease. Vehicles should 
be spaced about eight -car lengths apart. Level D should be a common goal for urban streets during peak 
hours, as attaining LOS C would require prohibitive costs and social impact via bypass roads and lane 
additions.  
E. Unstable flow:  operating at capacity: Flow becomes irregular and speed varies rapidly because there are 
virtually no usable gaps to manoeuvre in the traffic stream and speeds rarely reach the posted limit. 
Vehicle spacing should be about six -car lengths. Any disruption to traffic flow, such as merging ramp 
traffic or lane changes, can create a shock wave affecting downstream traffic. Any incident can create 
serious delays. The drivers' level of comfort becomes poor.  
F. Forced or breakdown flow:  Every vehicle moves in lockstep with the vehicle in front of it, with frequent 
slowing required. Travel time should not be predicted, with generally more demand capacity.  
The choice of level of service shall generally be based on economic considerations. Table 9 gives the 
guidelines for selection of level of service depending on the class of road being designed and the category of 
terrain.   
16 © EAC  2024  – All rights reserved  
  
 
Table 9 — Guidelines for selection of design levels of service  
 Level of service for specified combinations of terrain and area type  
Function  Class  Flat terrain  Rolling 
terrain  Mountainous 
terrain  Steep terrain  
Mobility 
roads  Class 1  B B C C 
Class 2  B C D D 
Class 3  C C D D 
Access roads  Class 4  D D D D 
Class 5  D D D D 
 
6.7.2  The maximum volume that can be carried at any selected level of service is referred to as the service 
volume for that level. The traffic flow rates that can be served at each level of service are termed as service 
flow rate.  
6.7.2  Once a level of service has been identified as applicable for design, the accompanying service flow 
rate logically become the design service flow rate, implying that if the traffic flow rate using the facility exceeds 
that value, operating conditions will fall below the level of service for which the facility was designed.  
6.8 Access Control and Access Management  
6.8.1  Partner states should put in place statutes, land -use ordinances, geometric design policies, and 
driveway regulations for managing and controlling access  
6.8.2  The principal advantages of controlling access are the preservation or improvement of service and 
the reduction of crash frequency and severity. The proper access control and access management is usually 
achieved through a number of controls which can be categorized as follow:  
a) full control of access,  which means that preference is given to through traffic by providing access 
connections by means of ramps with only selected public roads and by prohibiting crossings at grade and 
direct private driveway connections.  
b) partial control of access , some preference should be given to through traffic. Access connections, which 
may be at -grade or grade -separated, are provided with selected public roads and private driveways  
c) access management,  that involves providing (or managing) access to land development while 
simultaneously preserving the flow of traffic on the surrounding road system in terms of capacity, speed, 
and low crash frequency and severity. Access management applies to all types of roads and streets. It 
calls for setting access policies for various types of roadways, keying designs to these policies, having the 
access policies incorporated into legislation, and having the legislation upheld in the courts.  
d) driveway/entrance regulations , may be applied even though no control of access is obtained. Each 
abutting property is permitted access to the street or highway; however, the location, number, and 
geometric design of the access points are governed by the regulations  
6.8.3  The extent of access management depends upon the location, type, and density of development, 
and the nature of the highway system. Access management actions involve both the planning and design of 
new roads and the retrofitting of existing roads and driveways. The following principles define access 
management techniques.  
a. Classify the road system by the primary function of each roadway. traffic movement.   
© EAC 2024  – All rights reserved  17 
 b. Limit direct access to roads with higher functional classifications.  
c. Locate traffic signals to emphasize through traffic movements.  
d. Locate driveways and major entrances to minimize interference with traffic operations.  
e. Use curbed medians and locate median openings to manage access movements and minimize conflicts.  
6.8.4  Access can be managed and controlled by means of statutes or transport agency, land -use 
ordinances, geometric design policies, and driveway regulations. Key elements of access management 
include  
a) defining the allowable access and access spacing for various classes of highways,  
b) providing a mechanism for granting variances when reasonable access cannot otherwise be provided, 
and 
c) establishing means of enforcing policies and decisions.  
These key elements, along with appropriate design policies, should be implemented through a legal code that 
provides a systematic and supportable basis for making access decisions.  
7 Design elements  
7.1 Sight distances  
7.1.1  Stopping Sight Distance (SSD)  
7.1.1.1  The road shall be designed to have adequate stopping sight distance (SSD) to allow a driver to see 
objects in the road with sufficient time to manoeuvre around them or to stop.  
7.1.1.2  The stopping sight distance (SSD) is given by the formula below, and the typical design values 
corresponding to different design speed are given Table 10.  
 
where  
  is Stopping Sight Distance  
V  is Design speed, km/h  
t  is Brake reaction time, 2.5 sec  
  is Deceleration rate, m/s2 
Table 10 — Stopping site distance on level road  
         
         
 
Design Brake reaction Braking distance Stopping sight distance (m)  
18 © EAC  2024  – All rights reserved  
 speed 
(km/h)  distance (m)  
 on level (m)  Calculated (m)  Design (m)  
20 13.9 4.6 18.5 20 
30 20.9 10.3 31.2 35 
40 27.8 18.4 46.2 50 
50 34.8 28.7 63.5 65 
60 41.7 41.3 83.0 85 
70 48.7 56.2 104.9  105 
80 55.6 73.4 129.0  130 
90 62.6 92.9 155.5  160 
100 69.5 114.7  184.2  185 
110 76.5 138.8  215.3  220 
120 83.4 165.2  248.6  250 
130 90.4 193.8  284.2  285 
NOTE     Brake reaction distance prediction on a time of 2.5s, deceleration rate of 3.4m/s used to determine 
calculated sight distance   
 
 
7.1.1.3   The brake reaction time commonly considered as adequate for complex conditions as those of 
laboratory test should be 2.5 sec. The need for greater reaction time can be encountered in most complex 
situations such as those found at multiphase at grade intersections.  
7.1.1.4  The approximate braking distance of a vehicle level roadway travelling at a design speed of the 
roadway may be determined from the following equation:  
 
where  
 is braking distance, m;  
V is design speed, km/h;   
 is deceleration rate, m/ s2. 
Note  The braking distance shall be computed on base of the applicable design speed using a 
deceleration rate of 3.4 m/s2 and a reaction time of 2.5 sec.  
7.1.2  Passing Sight Distance (PSD)  
7.1.2.1  Passing Sight Distance shall be the minimum sight distance on two -way single roadway roads that 
shall be available to enable the driver of one vehicle to pass another vehicle safely without interfering with the 
speed of an oncoming vehicle travelling at the design speed.  
7.1.2.2  Within the sight area, the terrain should be the same level or a level lower than the roadway. 
Otherwise, for horizontal curves, it may be necessary to remove obstructions and widen cuttings on the 
insides of curves to obtain the required sight distance.  
7.1.2.3  Care shall be taken in specifying passing/no -passing zones in areas where the sight distance 
may be obscured in the future due to vegetative growth.  
7.1.2.4  Passing sight distance shall be measured between an eye height of 1.08 m and an object height 
of 1.08 m. The sight line near the centre of the area inside the curve should be approximately 0.24 m higher  
© EAC 2024  – All rights reserved  19 
 than the stopping sight distance. In cut sections, the resultant lateral dimension for normal highway cross 
sections (1V:2H to 1V:6H back slopes) between the centreline of the inside lane and the midpoint of the sight 
line is from 0.5 m to 1.5 m greater than that for stopping sight distance.   
7.1.2.5  The passing sight distance should be calculated on the basis of a distance required for a 
successful overtaking manoeuvre and makes adequate provision for an aborted manoeuvre in the case of a 
truck attempting to pass another truck. The minimum passing sight distance should be the total of four 
components: d1 + d2 + d3 + d4  
where  
d1 is the distance travelled during the perception -reaction time and during initial acceleration to the point 
where the passing vehicle just enters the right lane;  
d2 is the distance travelled during the time passing vehicle is travelling in the right lane;  
d3- distance between the passing vehicle and the opposing vehicle at the end of the passing manoeuvre;   
d4 is the distance moved by the opposing vehicle during two thirds of the time the passing vehicle is the 
left lane (usually taken to be 2/3 d2).  
7.1.2.6  The recommended minimum passing sight distance for two lane -roads are given in Table 11.  
Table 11 — Passing sight distance for design of two -lane highways  
Design 
speed 
(km/h)  Assumed speeds(km/h)  Passing sight distance (m)  
Passed vehicle  Passing vehicle  Total  passing  
sight distance  Rounded 
for design  
30 29 44 200 200 
40 36 51 266 270 
50 44 59 341 345 
60 51 66 407 410 
70 59 74 482 485 
80 65 80 538 540 
90 73 88 613 615 
100 79 94 670 670 
110 85 100 727 730 
120 90 105 774 775 
130 94 109 812 815 
7.1.3 Decision Sight Distance (DSD)  
Decision sight distance shall offer drivers additional margin for error, afford them sufficient length to 
manoeuvre their vehicles at the same reduced speed, other than to just stop, its values are substantially 
greater than stopping sight distances (See Table 12).  
Table 12 — Decision Sight Distance  
Design  speed   
(km/h)  Decision Sight Distance  
(m)   
Avoidance manoeuvre   
1   2   3  4  5  
50  70  155  145  170  195   
20 © EAC  2024  – All rights reserved  
 60  95  195  170  205  235  
70  115  325  200  235  275  
80  140  280  230  270  315  
90 170 325 270 315 360 
100 200 370 315 355 400 
110 235 420 330 380 430 
120 265 470 360 415 470 
      
NOTE 1  Avoidance manoeuvre A: Stop on rural road –t = 3.0 s;  
NOTE 2  Avoidance manoeuvre B: Stop on urban road –t = 9.1 s;  
NOTE 3  Avoidance manoeuvre C: Speed/path/direction change on rural road 10.2 s < t < 11.2 s  
NOTE 4  Avoidance manoeuvre D: Speed/path/direction change on rural road 12.1 s < t < 12.9 s  
NOTE 5  Avoidance manoeuvre E: Speed/path/direction change on rural road 14.0 s < t < 14.5 s  
NOTE 6  Avoidance manoeuvres A and B are given by the following relationship :  
 
where  
DSD      is Decision Sight Distance, m  
t           is pre -manoeuvre time, sec  
V         is design speed, km/h  
         is driver deceleration, m/s2 
NOTE 7  Avoidance manoeuvres C, D and E are given by the following relationship:  
 
   
where  
DSD      is Decision Sight Distance, m  
t           is total pre -manoeuvre and manoeuvre time, sec  
V         is design speed, km/h  
 
7.2 Horizontal alignment  
7.2.1  Alignment between control points  
Alignment between control points should be designed to be as favourable as practical, consistent with the 
environmental impact, topography, terrain, design traffic volume, and the amount of reasonably obtainable 
right-of-way. Sudden changes between curves of widely different radii or between tangents and sharp curves 
should be avoided. Where crest vertical curves and horizontal curves occur together, greater than minimum 
sight distance should be provided so that the horizontal curves are visible to approaching drivers. The 
following basic curve formula for moving mass on a curve features the relationship between the side friction, 
the rate of roadway superelevation, the radius of curve and the design.  
 
where   
© EAC 2024  – All rights reserved  21 
 e  is rate of roadway superelevation, per cent;   
f  is side friction (demand) factor;  
v  is vehicle speed, m/s;   
g  is gravitational constant, 9.81 m/s2;  
V is vehicle speed, km/h;  
R is radius of curve, measured to a vehicle’s centre of gravity, m.  
NOTE: For unpaved roads, the range depends on the type of terrain  
7.2.2  The straight  
Long straight shall be avoided as it increases the danger from headlight glare and usually lead to excessive 
speeding. In hot climate areas, long tangents have been shown to increase driver fatigue and hence cause 
accidents.  
The straight section shall not have length greater than 20xV D (where V D is design speed in km/h, and a 
minimum length of 6xV D m should also be adopted between circular curves following the same direction.  
7.2.3  Minimum Radius of Horizontal Curve  
7.2.3.1  The minimum radius of horizontal curve design, for a given design speed, shall be limited by 
maximum allowable side friction ( fmax), which is usually based on comfort standard, maximum super elevation 
rate ( emax) and the necessity to maintain stopping sight distance.  
7.2.3.2  For a given speed, minimum curve radius is limited by maximum allowable side friction, which is 
usually based on comfort required, maximum super - elevation rate for the curve, and the necessity to 
maintain stopping sight distance. The minimum radius can be calculated directly from the simplified curve 
equation as:  
 
where 
Rmin  minimum radius of horizontal curve  
fmax   maximum allowable side friction  
emax   maximum super elevation  
7.2.3.3  Maximum superelevation rates shall be controlled by such factors as climate conditions 
(frequency and amount of rain); terrain conditions (flat, rolling or mountainous); type of area (rural, urban); and 
frequency of slow -moving vehicles whose operation might be affected by high superelevation rates. The 
maximum super elevation for roads in urban roads shall be 4% and 10% on roads in the rural areas.  
7.2.3.4  The maximum side friction factors corresponding to the given design speed shall be as per the 
Table 13.  
Table 13 — Side friction factors for different design speeds  
Design Speed 
(km/h)  Maximum  
super Limiting values 
of side friction  Total  side 
friction Calculated 
radius (m)  Rounded radius 
(m)  
22 © EAC  2024  – All rights reserved  
 elevation   
e (%) (f) (e/100 + f)  
      
50 4 0.16 0.20 98.4 100 
60 4 0.15 0.19 149.2  150 
70 4 0.14 0.18 214.3  215 
80 4 0.14 0.18 280.0  280 
90 4 0.13 0.17 375.2  375 
100 4 0.12 0.16 492.1  490 
110 4 0.11 0.15 635.2  635 
120 4 0.09 0.13 872.2  870 
      
50 6 0.16 0.22 89.47745  90 
60 6 0.15 0.21 134.9831  135 
70 6 0.14 0.2 192.9134  195 
80 6 0.14 0.2 251.9685  250 
90 6 0.13 0.19 335.6817  335 
100 6 0.12 0.18 437.4453  435 
110 6 0.11 0.17 560.4447  560 
120 6 0.09 0.15 755.9055  755 
      
50 8 0.16 0.24 82.021  80 
60 8 0.15 0.23 123.2455  125 
70 8 0.14 0.22 175.3758  175 
80 8 0.14 0.22 229.0623  230 
90 8 0.13 0.21 303.712  305 
100 8 0.12 0.2 393.7008  395 
110 8 0.11 0.19 501.4505  500 
120 8 0.09 0.17 666.9755  665 
      
      
50 10 0.16 0.26 75.71169  75 
60 10 0.15 0.25 113.3858  115 
70 10 0.14 0.24 160.7612  160 
80 10 0.14 0.24 209.9738  210 
90 10 0.13 0.23 277.3023  275 
100 10 0.12 0.22 357.9098  360 
110 10 0.11 0.21 453.6933  455 
120 10 0.09 0.19 596.7675  595 
      
50 12 0.16 0.28 70.30371  70 
60 12 0.15 0.27 104.9869  105 
70 12 0.14 0.26 148.3949  150 
80 12 0.14 0.26 193.8219  195 
90 12 0.13 0.25 255.1181  255 
100 12 0.12 0.24 328.084  330 
110 12 0.11 0.23 414.2417  415 
120 12 0.09 0.21 539.9325  540  
© EAC 2024  – All rights reserved  23 
       
 
7.2.4  Minimum and maximum lengths of curve  
For small deflection angles, it is required that curves shall be long enough to avoid the appearance of a kink. 
On the other hand, a long curve, particularly if it is of near -minimum radius, may causes tracking problems.  
The maximum length of curve shall not exceed 1000 m while the minimum length shall not be less than 300 
m. 
7.2.5  Transition curve  
7.2.5.1 A transition curve shall be provided to give the driver a natural path that is easy to follow as a vehicle 
enters or leaves a circular horizontal curve and at the same time provides a suitable arrangement for affecting 
the super elevation runoff.  
7.2.5.2 The minimum length of transition curve shall be defined based on consideration of driver comfort and 
shifts in the lateral position of vehicles and these two criteria are used together to determine the minimum 
length of spiral which can be computed as follow:  
 
  
Where:  
Ls,mim  minimum length of spiral, m  
pmin  minimum lateral offset between the tangent and circular curve (0.20 m)  
R  radius of circular curve, m  
V  design speed, km/h  
C maximum rate of change in lateral acceleration (1.2 m/s3)  
7.2.5.3 The maximum relative gradient between pavement edges as given in Table 14 will determine the 
minimum length for the runout and runoff and hence the minimum length of the transition curves.  
Table 14 — Maximum relative gradient  
Design Speed (km/h)  30 40 50 60 80 100 110 120 
Maximum relative gradient  1:133  1:143  1:154  1:167  1:200  1:227  1:244  1:263  
7.2.5.4 The length of spiral transition curve shall not be so long (relative to the length of the circular curve) 
that drivers are misled about the sharpness of the approaching curve. A conservative maximum length of 
spiral that can minimize the likelihood of such concerns can be computed as:  
 
Where:  
Ls,max maximum length of spiral, m  
 pmaxminimum lateral offset between the tangent and circular curve (1.0 m)  
R radius of circular curve, m   
24 © EAC  2024  – All rights reserved  
 7.2.5.5 The desirable lengths of spiral transition curves for street and highway design, that correspond to 2.0 
s of travel time at the design speed of the roadway are shown in Table 15.  
 Table 15 — Desirable length of spiral curve transition  
Design speed (km/h  20 30 40 50 60 70 80 90 100 110 120 130 
Spiral length (m)  11 17 22 28 33 39 44 50 56 61 67 72 
 
7.2.5.5 The use of longer spiral curve lengths that are less than Ls,max than is acceptable. However, where 
such longer spiral curve lengths are used, consideration should be given to increasing the width of the 
travelled way on the curve to minimize the potential for encroachments into adjacent lanes. Further, if the 
desirable spiral curve length shown in Table 15 is less than the minimum spiral curve length Ls,min , the 
minimum spiral curve length should be used in design.  
7.2.5.6  For the various design speeds, a radius corresponding to a specified centripetal acceleration can be 
calculated. Thus, a changing radius at a specific speed corresponds to a specific rate of change of centripetal 
acceleration. For comfort, the range varies between 0.4 and 1.3 m/s2 . If the radius of the circular curve is less 
than the values shown in Table 5 -8, then transition curves are required to achieve this degree of comfort. For 
curves of large radius, the rate of change of lateral acceleration is small and transition curves are not normally 
required. Transition curves are also unnecessary for roads of low design speeds or low classification.  
7.2.5.7  The typical transition curve is illustrated in Figure  2 and requirements for transition curve are given in 
Table 16 
 
Table 1 6 — Transition curve requirements  
Design Speed (km/h)  Transition required if radius of curve is less 
than:  
70 290 
80 380 
85 428 
90 480 
100 590 
110 720 
120 850 
Table 5 -8: Transition curve requirements (m)   
© EAC 2024  – All rights reserved  25 
 
 
  
  
Figure 2 — Elements of Super -elevation and transition curve   
7.2.5.8  Generally, the Euler spiral, which is also known as the clothoid, is used in the design of transition 
curves . The radius of curvature at a point on a curve is the radius of the circle that fits the curve at that point. It 
is calculated from:  
                                                       
  
The equation for the applicable clothoid transition curve is:  
A2   = R.L   
26 © EAC  2024  – All rights reserved  
 Where,  
A the clothoid parameter,  
R the radius at the end of the clothoid,  
 L the length of the clothoid.  
7.2.5.9  The radius (R) varies from infinity at the straight end of the spiral to the radius of the circular arc at 
the end of the transition.  The clothoid parameter, A, expresses the rate of change of curvature along the 
clothoid. Large values of A represent slow rates of change of curvature while small values of A represent rapid 
rates of change of curvature.  The most rapid rate of change of curvature is represented by the minimum 
permissible value of A min. The determination of A min is based upon considerations of the rate of change of 
centrifugal acceleration, super -elevation run -off, aesthetics, and the ratio of the radii of consecutive curves of 
a com pound curve as described below: - 
𝐴𝑚𝑖𝑛 = 0.21(𝑉𝐷)1.5 
Where,  
VD = Design Speed (km/h)  
The clothoid must have sufficient length to accommodate the super -elevation run -off That is:  
𝐴𝑚𝑖𝑛 = (𝑅. 𝐿𝑚𝑖𝑛) 0.5 
and for aesthetic reasons the shape of t he curve should be clearly visible therefore  
𝐴𝑚𝑖𝑛 = 𝑅/ 3 
Finally, the two branches of the clothoid (at either end of the tangent section) should have approximately the 
same parameters that would therefore produce a similar rate of change of curvature.  
 
7.2.6  Reverse Curves, Broken -back Curves, and Compound Curves  
7.2.6.1  Reverse curve  
This is a curve followed by another curve in the opposite direction (see Figure 3 ) The occurrence of abrupt 
reverse curves (i.e., a short tangent between two curves in opposite directions) should be avoided. Such 
geometrics make it difficult for the driver to remain within the lane. It is also difficult to super -elevate both 
curves adequately, and this may result in erratic operation.   
© EAC 2024  – All rights reserved  27 
 
 
 Figure 3 — Super -elevation of Reverse Curves  
 
7.2.6.2  Broken -back curve  
This is a curve followed by another curve in the same direction but with only a short tangent in between. 
Broken -back curves should be avoided except where very unusual topographical or right -of-way conditions 
dictate otherwise. Drivers do not generally anticipate successive curves in the same direction hence safety is 
compromised. Problems with super -elevation and drainage can also arise. A single curve is preferred if it is 
physically and economically feasible. The super -elevation is illustrated in Figure 4.  
28 © EAC  2024  – All rights reserved  
 
 
 Figure 4 — Super -elevation of broken -back curve  
 
 
7.2.6.3  Compound curve  
Compound curves as illustrated in (see Figure 5 ) are curves in the same direction but of different radii, and 
without any intervening tangent section. The use of compound curves provides flexibility in fitting the road to 
the terrain and other controls. Caution should, however, be exercised in the use of compound curves because 
the driver does not expect to be confronted by a change in radius once a curve has been entered, hence 
safety is compromised. Their use should be avoided especially where the curves need to be of short radius.  
If two successive circular curves in the same direction cannot be avoided, the connecting tangent should be at 
least 150 m long. The tangent should have a single cross fall rather than reverting to a normal camber for a 
short distance.  
Compound curves with large differences in curvature introduce the same problems as are found at the 
transition from a tangent to a small -radius curve. Where the use of compound curves cannot be avoided, the 
radius of the flatter circular arc should not be more than 50 % greater than the radius of the sharper arc, i.e., 
R1 should not exceed 1.5.R 2. A compound arc on this basis is suitable as a form of transition from either a flat 
curve or a tangent to a sharper curve, although a spiral transition curve is preferred.   
© EAC 2024  – All rights reserved  29 
 
 
Figure 5 — Super -elevation of compound curve  
 
7.2.7 Sight distance in horizontal curve  
As illustrated on Figure 6, the sight line for general use in design of horizontal curve is a chord of the curve, 
and the stopping sight distance is measured along the centreline of the inside lane around the curve. Figure 2 
is a design chart showing the horizontal sight line offsets needed for clear sight areas that satisfy stopping 
sight distance criteria for horizontal curves of various radii on flat grades.  
 
 
30 © EAC  2024  – All rights reserved  
 Figure 6 — Diagram illustrating components for determining horizontal sight distance   
 
Figure 7 — Design Controls for Stopping Sight Distance on Horizontal Curves  
The horizontal sightline offset is calculated using the following equ ation:  
] 
where  
HSO    is horizontal sight line offset, m;  
S         is stopping sight distance, m;  
R        is radius of curve, m.  
The chart from Figure 7 shall be used for corresponding applicable to design speeds and superelvations  
  
© EAC 2024  – All rights reserved  31 
 7.2.8 Super elevation  
7.2.8.1  Super -elevation development  
Super -elevation development has two components: Tangent run -out and super -elevation runoff. Tangent run -
out involves the rotation of the outside lane(s) of the cross -section from the normal camber, usually 2.5 per 
cent, to a zero cross fall. Super -elevation runoff then continues this rotation until a cross fall equal to the slope 
of the normal camber across the full width of the travelled way is achieved. From this point further, the entire 
width of the travelled way is rotated until the full super -elevation appropriate to the design speed and radius of 
curvature is achieved. The process is illustrated in Figure  8 for the case of rotation around the centerline.  
The axis of rotation can, in fact, be located anywhere across the cross -section or even outside it. Selection of 
its location is dependent on the constraints under which the super elevation has to be developed. This is 
particularly so in the case of super -elevation development in urban areas. These constraints could involve 
issues of drainage, aesthetics or fitting the cross -section to the topography. The problem to be solved is 
largely one of the location of the road edges relative to the ground line. In the case of a two -lane road, the axis 
of rotation would typically be located on the centerline. Other standard locations are the inside and outside 
edges of the travelled way.  
The rotation of dual carriageways often takes place around the outer edge of the median island so that the 
median shoulders rotate in concert with the travelled lanes.  
However, as in the case of the two -way road, no hard and fast rules can be laid down concerning the selection 
of the location of the axis of rotation. It could be at the centerline of the median, at the edge of the median or 
even having the entire cross -section rotating as a unit around one of other of the outer edges of the cross -
section. Each case would have to be considered on its own merits.  
 
 Figure 8 — Super -elevation of a two -lane carriageway about the centerline  
 
  
32 © EAC  2024  – All rights reserved  
 7.2.8.2   Super -elevation runoff  
The super -elevation runoff is the length of road needed to accomplish the change in cross slope from the first 
section in which the adverse crown was removed to the fully super -elevated section and is effected over the 
whole length of the spiral transition curve.  
Its end point is the beginning of the circular curve itself which is denoted by SC (the Spiral to Curve transition 
point) or, alternatively called PC (the Point of Curvature i.e., the point where the circular curve begins).  
The length of runoff, shown in Table 17 and illustrated in Figure 2and Figure 8, is the spiral length with the 
tangent -to-spiral point (TS) at the beginning and the spiral -to-curve point (SC) at the end. The length of the 
transition curve is proportional to the total super -elevation and should not be less than the values shown in 
Table 17. A simple practical rule is that it must not be less than the distance travelled in 2 seconds at the 
design speed.  
Table 1 7 — Minimum length of super -elevation run -off for two -lane roads  
Design Speed (km/h)  Run off (m)  
120 70 
110 65 
100 60 
90 50 
80 45 
70 40 
60 35 
50 30 
40 25 
30 20 
 
           Table 5 -7  
 
7.2.9 Curve widening  
7.2.9.1  Widening on curves and embankments  
Offtracking, common to all vehicles, especially prominent in larger ones, occurs when rear wheels don't 
precisely follow front wheels on curves. On flat curves at low speed, rear wheels track inside front wheels; on 
superelevated curves, this offtracking may vary due to tire slip angle and side friction. Wheel track positioning 
depends on speed and friction, compensating for lateral force on superelevation. At higher speeds, rear 
wheels may track outside front wheels. Offtracking, influenced by curve type, speed, and tire behavior, affects 
vehicle maneuverability and road safety, necessitating careful design considerations for curves, especially in 
accommodating larger vehicles.   
© EAC 2024  – All rights reserved  33 
 The widening on curves and high fills for Class 1 and 2 shall be as given in Table  18  
Table 18 — Widening on curves and high fills for class 1 and 2 roads  
Radius of 
curvature  Curve widening: 
Single lane (m)  Curve widening: 
Two lanes (m)  Maximum gradient (%)  
Height of fill (m)  Amount (m)  
>250  0.0 0.0 0.0 - 3.0 0.0 
120-250 0.0 0.6 3.0 - 6.0 0.3 
60-120 0.0 0.9 6.0 - 9.0 0.6 
40-60 0.6 1.2 Over 9.0  0.9 
20-40 0.6 1.5 Over 9.0  0.9 
Note   Curve with radius less than 20m are considered to be 20m (see 7.2.9.3 ) 
 
7.2.9.2   Derivation of design values for widening on horizontal curves  
 
7.2.9.2.1   The track width (U)  (see Figure 9) , also called swept path width, for a vehicle on a curve or turn 
is the sum of the tangent track width (u) and offtracking. Offtracking varies based on curve radius, articulation 
points, and wheelbase lengths. The formula for calculating track width on a curve (U) includes these factors.  
 
where:  
U track width on curve, m;  
U track width on tangent (out -to-out of tires), m;  
R radius of curve or turn, m; and  
Li wheelbase of design vehicle between consecutive axles (or sets of tandem axles) and articulation points, 
m. 
7.2.9.2.2   This equation applies to various curve configurations and considers wheelbase lengths and 
curve radii. For simplicity, on two -lane highways, the curve radius at the centerline is often used. On turning 
roadways, the radius is based on the path of the outer front wheel. Wheelbase lengths (Li) include distances 
between axles and articulation points. Different types of vehicles, like single -unit trucks or articulated vehicles, 
have specific Li values. Negative terms may arise if articulation points are in front of drive axles or due to rear -
axle overhang. Lateral clearance allowance (C) ensures safe vehicle passage, with assumed values varying 
based on lane widths.   
34 © EAC  2024  – All rights reserved  
 
 
Figure 9. Track Width for Widening of Traveled Way on Curves  
7.2.9.2.3   The front overhang width (FA)  (see Figure 10)  is the distance from the outer edge of the tire path 
of the outer front wheel to the outer front edge of the vehicle body. FA on curves and turning roads is 
influenced by curve radius, vehicle's front overhang, and wheelbase. For tractor -trailer setups, only the tractor 
unit's wheelbase matters.  
 
where:  
A  front overhang of inner lane vehicle, m;  
L   wheelbase of single unit or tractor, m.  
7.2.9.2.3  The rear overhang width (FB) is the radial distance between the inner rear tire path edge and the 
vehicle body's inside edge. For passenger cars (P), the body width exceeds the rear wheels' out -to-out width 
by 0.3 m [1 ft], resulting in FB = 0.15 m [0.5 ft]. In truck design vehicles, the body width matches the rear 
wheels' out -to-out width, hence FB = 0.  
 
7.2.9.2.4   The extra width allowance (Z) accounts for challenges in maneuvering on curves and driver 
behavior variations. It's an empirical value influenced by traffic speed and curve radius. Z provides additional 
radial pavement width to accommodate these factors.   
© EAC 2024  – All rights reserved  35 
 
 
Figure  10 Front Overhang for Widening of Traveled Way on Curves  
 
Figure  11 —Extra Width Allowance for Difficulty of Driving on Traveled Way  
7.2.9.2.5   The required width of the traveled way on a curve (Wc) considers various factors including track 
width (U), lateral clearance (C), front overhang width (FA), and a provision for driving difficulty (Z)  (see Figure 
11). These components are detailed in the section on "Derivation of Design Values for Widening on Horizontal 
Curves." Selecting an appropriate design vehicle, typically a truck due to its significant offtracking, is 
necessary to calculate Wc. The WB -15 [WB -50] design vehicle is commonly used for two -lane highways, but 
alternative vehicles may be chosen based on specific traffic conditions.   
36 © EAC  2024  – All rights reserved  
 
 
where,  
w  widening of traveled way on curve, m;  
Wc  width of traveled way on curve, m;  
Wn  width of traveled way on tangent, m  
The width Wc is calculated by the  following equation : 
 
where:  
N = number of lanes;  
U = track width of design vehicle (out -to-out tires), m;  
C = lateral clearance, m;  
FA = width of front overhang of inner -lane vehicle, m;  
Z = extra width allowance, m.  
7.2.9.2.6   The calculated and design values for travaled way widening on two lane open highway curves 
are given in Table  19. 
Table  19Calculated and design values for traveled way widening on open highway curves (two -lane)  
 
Notes: Values shown are for WB -15 design vehicle and represent widening in meters. For other 
design vehicles, use adjustments in Table 3. Values less than 0.6 m may be disregarded. For 3 -lane 
roadways, multiply above values by 1.5. For 4 -lane roadways, multiply above values by 2.   
© EAC 2024  – All rights reserved  37 
  
 
 
 
 
 
 
 
 
 
 
 
7.2.9.2.7   The traveled way widening values on open highwa y curves shall be adjusted as per Table  20 
Table  20— Adjustments for Traveled Way Widening Values on Open Highway Curves (Two -Lane 
Highways,  
 
Notes: Adjustments are applied by adding to or subtracting from the values in Table 2. 
Adjustments depend only on radius and design vehicle; they are independent of roadway width  
38 © EAC  2024  – All rights reserved  
 and design speed. For 3 -lane roadways, multiply above values by 1.5. For 4 -lane roadways, 
multiply above values by 2.0.  
 
7.2.9.3   Width for turning roadways at intersections  
The typical pavement width shall for turning roadways shall be as per Table  21 Intersection turning roadways 
accommodate vehicles based on size, curvature radius, and speed, allowing for one - or two -way operation.   
Table  21 —Derived pavement widths for turning roadways for different design vehicles  
 
7.2.9.3   Switchback curves  
Switchback or hairpin curves  (see Figure  12) are curves with a radius of 20m or less but not less than 15m. 
They are mostly necessary in traversing mountainous and steep terrain,  
Switchback curves require a careful design to ensure that all design vehicles can travel through the curve. 
They must therefore provide for the tracking widths of the design vehicles as it indicated in the figure 1 & 2. 
Minimum outer radii for design vehicles DV2 through DV4 are 12.5m, 14.1m, and 12.5m, respectively. 
Minimum inner radii are 8m, 7.4m, and 6m, respectively. Switchback requirements can be determined which 
allow for:  
a) Passage of two opposing DV4 vehicles. This is recommended for road Classes 1 -2; however, these road 
classes should not use switchback curves  
b) Passage of a single DV4 and a DV1. This is recommended for road Classes 3 -4   
© EAC 2024  – All rights reserved  39 
 c) Passage of only a single DV4. This is recommended for road Classes 5 and single lane roads Figure 2. 
illustrates the elements of a switchback curve. By superimposing for design vehicle DV4 vehicle at a time 
on a Class 5 road.  
Note that this figure shows that the vehicle must use both lanes to make the maneuver. The radii minimum 
values which allow for this are: R = 10m, Ri = 6m, Rs = 14m  
 
Figure 12. Switchback Curve Layout  
Thus, although the normal carriageway width for a Class 5 road is 6.0m, at the switchback curve a width of 
least 8m is required if the road needs to allow for the passage of a single DV4 vehicle at a time. For provision 
for opposing DV4 vehicles passing at the same time, the width must be much greater. Requirements vary 
depending on road class, passage requirements, radius, deflection angle, and design standard, and a 
template should be used based on the design vehicle turning radii to ensure that all design vehicles can 
negotiate each switchback. It is important to provide relief from a severe gradient through the switchback.  
 7.3 Vertical alignment  
7.3.1  Gradients  
Maximum grades vary, depending on the type of facility, and usually do not constitute an absolute standard. 
The effect of a steep grade is to slow down the heavier vehicles and increasing operating costs and the extent 
to which any heavier vehicle is slowed depends on both the steepness and length of the grade. The minimum 
gradient for roads in cutting taken into account to avoid standing water in side ditches, is recommended to  
The recommended minimum grade for the roadway design is 0.5% and the recommended maximum grades 
which are based on which varies with the terrain category are given in Table 22. 
Table 22 — Recommended maximum grade  
Terrain  Maximum gradient (%)  
Flat 6 
Rolling  4 – 8  
40 © EAC  2024  – All rights reserved  
 Mountainous  7 – 12 
Steep  12 - 18 
  
 
7.3.2  Climbing lanes  
The climbing lane shall be provided with the effect of reducing congestion in the through lanes by removing 
slower -moving vehicles from the traffic stream. As such, it is used to match the Level of Service on the rising 
grade to that prevailing on the level sections of the route. The climbing lane help the maintenance of an 
acceptable level of service over a section of the route and the enhancement of road safety by the reduction of 
the speed differential in the through lane.  
The following are warrants for climbing lanes, that are based on both the speed and volume of the traffic:  
i) Upgrade traffic flow rate in excess of 200 vehicles per hour  
ii) Upgrade truck flow rate in excess of 20 vehicles per hour  
iii) One of the following conditions exists:  
a) A 15 km/h or greater speed reduction is expected for a typical heavy truck  
b) Level of service E or F exists on the grade  
c) A reduction of two or more levels of service is experienced when moving from the approach segment to 
the grade.  
In addition, safety considerations may justify the addition of a climbing lane regardless of grade or traffic 
volumes enhancement of road safety by the reduction of the speed differential in the through lane.  
7.3.3  Vertical curves  
7.3.3.1  General  
Minimum lengths of vertical curves shall be based on stopping sight distance criteria and the length of a curve 
(in metres) should not be shorter than the design speed. In the case of freeways, the minimum length shall not 
be less than twice the design speed in km/h.  
For crest vertical curves, the minimum length standard depends on the stopping sight distance, the height of 
the driver’s eye measured from an eye height of 1.05 m, and the height of the object of 0.6 m to be seen over 
the crest of the curve.  
For sag curves, stopping sight distance is based on the distance illuminated by the headlight height of 0.6 m 
at night.  
7.3.3.2  Crest curve  
7.3.3.1  Vertical curves shall be designed to be simple in application and should result in a design that is safe 
and comfortable in operation, pleasing in appearance, and adequate for drainage. The major control for safe 
operation on crest curves is the provision of ample sight distances for the design speed.  
7.3.3.2  It is recommended that all vertical curves should be designed to provide at least the stopping sight 
distances and additional sight distance should be provided at decision points. For driver comfort, the rate of 
change of grade should be kept within tolerable limits. This consideration is most important in sag vertical 
curves where gravitational and vertical centripetal forces act in opposite directions. Appearance also should 
be considered in designing vertical curves. A long curve has a more pleasing appearance than a short one;  
© EAC 2024  – All rights reserved  41 
 short vertical curves may give the appearance of a sudden break in the profile due to the effect of 
foreshortening.   
7.3.3.3  Parameters illustrated in Figure 13 shall be used in determining the length of a parabolic vertical 
curve needed to provide a specified value of sight distance. The basic equation of a crest vertical curve in 
terms of algebraic difference in grade and sight distance is given below:  
When S is less than L,  
 
When S is greater than L,  
 
where  
L         is length of vertical curve, m;  
A  is algebraic difference in grades, %;   
S is sight distance, m;         
h1 height of eye above roadway surface, m;  
h2 height of object above roadway surface, m.  
7.3.3.4   When the height of the eye and of the object are respectively 1.08 m and 0.6 m as used for 
stopping sight distance; equation should be simplified to the following:  
When S is less than L,  
 
When S is greater than L,   
42 © EAC  2024  – All rights reserved  
 
 
Figure 13 — Parameters considered in determining the length of the crest vertical curve to provide 
sight distance  
The horizontal circular curve provides a constant  rate of change of bearing. Analogous to this is the vertical 
parabola which provides a constant rate of change of gradient. The reciprocal of the rate of change of grade, 
K, is thus the distance required to effect a unit change of grade. Vertical curves are specified in terms of this 
factor, K, and their horizontal length calculated by multiplying K by the algebraic difference, A, in percentage 
between the gradients on either side of the curves so that:  
L = A × K.  
Table 23 and Table 24 gives the computed K values for lengths of vertical curves corresponding to the 
stopping sight distances and passing sight distance respectively.  
Table  23 — Design controls f or stopping sight distance and crest vertical c urves  
Design speed 
(km/h)  Stopping sight distance (m)  Rate of vertical curvature, Ka 
Calculated  Design  
20 20 0.6 1 
30 35 1.9 2 
40 50 3.8  4  
© EAC 2024  – All rights reserved  43 
 50 65 6.4 7 
60 85 11.0 11 
70 105 16.8 17 
80 130 25.7 26 
90 160 38.9 39 
100 185 52.0 52 
110 220 73.6 74 
120 250 95.0 95 
130 285 123.4  124 
a Rate of vertical curvature, k, is the length of curve per percent algebraic difference in intertsecting 
grades (A). K=L/A  
 
The design control in terms of K covers all combinations of A and L for any one design speed and therefore A 
and L need not be indicated separately in a tabulation of design value. The selection of design curves is 
facilitated because the minimum length of curve in metre is equal to K times the algebraic difference in grades 
in percent, L=KA. Conversely, the checking of plans is simplified by comparing all curves with the design 
value for K.  
Table  24 — Design controls for crest vertical curves based on passing sight Distance  
Design speed 
(km/h)  Stopping sight distance (m)  Rate of vertical 
curvature, Ka, Design   
30 200 46 
40 270 84 
50 345 138 
60 410 195 
70 485 272 
80 540 338 
90 615 438 
100 670 520 
110 730 617 
120 775 695 
130 815 769 
a Rate of vertical curvature, K, is the length of curve per percent algabraic difference 
in the intersecting grades (A). K=L/A  
 
7.3.3.3  Sag curve  
7.3.3. 3.1 Four different criteria for establishing lengths of sag vertical curves should be recognized to some 
extent. These are:  
a) headlight sight distance;   
44 © EAC  2024  – All rights reserved  
 b) passenger comfort;  
c) drainage control; and  
d) general appearance.  
7.3.3 .3.2 A headlight height of 0.6 m and a one -degree upward divergence of the light beam from the 
longitudinal axis of the vehicle is commonly assumed. The following relationship between S, L and A, using S 
as a distance between the vehicle and the point where one -degree upward angle of the light beam intercepts 
the surface of the roadway:  
When S is less than L,  
 
Or, 
 
When S is greater than L,  
 
Or, 
 
where  
 L  is length of sag vertical curve;   
 A  is algebraic difference in grades, %;   
 S  is light beam distance, m.  
7.3.3.3 .3 For drivers to see the roadway ahead, a sag vertical curve should be long enough that the light 
beam distance is approximately the same at the stopping sight distance. Accordingly, it is appropriate to use 
stopping sight distances for different design speeds as the value of S in the above equations. The resulting 
lengths of the sag vertical curves for the recommended stopping sight distances for each design speed are 
shown in Figure 14 with solid lines using rounded values of K as was done for crest vertical curves.   
© EAC 2024  – All rights reserved  45 
 
 
Figure 14 — Length of sag vertical curve as a function of algebraic difference in grade A (%)  
7.3.3 .3.4 Same lengths of sag vertical curves as that of the stopping sight distance should be used and the 
rate of vertical curves should be determined accordingly.  
Table  25 — Design controls for Sag Vertical curves  
Design speed 
(km/h)  Stopping sight distance (m)  Rate of vertical curvature, Ka 
Calculated  Design  
20 20 2.1 3 
30 35 5.1 6 
40 50 8.5 9 
50 65 12.2 13 
60 85 17.3 18 
70 105 22.6 23 
80 130 29.4 30 
90 160 37.6 38 
100 185 44.6 45 
110 220 54.4 55 
120 250 62.8 63 
130 285 72.7 73 
a Rate of vertical vertical curvature, K, is the length of curve (m) per percent algebraic difference 
intersecting (A). K=L/A   
46 © EAC  2024  – All rights reserved  
 7.3.3.4   Sight distances at undercrossing  
7.3.3.4.1  In some conditions, it is recommended to check the available sight distance at undercrossing 
such as at an undercrossing without ramp where passing sight distance need to be provided (Figure 15).  
 
Figure 15 — Schematic view of sight distance at undercrossing  
7.3.3.4.1 General equations should be developed for two cases as follows:  
A. Case 1 Sight distance greater than length of vertical curve ( S > L )  
 
 
where  
L  is length of vertical curve, m;                   
S  is sight distance, m;                   
C  is vertical clearance, m;                 
h1 is height of eye, m;                  
h2 is height of object, m;                 
A  is algebraic difference in grades, %.  
B. Case 2 Sight distance less than length of vertical curve ( S < L )  
 
where  
L length of vertical curve, m;   
© EAC 2024  – All rights reserved  47 
 A is algebraic difference in grades, %;   
S is sight distance, m;       
h1 is height of eye, m;       
h2 is height of object, m.  
7.3.3.4.2  Using an eye height of 2.4 m for truck driver and an object of height of 0.6 m for the taillights of a 
vehicle, the following equations can be derived:  
A. Case 1 Sight distance greater than length of vertical curve ( S > L )  
 
B. Case 2 Sight distance less than length of vertical curve ( S< L)  
 
7.3.3.4.3  The sight distance undercrossing shall be determined for two cases where:  
A. is the sight distance is greater than the vertical curve;   
B. is the sight distance is less than the vertical curve.   
7.3.3.4.4  The cases of a truck driver’s eye from 2.4 m staring an object of 0.6 m height for the taillights of a 
vehicle shall as well be considered.  
8 Cross Section Elements  
8.1 Lane Width  
8.1.1  The width travel lane shall be sufficient to accommodate the design vehicle, allow for imprecise 
steering manoeuvres, and provide clearance for opposing flow in adjacent lanes. In cases of paved road, lane 
width refers to the net width excluding width of edge strip, edge line and centreline markings.  
8.1.2  The design width of a travelled lane shall be:  
a) 3.5 to 3.75 m wide, for mobility roads of Class 1, Class 2 and Class 3, and  
b) 2.5 to 3.5 m wide for roads of Class 4 and Class 5.  
In suburbs and at the entrance of a town or city, of a grouped settlement or agglomeration, the road width may 
be increased where necessary.  
8.1.3  Wide lanes promote the safety of the occupants of vehicles, although current evidence suggests that 
there is an upper limit beyond which safety is reduced by further increases in lane width. The selection of lane 
width is based on traffic volume and vehicle type and speed.  
8.2 Shoulders  
8.2.1  Shoulders should be wide enough to adequately fulfil their purpose, but excessive width encourages 
drivers to use them as an additional travel lane. Shoulders are used for emergency stopping, for parking of 
disabled vehicles, and for lateral support of the subbase, base, and surface courses of the travel roadway. In  
48 © EAC  2024  – All rights reserved  
 cases of paved road, lane width covers net width excluding width of edge strip, edge line and centreline 
markings.  
8.2.2 The minimum width of shoulders for mobility roads of Class 1, shall be 2.5  m 
8.2.3  The minimum width of shoulders for roads of Class 2 and Class 3, shall be  at least  2 m. 
8.2.4 The width of shoulders for roads in rolling mountainous  and steep  terrain shoulders shall be 1 to 
1.5m.  
8.2.5 For access roads of Class 4 and Class 5, shoulders shall be  at least 1.0 m wide  for both sides of the 
road. 
8.2.6 The shoulder area shall be free from any object that can obstruct  the movement of pedestrians. Any 
roadway structure including bridges head walls and any physical structure projecting upward that can disrupt 
the pedestrian movement shall be fixed outside the shoulder width.  
8.3  Normal Cross Fall (NCF)  
8.3.1  The divided roadway is designed as two separate roadways, each having a centre crown bordered 
by slopes to the outside of the pavement, or each direction of travel can be sloped unidirectional . The cross 
slope is used on traffic lanes to promote drainage of surface water.  
8.3.2 The recommended cross slope varies depending on the surface of pavement ranging from asphalt 
concrete surface, surface dressing surface, stone paved surface and gravel and earth surface, and hall be as 
follow:  
a) 2.5% for asphalt concrete surfaces (or 3% in areas where heavy rainfall is common)  
b) Surface dressing surfaces 3.0%  
c) Stone paved surfaces 3.0%  
d) Gravel and earth surfaces 4.0%  
8.3.3 The shoulder should have the same slope as the carriageway.  
8.4 Side Slope and back slope  
8.4.1  Side slopes shall be designed to ensure roadway stability and provide a reasonable opportunity for 
recovery for an out -of-control vehicle. Three regions of the side slopes shown in Figure 16 are important to 
safety: the top of the slope (hinge point), the fore slope, and the toe of the slope (intersection of the fore slope 
with level ground or with a back slope, forming a ditch), the  
  
© EAC 2024  – All rights reserved  49 
 Figure  16 — Typical illustration of regions of side slopes  
8.4.2  The slope steeper than 1V:4H are not desirable because their use severely limits the choice of back 
slopes. Slope 1V:3H or steeper are recommended only where site conditions do not permit use of flatter 
slopes. When slopes steeper than 1V:3H are used, consideration should be given to the use of a roadside 
barrier.  
8.4.3  Back slopes should be 1V:3H or flatter, to accommodate maintenance equipment. Back slopes 
steeper than 1V:3H should be evaluated with regard to soil stability and traffic safety. Retaining walls should 
be considered where space restrictions would otherwise result in slopes steeper than 1V:2H.  
8.4.4  For freeways and other arterials, it is reccomended that the rate of slope of 1V:6H or flatter be 
provided on embankments since they can be negotiated by a vehicle with a good chance of recovery. For 
moderate heights with good roundings, steeper slopes up to about 1V:3H can also be traversable, though not 
recoverable.  
8.4.5  It is recommended that:  
a) For side slopes steeper than 1V:3H, roadside barrier should be used.  
b) Back slopes should be flatter than 1V:3H to accommodate maintenance equipment  
c) The slopes steeper than 1V:3H should be evaluated for slope stability and traffic safety  
8.5 Clear Zone  
The is unobstructed, relatively flat area provided beyond the edge of the roadway for the recovery of errant 
vehicles. It includes any shoulders or auxiliary lanes. Clear zone widths are related to speed, volume, 
embankment slope, and horizontal geometry. The need for clear zones increases with speed and curvature.  
The recommended clear zone width design values in relation to speed limits shall be as per Table 26.  
 Table 26 — Clear zone widths  
Speed limit  (km/h)  Standard  
Desired (m)  Minimum (m)  
70 5 3 
80 6 4 
100 9 6 
The values shown in Table 20  should be increased at sharp bends on high speed roads by a correction factor.  
8.6 Median  
8.6.1  Median width is expressed as the dimension between the edges of travelled way and includes the 
left/right shoulder (depending on whether driving is on the right/left, respectively), if any. Medians are highly 
desirable on multilane divided arterials carrying four or more lanes.  
8.6.2  The median shall be appropriate so as to properly fulfil its main functions including to separate 
opposing traffic, provide recovery area for out -of-control vehicles, provide a stopping are in case of 
emergencies, allow space for speed changes and storage of left/right -turning (depending on whether driving is 
on the right/left, respectively) and U -turning vehicles, minimise headlight glare, provide width for future lanes, 
and in urban areas to offer an open green space, refuge area for pedestrians crossing the street, and control 
the location of intersection traffic conflicts.  
The design width dimensions in relation to the speed limits shall be as per the Table 27.  
50 © EAC  2024  – All rights reserved  
 Table 27 — Recommended median widths  
Speed limit  (km/h)   Median   Widths  
Desirable (m)   Minimum (m)  
100  12 6 
80  9 4 
8.7 Service Roads  
8.7.1  A service road, also known as access road or frontage roads, serves numerous functions, depending 
on the type of arterial they serve and the character of the surrounding area. They may be used to control 
access to the arterial, function as a street facility serving adjoining properties, and maintain circulation of traffic 
on each side of the arterial. Service roads segregate local traffic from the higher speed through -traffic and 
intercept driveways of residences and commercial establishments along the highway.  
8.7.2  Cross connections provide access between the roadway and service roads and are usually located 
in the vicinity of the crossroads. Thus, the through character of the highway is preserved and unaffected by 
subsequent development of the roadsides. Service roads are used on all types of highways. Despite the 
advantages of using service roads on arterials roads, the use of continuous service roads on relatively high -
speed arterial roads with intersections may be undesirable.  
8.7.3  The design of service road is influenced by the type of service it is intended to provide. The 
recommended service road width for larger trading centres and towns is 6.0 m, however, lower widths may be 
adopted depending on the intended function of the service road. A spacing of about 50 m between the arterial 
and the service roads in urban areas is recommended.  
8.8 Headroom and Lateral Clearance 8.8.1  The roadway shall be designed with enough headroom to allow 
the traffic to pass safely under objects restricting the height, mainly under bridge structures, high -power cable 
and low -power cable.  
8.8.2  The recommended headroom shall be 5.5 m under bridge structures, 7.0 m under high -power cables 
and 6.0 m under low -power cables.  
8.8.3  The minimum lateral clearance shall be provided and shall have the width of;  
(i) width of 0.6 m on each side of a bridge for traffic volume of less than 400 veh/day,  
(ii) width of 1.0 to 1.2 m on each side of a bridge for traffic volume of 400 to 2000 veh/day, and  
(iii) width of approach roadway width for traffic volume of over 2000 veh/day  
8.8.4  For bridges over 30 m span, the width of lateral clearance shall be increased by 1 m.  
8.9 Typical Cross Section Dimensions  
The typical cross section dimensions for road design given in Table 28 are developed based on the proposed 
standards discussed in the previous sections. Figure 17 presents typical cross sectional elements of a dual 
carriageway.  
Table 28 — Typical cross section dimensions for a road design  
Road class    Dimension (m)    Slope (%)  
Right of 
way Roadway 
width  Carriageway 
width  Number 
of lanes  Lane 
width  Shoulder 
width  Cross 
slope  Shoulder 
slope  
Mobility roads 
class 1  60.0 11.4 7.4 2 3.7 2.0 2.5 4.0  
© EAC 2024  – All rights reserved  51 
 Mobility roads 
class 3  40.0 10.0 7.0 2 3.5 1.5 2.5 4.0 
Access roads 
class 4  30.0 8.6 6.6 2 3.3 1.0 4.0 4.0 
 
 
Figure 17 — Typical cross sectional elements of a dual carriageway  
8.10 Motorcycle Lanes  
8.10.1  The motorcycle lanes can be either restricted track type of exclusive track type.  
8.10.2  Restricted motorcycle track can be developed within the carriageway of an existing road. It is usually 
sited on the left side of the road, preferably between the kerbs and the parking lane. Some form of physical 
barrier or pavement markings defines the corridor set aside for the cyclists and route markings are necessary 
to define the route and reduce potential conflicts. However, at crossings and intersections, this kind of cycle 
track ceases as a separate mode and conflicts may occur with other forms of movement.  
8.10.3  An exclusive motorcycle track is a complete separate right -of-way established for the sole use of 
cyclists. It differs from the restricted cycle track in that it normally has a wide right -of-way and is not developed 
from existing carriageway of a wide road and it separates the cyclists from other motorists. It helps to separate 
conflicts at crossings and intersections with the provision of underpasses and other related facilities.  
8.10.4  In areas, where there is usually a high proportion of motorcyclist, the volume may be so substantial 
as to affect the smooth flow of traffic. In such instances, the provision of separate and exclusive cycle lanes 
should be considered. The warrants for determining the need for an exclusive cycle lane are:  
a) when total volume of traffic exceeds the provided lane capacity, and  
b) when volume of motorcycles exceeds 20% of the total volume of traffic.  
8.11 Emergency Escape Ramps  
8.11.1  General  
The emergency escape ramp is desired at appropriate location to provide a location for out -of-control 
vehicles, particularly trucks, to slow and stop away from the main traffic stream.  
If the conditions indicate the need for more than one, should be located wherever grades are of steepness 
and length that present a substantial risk of runaway trucks and topographic conditions permit construction.  
NOTE Out -of-control vehicles are generally the result of a driver losing braking ability either through overheating of the 
brakes due to mechanical failure or failure to downshift at the appropriate time. Each grade has its own unique 
characteristics. Highway alignment, gradient, length, and descent speed contribute to the potential for out -of-control 
vehicles.   
52 © EAC  2024  – All rights reserved  
 The principal factor in determining the need for an emergency escape ramp should be the safety of the other 
traffic on the roadway, the driver of the out -of-control vehicle, and the residents along and at the bottom of the 
grade. An escape ramp, or ramps if the conditions indicate the need for more than one, should be located 
wherever grades are of a steepness and length that present a substantial risk of runaway trucks and 
topographic conditions will permit construction.  
Crash experience (or, for new facilities, crash experience on similar facilities) and truck operations on the 
grade combined with engineering judgment are frequently used to determine the need for a truck escape 
ramp. Often the impact of a potential runaway truck on adjacent activities or population centres will provide 
sufficient reason to construct an escape ramp. For existing facilities, an escape ramp should be provided as 
soon as a need is established.  
8.11.2  Design Considerations  
The design and construction of effective escape ramps involve a number of considerations as follows:  
a) To safely stop an out -of-control vehicle, the length of the ramp should be sufficient to dissipate the kinetic 
energy of the moving vehicle.  
b) The alignment of the escape ramp should be tangent or on very flat curvature to minimize the driver’s difficulty 
in controlling the vehicle. Escape ramps should exit to the right of the roadway.  
c) The width of the ramp should be adequate to accommodate more than one vehicle because it is not 
uncommon for two or more vehicles to have need of the escape ramp within a short time. A minimum width of 
8 m may be all that is practical in some areas, though greater widths are preferred. Desirably, a width of 9 to 
12 m would more adequately accommodate two or more out -of control vehicles.  
d) To assist in decelerating the vehicle smoothly, the depth of the arrester bed should be tapered from a 
minimum of 75 mm at the entry point to the full depth of aggregate in the initial 30 to 60 m of the bed. As a 
vehicle rolls upgrade, it loses momentum and will eventually stop because of the effect of gravity.  
e) To determine the distance needed to bring the vehicle to a stop with consideration of the rolling resistance 
and gradient resistance, the following equation may be used:  
 
where 
L is length of arrester bed, m  
V is enterring velocity  
R is rolling resistance, expressed as equivalent percentage gradient divided by 100  
G is percent gare divided by 100  
The final speed for one section of the ramp is subtracted from the entering speed to determine a new entering 
speed for the next section of the ramp and the calculation repeated at each change in grade on the ramp until 
sufficient length is provided to reduce the speed of the out -of-control vehicle to zero. Figure 18 shows a plan 
and profile of an emergency escape ramp with typical appurtenances.   
© EAC 2024  – All rights reserved  53 
 
 
Figure  18 —Typical emergency escape ramp  
8.11.3  Brake -Check Areas  
Turnouts or pull -off areas at the summit of a grade can be used for brake -check areas or mandatory -stop 
areas to provide an opportunity for a driver to inspect equipment on the vehicle and check that the brakes are 
not overheated at the beginning of the descent. An elaborate design is not needed for these areas. A brake -
check area can be a paved lane behind and separated from the shoulder or a widened shoulder where a truck 
can stop. Appropriate signing should be used to discourage casual stopping by the public.  
9 Intersections  
9.1 General  
9.1.1  An intersection is an important part of a road network because the safety, speed, efficiency, and cost 
of operation of vehicles on and capacity of the network are greatly influenced by the effectiveness of its 
intersections. Intersections vary in complexity from a simple intersection, which has only two roads crossing at 
a right angle to each other, to a more complex intersection, at which three or more roads cross within the 
same area.  
9.1.2  The highways intersection can be of the following categories:  
a) At-grade intersections , which do not provide for the flow of traffic at different levels of elevation  
b) Grade separated without ramps , which provide spatial separation between the crossing movements but 
do not make provision for turning movements  
c) grade separated with ramps (interchanges).  The principal difference between interchanges and other 
forms of intersection is that, in interchanges, crossing movements are separated in space whereas, in the 
other forms of intersection, they are separated in time   
54 © EAC  2024  – All rights reserved  
 9.2 The Choice of an Intersection Type  
9.2.1  All intersections shall be compatible with the operating conditions of the type of road on which it is 
installed with a view to apply grade separation, if warranted. Furthermore, the type of intersection must be 
compatible with the specific conditions of the site and its operation including traffic volume, closeness to a 
built-up area, transition between two types of road, and safety problems.  
9.2.2  The choice of the right type of intersection shall be made based on the capacity required, safety, 
environment and uniformity, spatial limitations, topographical conditions and construction and maintenance 
costs, road user costs. In the case of a new construction, the decision will frequently be based on general 
knowledge about how a given type of intersection affects, for instance, the number of accidents and delays 
while at a site with an existing junction, a safety diagnosis is an essential basis for decision -making.  
Figure 19 shows a typical approach to selection of appropriate intersection type.   
 
Figure  19 —Types of intersections appropriate to different traffic flows  
9.2.3  Many factors enter into the choice of type of intersection and the extent of design of a given type, but 
the principal controls are the design -hour traffic volume, the character or composition of traffic, and the design 
speed. In general, traffic service, highway design designation, physical conditions, and cost of right -of-way are 
considered jointly in choosing the type of intersection.  
9.2.4  The selection can be performed through two -steps procedure for selection of at -grade intersection; 
these are selection of intersection category (priority or control) and selection of intersection type and is based 
on the following assumptions  
 
a) Priority intersections can be safe and give sufficient capacity for certain traffic volumes and speed 
limits  
b) If a priority intersection is not sufficient for safety and capacity, the major road traffic must also be 
controlled  
c) Depending on location, traffic conditions and speed limits, different types of priority or control 
intersection should be selected   
© EAC 2024  – All rights reserved  55 
 9.3 Basic At -Grade Intersection Elements  
9.3.1  General  
The intersections shall be designed with the main objective of ensuring effective utilization of the road network 
and to reducing the severity of potential conflicts between vehicles or between vehicles and pedestrians, while 
facilitating the necessary manoeuvres.  
The basic types of at grade intersection are:  
a) three -leg intersection  (T-junction), which generates six vehicle conflict points and ten vehicle -
pedestrian conflict points;  
b) four-leg or cross intersection , which has twenty -four vehicle conflicts and twenty -four vehicle -
pedestrian conflict points; and  
c) multi -leg intersection .  
9.3.2  The Distance between Intersections  
9.3.2.1  The spacing of intersections impacts significantly on the operation, level of service and capacity of a 
roadway. For safe and efficient traffic flow, intersections should not be placed too close together. Designers 
seldom have influence on the spacing of roadways in a network as it is largely predicated by the original or 
developed land use.  
9.3.2.2  The proper intersection spacing, particularly for signalized intersections, is critical for providing 
coordinated signal timing, optimal timing progression for two -way movements should allow travel time 
between intersections to be about half of the cycle length.  
9.3.2.3  The signal spacing shall be at least 400 m in urban areas and 800 m in suburban areas for optimum 
two-way progression, and for unsignalised intersections the minimum distance should be at least 10*V D 
meters.  
9.3.3  Location  
The intersection shall be located in appropriate locations to ensure their safety. It is suggested that where 
possible, intersections should not be located on a horizontal curve if possible or near a crest vertical curve, 
due to restrictions on sight distance.  
9.3.4  Angle  
9.3.4.1  The angle of intersection is important to operations. A right angle intersection provides the most 
favourable conditions for intersecting and turning traffic movements.  Specifically, a right angle (90°) provides 
(a) the shortest crossing distance for motor vehicles, bicycles, and pedestrians, and (b) sight lines which 
optimize corner sight distance and the ability of drivers to judge the relative position and speed of approach 
vehicles.  
9.3.4.2  Minor deviations from right angles are generally acceptable provided that the potentially detrimental 
impact on visibility and turning movements for large trucks can be mitigated. However, large deviations from 
right angles may decrease visibility, hamper certain turning operations, and will increase the size of the 
intersection and therefore crossing distances for bicyclists and pedestrians.  
9.3.4.3  This makes vehicle and pedestrian time in the area conflicting with other traffic streams increase so 
delays and collisions increase. In addition, the paved area —and therefore construction and maintenance 
cost—will increase and these effects increase dramatically with angles less than 60 or greater than 120.  
9.3.4.4  Several retrofit improvement strategies exist for treating existing or proposed intersections with 
unfavourable angles. One (minor leg) or both roads could be realigned with horizontal curves to create a more  
56 © EAC  2024  – All rights reserved  
 favourable angle. In some cases, one intersection can be made into two, creating an offset intersection. 
Another option is to use islands to guide drivers and pedestrians safe manoeuvre through the intersection and 
reduce the paved area.  
9.3.4.5  The angle of existing intersections should be checked for possible implementation of retrofitting 
improvement strategies, if necessary.  
9.3.5  Warrants for Turning Lanes  
9.3.5.1  Turning lanes are provided to give room for left -turning or right -turning ve hicles to decelerate before 
their turns and/ or to queue while waiting to turn. They are particularly effective at reducing delay and 
collisions by getting those vehicles out of the way of through vehicles.  
9.3.5.2  When designing an intersection, left -turning traffic should be removed from the through lanes, 
whenever practical. Therefore, provision for left turn lanes has widespread application. Ideally, left -turn lanes 
should be provided at driveways and street intersections along major arterial and collector roads wherever left 
turns are permitted.  
 
9.3.5.3   Left turn facilities should be established on roadways where traffic volumes are high enough or 
safety considerations are sufficient to warrant them. They are often needed to ensure adequate service levels 
for the intersections and the various turning movements.  
 
9.3.5.4  Table 29 presents proposed guide to traffic volumes where left turn lanes should be considered on 
two-lane highways. For the volumes shown, left turns and right turns from the minor street can be equal to, 
but not greater than, the left turns from the major street. In the case of the double left turn lanes, a capacity 
analysis of the intersection should be permitted to determine what traffic controls are needed in order for it to 
function properly.  
Table 29 — Guide for left -turn lanes on two -lane highways  
 
Opposing volume 
(veh/h)   Advancing volume (veh/h)   
5% left turns   10% left turns  20% left turns  30% left turns  
  60 km/h operating speed   
800 330  240 180 160 
600 410  305 225 200 
400 510  380 275 245 
200 640  470 350 305 
100 720  515 390 340 
  80 km/h operating speed   
800 280  210 165 135 
600 350  260 195 170 
400 430  320 240 210 
200 550  400 300 270 
100 615  445 335 295  
© EAC 2024  – All rights reserved  57 
   100 km/h operating speed   
800 230  170 125 115 
600 290  210 160 140 
400 365  270 200 175 
200 450  330 250 215 
100 505  370 275 240 
 
 9.3.6  Grades  
9.3.6.1  Intersections require less steep grade to avoid hampering traffic operations at intersections. Steep 
downgrades on an intersection approach increase stopping distances and make turning more difficult. Steep 
upgrades on an intersection approach make idling difficult for vehicles with manual transmissions and make 
acceleration slower for all vehicles, which in turn increases necessary gap sizes and sight distances for 
crossing and turning movements.  
 9.3.6.2  It is recommended that Intersection shall at locations that have grades of less than 4%.  
9.4 Grade Separations and Interchanges  
9.4.1  General  
9.4.1.1  Grade separation shall be used to accommodate high volumes of intersecting traffic safely and 
efficiently through the arrangement of one or more interconnecting roadways. A system of interchange shall 
be used to provide for the movement of traffic between the roadways.  
9.4.1.2  For Interstate highways, interchanges shall be provided between all intersecting Interstate routes, 
between other selected access controlled highways and at other selected public highways to facilitate 
distribution of traffic. The design of its design shall be considered in conjunction with adjacent interchanges or 
grade separations on the project as a whole to provide uniformity and route continuity to avoid confusion in 
driver expectancy.  
9.4.1.3  Each interchange shall provide for all traffic movements. The type and design of grade separations 
and interchanges are influenced by many factors such as highway classification, character and composition of 
traffic, design speed and degree of access control. These controls plus signing needs, economics, terrain and 
right-of-way are of great importance in designing facilities with adequate capacity to safely accommodate 
traffic demands.  
9.4.1.4  Interchanges vary in type from single ramps connecting local streets to complex and comprehensive 
design layouts involving the intersection of multiple highways. The basic interchange configurations are 
indicated in Figure 20.  
58 © EAC  2024  – All rights reserved  
 
 
Figure  20 — Interchange Configurations   
 
9.4.1.5  An important element of interchange design which influences the efficiency, safety and capacity 
attained is the size and arrangement of ramps that connect two or more legs at an interchange.  
9.4.2  Warrants for Grade Separations and Interchanges  
9.4.2.1  The justification of an interchange at a given location is difficult due to the wide variety of site 
conditions, traffic volume, highway types and interchange layouts. The following six warrants should be 
considered together with undertaking of benefit cost analysis for determining if an interchange is justified at a 
particular site:   
© EAC 2024  – All rights reserved  59 
 a) Design designation:  The design designation of a roadway is one method used to help decision -makers 
decide whether or not grade -separated interchanges are justified. Once a controlled access facility has 
been designated on the major road, an interchange may be warranted for a crossing road based on the 
anticipated demand for access to the minor road. If a given route has been designated as a freeway 
corridor, then all conflicting approaches to the freeway must be evaluated individually. In other words, it 
must be determined whether a given crossroad should be terminated, rerouted, or provided with grade 
separation. This determination is based on the importance of the crossroad. A minor or local road would 
typically be terminated, or rerouted, due to their nature of having low traffic volumes. If an arterial or a 
collector crosses the freeway with significant traffic volumes, a grade separation may be provided. Since 
this determination is based, in part, on traffic volumes, a method for estimating the amount of traffic that 
justifies grade separation needs to be evaluated in more detail. The justification for implementing grade 
separations based solely on the design designation of a roadway is generally applicable in the case of 
freeway corridors only. If the arterial road has not been designated as a freeway corridor and does not 
conflict with any existing freeways, this warrant would not be applicable. Of course this does not imply 
that the designation of the arterial roadway cannot be changed in the future. In fact the alteration of an 
arterial road to a highflow arterial may merely be a stepping stone in the design evolution from a simple 
arterial street to a freeway. Thus, an intersection that might warrant only traffic signal, if considered as 
an isolated case, will warrant a grade separation or interchange when considered as part of a freeway.  
b) Reduction of bottlenecks or spot congestion : An interchange may be warranted if the congestion at 
an at grade intersection is intolerable, and the intersection cannot be redesigned to accommodate the 
traffic volumes. Intersections that cannot provide sufficient capacity for roadways with significantly large 
volumes will inevitably experience excessive levels of congestion on one or more of its approaches. 
Such intersections are typically classified as bottlenecks. A single congested intersection of an arterial 
network can easily affect nearby intersections or driveways if long queues of vehicles are allowed to 
spillback. Therefore, it is essential to minimize the delay that results from heavy congestion through the 
use of at -grade treatments. Surface treatments may include signal optimization, channelization, and 
pavement re -striping. If a bottleneck cannot be eliminated through simple means, then grade separation 
may be justified.  
c) Safety improvement:  For intersections where the crash rate is significantly high, grade separation may 
be justified. This is because more crossing or turning conflicts are encountered at surface intersections, 
and grade separations remove a significant portion of these vehicle conflicts. A cost -effective analysis 
may justify the expense of an interchange solely on the basis of safety benefits so that the likelihood of 
traffic crashes can be significantly decreased.  
d) Site topography:  The topography at some locations may be such that an interchange can be 
constructed at less than or comparable to the cost of an at -grade intersection. This is due primarily by 
the design constraints associated with vertical alignment.  
e) Road -user benefits:  An interchange may be warranted where the road user costs such as fuel and oil 
usage, wear on tires, repairs, delays, crashes that results from speed changes, stops, and waiting are 
significantly reduced when compared to at -grade intersections. The relation of road -user benefits to the 
cost of improvement indicates an economic warrant for that improvement. Comparison of benefit cost 
ratios for design alternatives is an important factor in determining the type and extent of improvement to 
be made.  
f) Traffic volumes:  This may be the most tangible of any interchange warrant. Although a specific volume 
of traffic at an intersection cannot be completely rationalized as the warrant for an interchange, it is an 
important guide, particularly when combined with the traffic distribution pattern and the effect of traffic 
behaviour. However, volumes in excess of the capacity of an at -grade intersection would certainly be a 
warrant.  
9.4.2.2  Not all warrants for grade separations are included in the warrants for interchanges. Additional 
warrants for grade separation include grade separation that would be installed at:  
a) Locations where the termination of local roads and streets is not feasible because of limitations in 
freeway right -of-way  
60 © EAC  2024  – All rights reserved  
 b) Areas that are not accessible by means of frontage roads or other sources of access.  
c) Eliminate a railroad -highway grade crossing  
d) Locations with unusual concentrations of pedestrian and/or bicycle traffic for instance, a city park 
developed on both sides of a major arterial  
e) Other areas with routine pedestrian and/or bike traffic, especially in school zones  
f) Mass transit stations that require access because of their location within the confines of a major 
arterial  
g) Places with free -flow characteristics of certain ramp configurations and completing the geometry of an 
interchange.  
benefits attributable to operational and design improvements made to arterial intersections.  
9.4.2  Spacing  
9.4.2.1  If an interchange is warranted for any of the listed reasons, interchange spacing is an additional 
consideration in the decision -making process. Interchange spacing has a pronounced effect on freeway 
operations. In areas of concentrated urban development, proper spacing usually is difficult to attain because 
of traffic demand for frequent access. Minimum spacing of arterial interchanges (distance between 
intersecting roads with ramps) is determined by weaving volumes, ability to sign, signal progression, and 
lengths of speed -change lanes.  
9.4.2.2  The rule of thumb for minimum interchange spacing is 1.5 km in urban areas, and 3.0 km in rural 
areas (between freeway -to-freeway interchanges and local roads interchanges). In urban areas spacing of 
less than 1.5 km may be developed by grade -separated ramps or by adding collector -distributor roads.  
9.5 Railroad – Highway Crossing  
9.5.1  A railroad -highway crossing, like any highway -highway intersection, involves either a separation of 
grades or a crossing at -grade. The geometrics of a highway and structure that involves the overcrossing or 
under -crossing of a railroad are substantially the same as those for a highway grade separation without 
ramps. The horizontal and vertical geometrics of a highway approaching a railroad grade crossing should be 
constructed in a manner that facilitates drivers’ attention to roadway conditions.  
9.5.2  Some of the safety, operational and emergency issues associated with an at grade crossing include 
traffic and emergency response may periodically be disrupted by train traffic. Operational issues with this type 
of crossing may be worsened as traffic volumes increase over time. Intersections handling a high volume of 
traffic and pedestrians (and possibly railroads) limit the capacity of the approaching roads.  
9.5.3  Grade -separating these conflict points allow an uninterrupted flow of traffic while also eliminating the 
safety threat posed by trains, pedestrians, or other vehicles. Three primary roadway improvement objectives 
are accomplished using grade separated intersections; increased capacity and uninterrupted flow, increased 
safety, and reduced vehicle -train conflict and delay.  
9.5.4  The decision to grade separate a highway -rail crossing is primarily a matter of economics. 
Investment in a grade separation structure is long -term and impacts many users. Such decisions should be 
based on long -term, fully allocated life -cycle costs, including both highway and railroad user costs, rather than 
on initial construction costs. Such analysis should consider:  
a) Eliminating train/vehicle collisions (including the resultant property damage and medical costs and 
liability).  
b) Savings in highway -rail grade crossing surface and crossing signal installation and maintenance costs.   
© EAC 2024  – All rights reserved  61 
 c) Driver delay cost savings.  
d) Costs associated with providing increased highway storage capacity (to accommodate traffic backed up 
by a train).  
e) Fuel and pollution mitigation cost savings (from idling queued vehicles).  
f) Effects of any “spillover” congestion on the rest of the roadway system.  
g) Benefits of improved emergency access.  
h) Potential for closing one or more additional adjacent crossings.  
i) Possible train derailment costs.  
10 Speed Management  
10.1 Speed Control Measures  
10.1.1  Speed management encompasses a range of measures aimed at balancing safety and efficiency of 
vehicle speeds on a road network. It aims to reduce the incidence of driving too fast for the prevailing 
conditions, and to maximize compliance with speed limits. Speed management aims to reduce the number of 
road traffic crashes and the serious injury and death that can result from them. There are many tools available 
for effective speed management. They include appropriate speed limits, engineering treatments, effective 
enforcement of speed limits by police and the use of extensive public information and education programmes 
to encourage compliance with speed limits.  
10.1.2  Numerous practices have been implemented in urban areas to reduce speeds:  
a) Pre-warnings: typically lines on the pavement with (rumble strips) or without punishment (lines and traffic 
signs)  
b) Gates: typically different pavement colour or structures that indicate transition between traffic 
environments, often augmented with signs and landscaping  
c) Narrowings: typically the available roadway width is reduced to narrower lane widths with the addition of 
islands, by eliminating one lane in two -lane roads or by using wider edge markings  
d) Humps and tables: with varied profiles including circular, sinusoidal, dome -shaped, or trapezoidal cross -
sections and varied lengths depending upon the desired speed reduction  
e) Raised areas: typically a trapezoidal hump with extended length to allow for longer vehicles to have all 
wheels on them  
f) Staggering: typically a lane is shifted over  
g) Chicanes: typically extensions of the curb at intersections to reduce approach lane widths  
h) Islands: typically raised elements along the centreline of the roadway to shelter pedestrians and ease 
street crossing  
i) Cushions: typically square humps in each travel lane  
10.2 Speed Humps  
10.2.1  Road humps are not recommended to be used on International Trunk Roads. They may be used on 
National Trunk Roads and Regional Roads where proven to be absolutely necessary. They are suitable for  
62 © EAC  2024  – All rights reserved  
 residential areas but are not acceptable and should not be used on national roads such as Trunk and 
Regional roads.  
10.2.2  There are two main types of road humps:  
a) circular , which are intended for traffic speed reduction only, and;  
b) flat-topped humps , which are intended for speed reduction and for use as a pedestrian crossing.  
Table 30a and Table 30b gives the recommended standards for design of circular speed hump and of flat -
topped speed hump  
Table 30a — Detailed design of circular speed hump  
 
 
Table 30b — Detailed design of flat – topped speed hump  
 
Car (truck) speed level  
(km/h)  Ramp length r (m)  Grade i (%)  
   30 (10)  1.0 10 
  35 (15)  1.3 7.5 
  40 (20)  1.7 6 
  45 (25)  2.0 5 
 50 (30)  2.5 4 
 (35) 3.3 3 
 (40) 4.0 2.5 
 
 
© EAC 2024  – All rights reserved  63 
 10.3 Rumble strips  
10.3.1  Rumble strips are transverse strips across the road used to alert and warn drivers with a vibratory 
and audible effect before a hazard such as a sharp bend, an intersection or a lower speed limit at the entry to 
a trading centre. Warning signs are not normally needed when the strips are built to the specifications given 
below. However, rumble strips create disturbing noises and can cause vibration problems on soft ground. 
Thus, it is recommended to avoid installing them near places such as houses, schools, and hospitals.  
10.3.2  Rumble strips can be used in such situation as, before a local speed limit, at an approach to a 
dangerous intersection, before a sharp bend, before a hump.  
10.3.3  The design of rumble strips may be done in either of the following two options. This first option 
recommends the following principles as they are presented on Figure 21: 
a) Rumble strips should normally be in groups of 4 strips  
b) The height of the strips shall be no more than 10 – 15 mm  
c) One set of rumble strips is usually enough within 50km/h sections  
d) The last or only strip should be located 30 to 50 m before the hazard  
e) Pre-warning sets can, if used, be located 20 to 80 m before the hazard depending on speeds  
f) Rumble strips should preferably have white  thermoplastic lines across the top for better visibility  
g) Strips should continue across the full width of the carriageway, including the shoulders but be 
terminated so that they do not interfere with drainage.  
 
Figure  21 — Design of rumble strips option one  
 
10.3.4  The design of rumble strips by option two recommend the following principles and the typical 
illustration is given on Figure 22. The rumble strips should have rounded profile, a maximum height of 15 mm, 
and installed in groups of four extending the full width of the carriageway including the shoulders. The 
standard layout comprises of three groups of strips, the first pair 90 m apart and the second pair 60 m apart. 
Where approach speeds are less than 80 km/hr, the number of groups can be reduced to two or one. The last 
group should be 25 -50 m in advance of the hazard. It is further recommended that the spacing and width of 
rumble strips should be 200 m and rumble strips should not be used on bends with radius of less than 1, 000 
m.  m for 50 km/hr  20  to 50 m  30 
and 80 m for 70 km/hr  
Shoulder  Hazard   
64 © EAC  2024  – All rights reserved  
 
 
Figure  22 — Design of rumble strips option two  
10.3.5  Other speed control measures other than the speed humps and rumble strips should be tried on 
major roads to reduce vehicle speeds. Such measures may include narrowings, raised zebra crossings, and 
use of combination of measures.  
11 Pedestrian and Cyclists Facilities/ NMT Facilities  
11.1 Pedestrians Facilities  
11.1.1  Pedestrian facilities include sidewalks, crosswalks, traffic control features, bus stops or other loading 
areas, sidewalk on grade separations, and the stairs, escalators, or elevators related to these facilities, and 
curb cuts (depressed curbs and ramped sidewalks) and ramps for older walkers and persons with mobility 
impairments.  
11.1.2  The pedestrian facility shall provide a network of pedestrian routes and crossing facilities that is 
convenient to use and avoids conflicts with vehicular traffic.  
11.2  Shoulders and Footways  
11.2.1  The shoulder shall be at least 1.5 m wide, though 1 m is just acceptable if there are constraints. The 
surface must be well drained and be as smooth as the traffic lanes, if not, pedestrians may prefer to walk in 
the traffic lane.  
11.2.2  The height of barrier kerb shall be 100 – 200 mm high to deter vehicle from encroaching sidewalks.  
11.2.3  In situation such as  high-speed and / or high volume roads, letting pedestrians use the shoulders is 
not entirely satisfactory, as there is nothing to protect the pedestrian from speeding traffic. In these situations, 
it is preferable to provide a separate footway several metres beyond the edge of the shoulder – and separated 
from it by a grass strip.  
Some criteria for the provision of footways are given in Table 31. However, but these should be used with 
caution – In some circumstances footways can be justified at lower pedestrian flows.  
 
Table 31 — Criteria for provision of footways  
Location of footway  Average daily 
vehicle traffic  Pedestrian flow per day  
Speed limit of 
60 – 80 km/h  Speed limit of  
80 – 100km/h   
© EAC 2024  – All rights reserved  65 
 One side only  400 to 1,400  300 200 
> 1,400  200 120 
Both sides  700 to 1,400  1,000  600 
> 1,400  600 400 
 
11.2.4  Standard footway widths shall be as follow:  
a) Absolute minimum:  1 m (two persons cannot pass each other)  
b) Desirable minimum:  1.8 m (two persons can pass each other closely)  
c) Light volume:  2.25 m (two persons can pass each other comfortably)  
d) Heavy volume:  3.5 m + (space for three persons)  
11.2 Pedestrian Bridges and Underpasses  
11.2.1  The minimum height for underpasses shall be 2.3 for short (< 15 m) and 3.0 m for long (> 15 m) 
underpasses.  
11.2.2  The recommended flights of stairs (between landings) is limited to 12 steps and 9 steps where there 
are significant numbers of disabled persons.  
11.2.3  The headroom under pedestrian bridges shall be 5.5m above the carriageway surface.  
11.3 Cycle Facilities  
11.3.1  The conventional view is that cyclists in rural areas can use the shoulders, and this is acceptable 
provided that the combined volume of pedestrians and cyclists is low (<400 per day) and the shoulder is at 
least 1.5 m wide. However, with heavier flows, and especially if there is high -speed traffic or a high proportion 
of heavy goods vehicles, it will be better to provide a separate cycleway or a combined cycleway and footway.  
11.3.2 The design manuals for widths for cycle facilities are given in Table 32 and clearances to wall, fence, 
barrier or other fixed object for various speed limits are given in Table 33. 
Table 32— Widths for cycle facilities  
Type  Minimum width (m)  Standard width (m)  Width for heavy usage (m)  
Cycleway  
(separate from 
carriageway)  2.0 2.5 3.5 
Combined cycleway and 
footway  2.0 3.0 4.5 
Cycle lane (one 
way) 1.5 2.0 2.5 
 
Table 33 — Clearances for cycle facilities  
Type  Recommended clearance  [m]  
Minimum overhead clearance  0.50 
Clearance to wall, fence, barrier or other fixed object  0.50 
Clearance to unfenced drop -off, e.g. embankment, river, wall  1.0  
66 © EAC  2024  – All rights reserved  
 Minimum clearance to edge of traffic lane for speed limit of:  
50 km/h  
80 km/h  
100 km/h  0.50 
1.00 
1.50 
 
11.3.3  It is also recommended to provide separate cycleway or combined cycleway and footway if there is 
high-traffic and the combined flow of pedestrians and cyclists is more than 400 per day.  
11.3.4  Cycleways shall have a smooth surface with good skid resistance.  
12 Traffic Control Devices and Other Facilities  
12.1 General  
Road furniture and safety represents a collection of marginal elements intended to improve the driver's 
perception and comprehension of the continually changing appearance of the road. Such elements include 
traffic signs, road markings, traffic signals marker posts, kilometre posts and road reserve marker posts.  
12.2 Marker Posts  
The maker posts are intended to make drivers aware of potential hazards. They are to be sited at 0.25 m 
outside the edge of the shoulder.  
12.3 Kilometre Posts  
12.3.1  These are blocks or pillars of concrete set up beside major roads to show distances from that point 
to town centres or major settlements along the road. They show the distances to the destination and that of 
the origin placed in such a way that the road user will only immediately see the distance to the destination.  
12.3.2  The kilometre posts are installed along the whole road at an interval of distance of 5 kilometres from 
each other. Also the posts should be placed in stagger thus forming a 10 km interval on each side of the road.  
12.4 Road Reserve Marker Posts  
12.4.1  Demarcation of the boarder of the road reserve shall be done by proper placement of road reserve 
markers posts to limit the unauthorised access tend to develop within the road reserve area.  
12.4.2  The posts are recommended to be erected on both sides of the road at intervals of 100 m from each 
other when traversing inhabited areas and at 300 m on other areas. Whenever new villages are formed along 
the roads, additional road reserve marker posts should be erected to meet the 100 m interval.  
12.5 Rest Areas  
There are three categories three types of rest areas: major, minor and truck parking bays. Having taken these 
requirements into account, we recommend that the following requirements should be taken into account when 
planning for rest areas.  
12.5.1  Planning for Rest Areas  
12.5.1.1  A detailed Rest Area Strategy Plan should be developed for all mobility roads and significant freight 
routes. Three categories of rest areas should be reflected in the Rest Area Strategy Plans developed by road 
agencies for all major highways and significant freight routes:   
© EAC 2024  – All rights reserved  67 
 a) Major Rest Areas:  These areas are designed for long rest breaks, offering a range of facilities and 
separate parking areas for heavy and light vehicles. These are designed to allow drivers to take rest 
and sleep breaks required under current driving hours regulations.  
b) Minor Rest Areas:  These areas are designed for shorter rest breaks, and at a minimum should 
provide sufficient parking space for both heavy and light vehicles. While it is not anticipated that these 
stops will be used for long rest breaks/sleep opportunities, separate parking areas for heavy and light 
vehicles may be required at some locations.  
c) Truck Parking Bays:  These areas are primarily designed to allow drivers of heavy vehicles to conduct 
short, purpose -based stops including load checks, completing logbooks and addressing associated 
operational needs.  
12.5.1.2  On a given highway or freight route, a mix of the three rest area categories shall be provided. In the 
development of new rest areas, the category and size of a selected rest area shall flow from the Rest Area 
Strategy Plan developed for the route. Selection shall be based on the mix and volume of traffic, the perceived 
demand for the rest area and the availability of existing rest opportunities, commercial service centres or 
nearby towns.  
12.5.2  Spacing Intervals  
12.5.2.1  Intervals between rest areas shall depend on the category of rest area selected, the volume and mix 
of traffic and the demand for parking and rest opportunities identified in the Rest Area Strategy Plan for a 
given highway or route. However, as a general rule:  
a) Major Rest Areas  shown in Figure 23, shall be located at maximum intervals of 100 – 120 km,  
b) Minor Rest Areas  shown in Figure 24 shall be located at maximum intervals of 50 – 60 km, and  
c) Truck Parking Bays  shall be located at maximum intervals of 30 – 40 km.  
 
Figure  23 — Typical layout design with truck and car parking of major rest areas –front to rear parking 
design  
 
  
68 © EAC  2024  – All rights reserved  
 Figure  24 —  Typical layout design with truck and car parking of minor rest areas –angle parking 
design  
12.5.2.2  Differences in traffic volume and rest area demand will impact on the spacing of rest areas, and the 
number and size of parking spaces provided. Spacing may need to be reduced on highly trafficked routes 
where demand is evident.  
12.5.2.3  On mobility roads it is recommended that Rest Areas, and especially commercial service centres, are 
located at mid -block locations on both carriageways, offering equal stopping opportunities in advance of sites 
servicing opposing traffic. Where duplicate rest areas are not available (due to insufficient funding or 
demand), facilities should be staggered in the direction of approaching traffic to discourage cross median 
vehicular movements and to deter drivers from parking on shoulders and walking across the carriageway to 
access facilities.  
12.5.2.4  On major, undivided highways it is recommended that Rest Areas and Truck Parking Bays are 
provided on both sides of the road. This is particularly important in order to eliminate the need for heavy 
vehicles to turn across oncoming traffic to enter a rest area on the opposite side of the road.  
12.5.3  Proximity to a Town  
The rest areas shall be located to promote the use of town facilities (including toilet and shower amenities and 
the purchase of food and fuel), where they are provided and accessible on a given route. Where the traffic 
volume and demand warrants, consideration should be given to providing a Truck Parking Bay within 20km to 
30km of a township to allow drivers the opportunity to take a rest break and check vehicle loads.  
12.5.4  Location  
12.5.4.1  In planning the location of new rest areas and the upgrading of existing rest areas, the status and 
physical characteristics of the environment of a potential or existing site must be examined. Issues associated 
with topography, landmarks or scenic viewpoints and environmental qualities should be considered. The 
location of watercourses and utilities, the proximity to major road interchanges and the need for additional 
land should also be considered.  
12.5.4.2  Whilst it is most desirable to comply with the siting requirements summarised above, there are many 
other features that influence the location of rest areas. It must be practicable to provide a rest area at a 
particular site in relation to factors such as topography, availability of utilities, environmental considerations, 
other road infrastructure, and traffic operations and safety.  
12.5.5  Proposed Layout  
The rest areas shall be designed to provide suitable facilities in an environment that promotes effective and 
safe rest and/or sleep opportunities, and to ensure that there is adequate provision for vehicles and 
pedestrians to move safely within the site.  
The number of spaces provided at a given rest area site shall be based on traffic volume and expected 
demand. As a general rule:  
a) major rest areas should provide sufficient parking space for at least 20 vehicles,  
b) minor rest areas should provide parking for up to 10 vehicles, and  
c) truck parking bays should provide sufficient area to accommodate at least four to five heavy vehicles 
at any one time.  
12.5.6  Pedestrian Access and Visibility  
Rest areas and service centres shall be designed to ensure that potential conflict between vehicles and 
pedestrians is minimised, and that any necessary interaction occurs at a very low speed.   
© EAC 2024  – All rights reserved  69 
 12.5.7  Speed  
The design speed for vehicle travel within a rest area shall be considered in conjunction with the type of rest 
area, the location within the area and environmental conditions.  
12.5.8  Access  
When designing rest area layouts, consideration should be given to the provision of safe and effective access 
to the facility required for different standards of roads. Features including acceleration and deceleration lanes, 
entrance and exit ramps and slip lanes need to be designed.  
12.5.9  Minimum Facilities  
12.5.9.1  The minimum facilities that shall be provided at major and minor rest areas are:  
a) All weather pavements, with sealed pavements for access and egress roads/ramps.  
b) Shade.  
c) Rubbish bins.  
d) Separate parking for heavy and light vehicles.  
e) Sheltered areas.  
f) Tables and/or benches.  
12.5.9.2  Truck parking bays shall include, at a minimum, rubbish bins, shade and all weather pavements. If 
possible, sheltered areas and tables and benches should be provided. Rest area selection is based on the 
provision of natural shade, rubbish bins, all weather surfaces and sealed pavements, and the quality and 
cleanliness of toilets and running water provided.  
12.5.9.3  Provision for People with Disabilities shall also be taken into account when design rest Area. This 
also includes access to and the use of rest area facilities, provision of suitable toilets, parking spaces, and 
appropriately graded and treated footways and ramps.  
12.5.10  Additional Facilities  
Additional facilities such as toilets and drinking water shall be provided in rest areas where they can be 
adequately justified based on estimated use and the cost of maintenance and cleaning. The provision of 
additional facilities, such as toilets, showers and drinking water, should be considered on a site by site basis, 
taking into consideration the availability of required utilities, the installation requirements and the costs 
associated with ongoing facility maintenance. The ongoing maintenance and upkeep of these facilities should 
be documented and implemented.  
12.5.11  Cross -Border Compatibility  
The State and Territory road authorities are recommended to adopt common guidelines for the provision of 
rest areas and service centres, and coordinate their provision across state and territory borders, so that a 
consistent level of service is provided along important routes.  
In order to maximise the benefits of rest areas and service centres it is imperative that the provision of these 
facilities be closely coordinated across state and territory borders. Duplication of such facilities may lead to 
under -utilisation of one or both facilities and adversely affect their viability.  DEAS 1206 : 2024    
 
© EAC  2024  – All rights reserved