ICS 93.080.10  
 
Reference number  
DRS  267-1: 20 21 
© RSB 20 21 
  
 RWANDA 
STANDARD  DRS 
267-1   
 
Second  edition  
2021 -mm-dd  
Feeder roads —  
Part 1:  
Guidelines for design   
 
   
DRS  267-1: 20 21 
©RSB 2021  - All rights reserved                  ii 
 In order to match with technological development and to keep continuous progress in industries, 
standards are subject to periodic review. Users shall ascertain that they are in possession of the latest 
edition  
 
 
 
 
 
 
 
 
© RSB 2021  
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized 
in any form or by any means, electronic or mechanical, including photocopying and microfilm, without 
prior written permission from R SB. 
Requests for permission to reproduce this document should be addressed to : 
Rwanda Standards  Board  
P.O Box 7099 Kigali -Rwanda  
KK 15 Rd, 49  
Tel. +250 788303492  
Toll Free: 3250  
E-mail: info@rsb.gov.rw  
Website: www.rsb.gov.rw  
ePortal: www.portal.rsb.gov.rw   DRS  267-1: 20 21 
iii                ©RSB 2021  - All rights reserved  
 Contents  Page 
1 Scope  ................................ ................................ ................................ ................................ .............  1 
2 It applies to paved and unpaved feeder roads. Normative references  ................................ ... 1 
3 Terms and definitions  ................................ ................................ ................................ ..................  1 
4 Symbols (and abbreviated terms)  ................................ ................................ .............................  10 
5 Elements of design  ................................ ................................ ................................ .....................  12 
5.1 General  ................................ ................................ ................................ ................................ ........  12 
5.2 Horizontal alignment  ................................ ................................ ................................ ..................  24 
5.3 Vertical alignment  ................................ ................................ ................................ .......................  36 
5.4 Cross section elements  ................................ ................................ ................................ .............  44 
5.5 Drainage design  ................................ ................................ ................................ ..........................  48 
5.6 Site and materials investigation  ................................ ................................ ...............................  58 
5.7 Pavement  ................................ ................................ ................................ ................................ ..... 62 
5.8 Design for clima te resilience  ................................ ................................ ................................ ..... 71 
5.9 Seismic considerations  ................................ ................................ ................................ .............  72 
5.10  Demographics  ................................ ................................ ................................ .............................  72 
5.11  All-Weather  Access  ................................ ................................ ................................ ....................  72 
5.12  Bridge design  ................................ ................................ ................................ ..............................  73 
5.13  Safety  ................................ ................................ ................................ ................................ ...........  74 
5.14  Quality control  ................................ ................................ ................................ ............................  75 
 DRS  267-1: 20 21 
©RSB 2021  - All rights reserved                  iv 
 Foreword   
Rwanda Standards are prepared by Technical Committees and approved by Rwanda Standards Board (RSB) 
Board of Directors in accordance with the procedures of RSB, in compliance with Annex 3 of the WTO/TBT 
agreement on  the preparation, adoption and application of standards.  
The main task of technical committees is to prepare national standards. Final Draft Rwanda Standards adopted 
by Technical committees are ratified by members of RSB Board of Directors for publication and gazettment as 
Rwanda Standards.  
DRS 267 -1 was prepared by Technical Committee RSB/TC 009, Building materials and civil engineering .  
In the preparation of this standard, reference was made to the following standards:  
1) Rwanda Feeder Roads Standards — RFRS/Vr14/05/07BL — Final Report  
2) ISO 6707 -1: 20 20, Building s and civil engineering works — Vocabulary — Part 1: General terms   
3) AASHTO 1993, Guide for design of pavement structures   
4) Low Volume Roads  — Volume 1: Pavement Design  —Republic of Malawi  
The assista nce de rived from the above source is hereby acknowledged with thanks.  
This second edition cancels and replaces the first edition (RS  267-1: 2015), of which has been technically 
revised.  
RS 267 consists of the following parts, under the general title Feeder roads : 
 Part 1: Guidelines for design   
 Part 2: Guidelines for maintenance   
Committee membership  
The following organizations were represented on the Technical Committee on Building materials and civil 
engineering  (RSB/TC 009) in the preparation of t his standard.  
Institution of Engineers Rwanda  
Ministry of Infrastructure (MININFRA)  
NPD Ltd  
Rwanda Transport Development Agency (RTDA)  DRS  267-1: 20 21 
v                ©RSB 2021  - All rights reserved  
 Rwanda Public Procurement Authority (RPPA)  
TECOS Ltd  
University of Rwanda -College of Science and Technology (UR -CST)  
Rwanda Standards Board (RSB) - Secretariat  DRS  267-1: 20 21 
©RSB 2021  - All rights reserved                  vi 
 Introduction   
Road is one of the transport infrastructure which facilitates local and international exchange of goods and 
services. Adequate transport of agricultural products between different parts of the country contributes to the 
socio -economic development of local population through easy access to markets, modernization and 
commercialization of agriculture, creating possibilities for transporting modern agricultural inputs and 
machineries to farms hence creating  new opportunities and value addition to raw products.  
To enhance connectivity to agricultural market cent res, guidelines on effective design, construction and 
maintenance of roads linking agricultural areas with commercial cent res and/or processing plant s are necessary 
to provide guidance on cost effectiveness of the roads as well as technical guidance to designers and 
contractors.  
Regardless of the hierarchal classification of roads in Rwanda,  road that links districts, productive areas or other 
urban centres to national roads and other low volume roads having less than 300 vehicle per a day and 
Equivalent standard Axle road less than 1 million during the design period  either paved or unpaved ar e defined 
as “Feeder roads ”.   
This standard provides designers and contractors of feeder roads with uniform set of guidelines, which address 
elements for design such as horizontal and vertical alignments, cross section, drainage, materials, pavement 
and s eismic considerations. It also provides requirements specific to paved and unpaved feeder roads.  
In this standard, the term” should” expresses recommendations, whereas “shall” expresses requirements.  
 
 DRS 267-1: 20 21 
1                ©RSB 2021  - All rights reserved  
 Feeder roads — Part 1: Guidelines for design  
1 Scope   
This Draft Rwanda S tandard provides guidelines for the design of feeder roads intended for a transport chain 
with one end in the agricultural fields  and the other on the farm gate, the transport chain from the farm gate 
network to collection centre, post -harvest processing and storage facilities and a transport chain from one of 
the latter to loca l markets, processing centres, post -harvest storage facility centres to district roads.  
This standard is applicable to classified roads. It excludes unclassified roads.  
2 It applies to paved and unpaved feeder roads. Normative references   
The following refer enced documents are indispensable for the application 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.  
RS ISO 22476 –2, Geotech nical investigation and testing — Field testing — Part 2: Dynamic probing  
RS 265, Precast reinforced concrete pipe culvert — Specification  
3 Terms and definitions   
For the purposes of this standard, the following terms and definitions apply.  
3.1 
feeder road  
road which links districts, productive areas or other urban centres to national roads   
3.2 
productive areas  
centres for agricultural production activities, public utilities, community development centres, business centres, 
economic zones, dumping sites, a dministrative offices, natural ressources processing, tourist sites  
3.3 
Low Volume Road (LVR)  
road having less than 300 vehicle per day and cumulative Equivalent standard Axle road less than 1 million for 
the design period  
3.4  
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 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;  
3.5  
unclassified road  
roads that are not  assigned to classes or cate gories  
3.6  
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  
3.7  
Average Annual Daily Traffic (AADT)  
total yearly tra ffic volume in both directions divided by the number of days in the year  
3.8 
back slope  
area proceeding from ditch bottom to the limit of the earthworks            
3.9 
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  
3.10  
carriageway  
part of the road constructed for use  by moving traffic, including auxiliary lanes, climbing lanes, and passing 
places but excluding shoulders  
3.11 
travelled way  
That part of the carriageway used for the movement of vehicles, exclusi ve of auxiliary lanes, bus -bays and  
shoulders.  DRS 267-1: 20 21 
3                ©RSB 2021  - All rights reserved  
 3.12 
circula r curve  
usual curve configuration used for horizontal curves  
3.13 
clear zone  
unencumbered roadside recovery area  
3.14  
climbing lane  
auxiliary lane in the upgrade direction for use by slow moving vehicles and to facilitate overtaking, thereby 
maintain ing capacity and freedom of operation on the carriageway  
3.15 
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  
3.16  
cross -fall  
tilt or transverse inclination of the cross -section of a carriageway which is not cambered, expressed as a 
percentage  
3.17  
cross -section  
vertical section of the road at right angles to the centre line  
3.18 
deflection angle  
successive angles from a tangent  subtending a chord and used in setting out curves  
3.19  
design speed  
a selected speed used to determine the various geometric design features of the road way to ensure a safe 
operation of vehicles .  
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 3.20  
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  
3.21  
design vehicle  
vehicle whose physical characteristics and proportions are used in setting geometric design  
3.22 
design volume  
volume determined for use in design, representing traffic expected to use the road  
3.23  
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  
3.24  
fill  
material which is used for the construction of embankments   
3.25  
road function  
objective of providing a particular road link in terms of being " national ", "districts  or "access"   
3.26 
horizontal curve/curvature  
curve/succession of curves, normally circular, in plan  
3.27 
transition curve  
curve in which the radius changes continuously along its length, used for the purpose of connecting a straigh t 
with a circular curve, or two circular curves of different radii  DRS 267-1: 20 21 
5                ©RSB 2021  - All rights reserved  
 3.28  
vertical curve  
curve on the longitudinal profile of a road,                               
   
3.29 
vertical alignment  
direction and course of the centre line in profile  
3.30 
lane  
strip of carriageway intended to accommodate a single line of moving vehicles  
3.31 
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 h orizontal distance  
3.32  
guardrail  
continuous barrier erected alongside a road to prevent traffic from accidentally leaving the roadway or from 
crossing the median  
3.33  
horizontal alignment  
direction and course of the road centreline in plan  
3.34 
horizontal curve  
curve in plan  
3.35  
object height  
assumed height of a notional object on the surface of the roadway used for the purpose of determining sight 
distance   
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 3.36 
Passenger Car Equivalent Unit (PCU)  
unit of road traffic, equivalent for capac ity 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  
3.37 
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 start ed  
3.38  
pavement  
part of a road designed to withstand the weight or loading by traffic  
3.39  
reaction time  
time taken by the driver to perceive the hazard ahead plus the time taken to activate the brake  
3.40  
safety barrier  
continuous barrier erecte d alongside a road to prevent traffic from accidentally leaving the carriageway or verge 
or from crossing the central reserve  
3.41  
hard shoulder  
part of the road outside the roadway, but at substantially the same level, for accommodation of stopped vehic les 
for emergency use, and for lateral support of base and surfacing courses  
3.42 
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.  DRS 267-1: 20 21 
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 3.43  
side slope  
area between the outer edge of shoulder or hinge point and the ditch bottom  
3.44  
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  
3.45 
single lane road  
road consisting of a si ngle traffic lane serving both directions, with passing bays  
3.46 
speed  
rate of movement of vehicular traffic or of specified components of traffic, expressed in kilometres per hour 
(km/h)  
3.47  
road hump  
physical obstruction, , placed transversely on the surface of the carriageway for the purpose of reducing traffic 
speed  
3.48 
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 f orce on a moving vehicle; expressed as a percentage        
                       
3.49 
functional classification   
grouping of streets and roads according to the character of the service they are intended to provide  
3.50  
design hourly volume   
projected  hourly volume that is used in the design    
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 3.51  
subgrade  
the naturally occurring material on which the pavement is constructed.  
3.52  
median  
portion  of a divided road separating the travelled ways for traffic in opposite directions including inside shoulders  
3.53  
roadway  
portion of a road, including shoulders,  intended  for vehicular use  
3.54  
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   
3.55  
street  
road which  has become partly or wholly defined by buildings established along one or both frontages   
3.56  
tangent  
portion of a horizontal alignment of straight geometrics   
3.57  
traffic  
vehicles, pedestrians and animals travelling along a route  
3.58  
traffic flo w  
number of vehicles or persons that pass a specific point in a stated time, in both directions unless otherwise 
stated  DRS 267-1: 20 21 
9                ©RSB 2021  - All rights reserved  
 3.59  
traffic lane  
part of a carriageway intended to accommodate a single stream of traffic in one direction  
3.60  
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   
3.61 
right -of-way or 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 
angles to the centreline   
3.62  
road  
way for vehicles and for other types of traffic which may or may not be lawfully usable by all traffic  
3.63  
competent authority local government authority, institution, agency or any other body entrusted by  the Ministry 
in charge of roads with the duties to develop, manage and maintain roads  
3.64 
road side  
general term denoting the areas adjoining the outer edges of the shoulders  
3.65 
flat terrain  
terrain with largely unrestricted horizontal and vertical  alignment; transverse terrain slope up to 10 %  
3.66  
rolling terrain  
terrain with low hills introducing moderate levels of rise and fall with some restrictions on vertical alignment; 
traverse terrain slope 25 %   
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 3.67  
mountainous terrain  
terrain that i s rugged and very hilly with substantial restrictions in both horizontal and vertical alignment; 
transverse terrain slope above 25 %  
3.68 
steep terrain  
steep country inclusive of switchback sections and side hill traverses; transverse terrain slope great er than 
60 %. The terrain with this geological features needs special geometric design guidelines because of the 
engineering risks involved  
3.69 
Level Of Service (LOS)  
measure  used to quantify traffic flow and congestion by assigning quality levels of traffic based on performance 
measure such as speed, density, etc.  
3.70 
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, sele ct an appropriate speed and path and initiate a complex manoeuvre  
3.71 
laterites  
soil types rich in iron and aluminium, formed in hot and wet tropical areas  
3. 72  
soil stabilisation  
process of mixing soil with materials to permanently alter their physic al and chemical properties to increase  or 
maintain the stability of soil or improve  its strength or bearing capacity.  
4 Symbols (and abbreviated terms)  
AADT   Annual Average Daily Traffic  
AASHTO  American Association of State Highw ay and Transportation Offi cials 
ADT    Average Daily Traffic  DRS 267-1: 20 21 
11                ©RSB 2021  - All rights reserved  
 BF   Basic Access Farms  
CBR    California Bearing Ratio  
CF      Cross Fall  
DCP    Dynamic Cone Penetrometer  
DHV    Design Hourly Volume  
DSD    Decision Sight Distance  
DTV    Design Traffic Volume  
EIA     Environmental Impact Assessment  
EIS     Environmental Impact Statement  
ESALs    Equivalent Single Axle Loads  
GM     Silty Gravel  
GWC    Gravel Wearing Course  
HFL     Highest Flood Level  
HSG    Hydraulic Soil Group  
IRI      International Roughness In dex  
LS      Linear Shrinkage  
LCVC    Length of the Crest Vertical Curve  
LL      Liquid Limit  
LVR    Low Volume Roads  
LVRR    Low Volume Rural Roads  
MDD    Maximum Dry Density  
NCF    Normal Cross Fall   
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 PCU    Passenger Car Equivalent Unit  
PL      Plastic Limit  
PSD    Passing Sight Distance  
ROW    Right -Of-Way  
SS      Side Slopes  
SSD    Stopping Sight Distance  
SW     shoulder Width  
TEF     Traffic load Equivalent Factor  
TFV     Ten percent Fines Value  
VPD    Vehicle Per Day 
5 Elements of design  
5.1 General  
5.1.1  Climatic zone  
5.1.1.1  For the purposes of feeder roads or LVR designs, climatic zone s shall be considered.   
NOTE  The places with mean annual rainfall greater than  or equal to  1000 mm are considered wet zones  
and all other places shall be considered as dry zones. Map of Rwanda Climatology.  
5.1.1.2  Designer should also consider the following four climatic seasons:  
i) Season 1 , short dry season which extends from December of the preceding year to 
the end of Februa ry of the succeeding year;   
ii) Season 2 ,  long rainy season, which extends from February to the end of May;  
iii) Season 3 ,  long dry season which extends from June to the end of August; and  
iv) Season 4 ,  short rainy season, which extends from October to the end of November . DRS 267-1: 20 21 
13                ©RSB 2021  - All rights reserved  
 5.1.2  Terrain classes  
Terrain class shall be determined by the number of five -meter contours crossed by a straight line connecting 
the two ends of the concerned LVR; independent of a road’s selected alignment. Terrain classification is shown 
in table 1.  
Table 1 — Terrain classification  
S/N Terrain class  cross slope  
(%) 
1 Plain  0 - 10 
2 Rolling  > 10 - 25 
3 Mountainous  > 25 - 60 
4 Steep  > 60 
 
5.1.3  Environmental considerations  
5.1.3.1  Prior to approval of any LVR construction project, an Environment Impact Assessment (EIA) shall 
be done by expert and approved by a Competent Authority to ascertain acceptable impact to the environment.  
5.1.3.2  There shall be a direct relationship between  good road engineering practices and sound 
environmental management. Choices that can affect the potential for adverse environmental impacts should be 
assessed, these should include the following:  
 finding the correct road alignment or location to cross a w atercourse or a wetland at the narrowest 
place;  
 being aware of the boundaries of protected areas;  
 considering the possibility of a road diversion around an urbanized village to minimize the potential 
for traffic safety or dust issues; and  
 considering th e quality and dimensions of a road going through an urbanized area, and the option 
to seal it, widen it, enhance the quality of the shoulders, create pedestrian lanes.  
5.1.3.3  The following environmental issues should be considered:  
 road construction related disruptions to the local hydrology and watershed function;  
 damage to fragile wetland ecosystems, stream courses, drainage ways, and the biodiversity assets 
they contain;  
 improper road construction or lack of maintenance creates standing or stagnant water situations;   
©RSB 2021  - All rights reserved                  14 
 
 proper environmental management of borrow pit and quarry sites;  
 improved road conditions may lead to better access to rural forestry assets such as woodlots,  small 
plantations, and even natural forest;  
 sourcing timber needed for road furniture or structures in an unsustainable manner;  
 social conflict because of perceived inequities with the road building;  
 improved road conditions and traffic safety;  
 provid ing traffic control devices, that is, road humps, to slow traffic through villages and other 
populated areas;  
 improved roads lead to more noise and road dust; and  
 road camps and their placement and decommissioning.   
5.1.4  Functional classification  
The road de sign parameters for low traffic volume in which the feeder roads fall in, should rely on the maximum 
design traffic volumes of 300  VPD.   
5.1.5  Projection for future demand  
5.1.5.1  Geometric design of new roads or improvement of existing roads should not usuall y be based on 
current traffic volumes alone, but should consider future traffic volumes expected to use the facility.  
5.1.5.2  The design life and traffic design volume should be for seven years on gravel roads corresponding 
to the period of re -gravelling on a regularly maintained gravel road.  
5.1.5.3  For paved feeder roads, the design life should depend on the type of surfacing selected.  
Type of surfacing  Design life  
Asphalt concrete  20 
Double surface treatment/double chip seal  12 
Cape seal  15 
Cement  concrete road  30 
Stone paved roads  Variable  
5.1.6  Level Of Service (LOS)  
5.1.6.1  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 should be six LOS:  DRS 267-1: 20 21 
15                ©RSB 2021  - All rights reserved  
 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 n oticeable 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. Vehi cle 
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 p oor.  
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.  
5.1.6.2   In the case of feeder roads, LOS D shall be  used for all types of terrains.  
5.1.7  Design vehicle  
5.1.7.1  Feeder roads should be designed to accommodate safely and conveniently all vehicles types and 
dimensions for which they have been designed. The design shall ensure that the vehicle can negotiate the  road 
geometry (see figure 1).  
5.1.7.2  Four general classes of design vehicles are indicated in table 2.  
 
 
  
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 Table 2 — Design vehicles  
Design vehicle in types  Symbol   Dimensions  
 
Length  Width   Height  Minimum 
turning radius  Wheel base  
Passenger  cars 
  
Passenger cars   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+6.52*+7.94  
Semi -trailer with long 
trailer  DV8  21.0  2.6   4.6  13.7  6.1 & 12.8  
*Distance between SU rear wheels and trailer front wheels  
  
5.1.7.3  The design vehicle should be  compared to the intermediate semi -trailer WB -12 (SI), (WB -40- 
imperial) as shown in figure 1a and Figure 1b.  
 
DRS 267-1: 20 21 
17                ©RSB 2021  - All rights reserved  
 Figure 1a — Typical tire size and space between tires applies to all trailers  
  
Figure 1b — Minimum turning path for intermediate semi -trailer  
Figure 1 — Geometry of the design vehicle and lateral clearance on curve  
5.1.7.4 For the design of feeder roads, the WB ‐12 (SI) ‐ WB‐40 (imperial) should be used. DV5 should be the 
minimum design vehicle for those types of vehicles travelling feeder roads. Furthermore, for design purpose, 
the width of the truck shall be assumed to be 2.6 m minimum.  
5.1.8  Passenger Car equivalent Unit (PCU)  
The number of PCUs associated with a single vehicle type is a measure of impedance that it offers to the 
passenger cars in the traffic stream. The recommended PCUs are presented in table 3.  
 
 
 
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 Table 3 — Passenger Car Equivalent Unit  
S/N Vehicle type   Terrain   
Flat  Rolling  Mountainous  
 PCU   
i.  Passenger cars  1.0  1.0  1.5  
ii.  Light goods vehicles  1.0  1.5  3.0  
iii.  Medium goods vehicle  2.5  5.0  10.0  
iv.  Heavy goods vehicles  3.5  8.0  20.0  
v.  Buses  2.0  4.0  6.0  
vi.  Motor cycles, scooters  0.5  1.0  1.5  
vii.  Pedal cycles  0.5  0.5  0.5  
5.1.9  Design hourly volume  
The design hourly volume represents the 30th highest hourly volume during the year. Field data should be 
gathered  to estimate the K factor. In the absence of these data, values of K of 0.15 for rural roads and 0.10 for 
urban roads should be used. Then DHV=0.15x ADT or 0.10 x ADT respectively.  
5.1.10  Design speeds  
Design speeds corresponding to feeder roads and respond to th e traffic volume criteria as a function of the four 
terrains classifications typical to Rwanda are indicated in table 4.   
Table 4 — Design speeds for  feeder roads as a function of the terrain type 
Road class   Design speed  (km/hr)   
Flat terrain  Rolling terrain  Mountainous terrain  Steep terrain  
Feeder roads   District 
class 1  80  60  40  40  
District 
class 2   60  40  30  30  
5.1.11  Brake reaction time  
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 inters ections.  
5.1.12  Brake distance  
5.1.12.1  The approximate braking distance of a vehicle level roadway travelling at a design speed of the 
roadway may be determined from the following equation:  DRS 267-1: 20 21 
19                ©RSB 2021  - All rights reserved  
 𝑑𝐵=0.039𝑉2
𝑎 
where 
𝑑𝐵 is braking distance, m ;  
V is design speed, km/h ;   
𝑎 is deceleration rate, m/ s2. 
5.1.12.2  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. The corresponding stopping sight distances as a 
function of the design speed likely to been countered on majority of the feeder roads network. For those 
segments of the feeder road network pertaining to higher class roads, corresponding values referred to are 
given in table 5.  The lowest design speed should be 30 k m/h.  
5.1.12.3  The stopping sight distance (SSD) is given by the formula below:  
𝑆𝑆𝐷 =0.278 𝑉𝑡+0.039𝑉2
𝑎 
where  
𝑆𝑆𝐷  is Stopping Sight Distance  
V  is Design speed, km/h  
t  is Brake reaction time, 2.5 sec  
𝑎  is Deceleration rate, m/s2 
Table 5 — Stopping sight distance for feeder roads  
Design speed 
(km/h)  Brake reaction 
distance   
(m)  Brake reaction distance 
on level  
(m)  Stopping sight distance  
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   
©RSB 2021  - All rights reserved                  20 
  
5.1.13  Effect of grade on stopping  
For road way  segment on a grade, the values of table 6 shall be used to take into consideration the grade G 
from the following relationship:  
𝑑𝐵=𝑉2
254 [(𝑎
9.81)+𝐺] 
where  
dB is braking distance on grade, m ;  
 V  is design speed, km/h ;  
  𝑎 is deceleration, m/s2;  
 G  is grade, rise/run, m/m .  
 
Table 6 — Stopping sight distance for feeder roads with effect of grade  
Design  
speed  
(km/h)    Stopping sight distance    
 Downgrades  
(%)    Upgrades  
(%)   
3  6  9  3  6  9  
20  20  20  20  19  18  18  
30  32  35  35  31  30  29  
40  50  50  53  45  44  43  
50  66  70  74  61  59  58  
60  87  92  97  80  77  75  
70  110  116  124  100  97  93  
80  136  144  154  123  118  114  
 
 DRS 267-1: 20 21 
21                ©RSB 2021  - All rights reserved  
 5.1.14  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 7).  
Table 7 — Decision Sight Distance  
Design sight 
distance  
(km/h)  Decision Sight Distance  
(m)   
Avoidance manoeuvre   
A  B  C  D  E  
50  70  155  145  170  195  
60  95  195  170  205  235  
70  115  325  200  235  275  
80  140  280  230  270  315   
©RSB 2021  - All rights reserved                  22 
 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 manoeuvr e 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 :  
 
𝐷𝑆𝐷 =0.278 𝑉𝑡+0.039𝑉2
𝑎 
where  
DSD      is Decision Sight Distance, m  
t           is pre -manoeuvre time, sec  
V         is design speed, km/h  
𝑎         is driver deceleration, m/s2 
NOTE 7  Avoidance manoeuvres C, D and E are given by the following relationship :  
 𝐷𝑆𝐷 =0.278 𝑉𝑡  
where  
DSD      is Decision Sight Distance, m  
t           is total pre -manoeuvre and manoeuvre time, sec  
V         is design speed, km/h  
  ;  
;  
;  
5.1.15  Object height design value  
The object height values shall be the ones indicated in Table 8 for the purpose  of stopping sight distance, 
decision sight distance and passing sight distance computation.   
 
 DRS 267-1: 20 21 
23                ©RSB 2021  - All rights reserved  
 Table 8 — Object height (m) for use in the computation of SSD, DSD and PSD  
 Object height  
(m)   
Stopping sight 
distance  Decision sight distance  Passing sight distance  
0.60  0.60  1.08  
5.1.16  Passing Sight Distance  
5.1.16. 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 th e design speed.  
5.1.16 .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.  
5.1.16 .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.  
5.1.16 .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 than 
the stopping sight distance. In cut sections, the resultant lateral dimension for normal highway cross section s 
(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.   
5.1.16. 5 The passing sight distance should be calculated on the basis of a d istance 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).  
5.1.16. 6 The recommended minimum passing sight distance for two lane -roads are given in Table 9.   
Table 9 — Passing Sight Distance in two -lane roads  
Design speed  
(km/h)  Assumed speeds  Passing sight distance  
(m)   
©RSB 2021  - All rights reserved                  24 
 Passed 
vehicle  Passing vehicle  Total passing sight 
distance  Rounded for 
design1  
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  
1The rounded values in Table 9 shall be used for Passing Sight Distance calculations   
5.2 Horizontal alignment  
5.2.1  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 s harp 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.  
0.01𝑒+𝑓
1−0.01𝑒𝑓=𝑣2
𝑔𝑅=0.0079 𝑉2
𝑅=𝑉2
127 𝑅 
where  
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 .  DRS 267-1: 20 21 
25                ©RSB 2021  - All rights reserved  
 5.2.2  Side friction factor ‘f’  
The value of (0.01 ef) being so small, the previous equation is re -arranged to give:  
𝑓=𝑉2
127 𝑅−0.01𝑒 
The relationship of the side friction and speed for low speed roads is given in figure 2 which can be simplified 
for design purposes.  
 
Figure 2 — Relationship between side friction and design speed on curve  
NOTE It can be seen that the side friction decreases as the speed increases. For the case of feeder roads, the first portion 
of the graph corresponding to speeds of 20 km/h to 70 km/h. For t hese same speeds, the side friction decreases from 0.35 
(at 20 km/h) to 0.15 (at 70 km/h).  
5.2.3  Relationship between the design speed and the running speed  
Values in table 10 for running speeds as a function of the design speed should be used.  
Table 10 — Rela tionship between design speed and running speed (km/h)  
Design speed  
(km/h)  Average running speed  
(km/h)  
20  20  
30  30  
40  40  
50  47  
60  55  
70  63  
80  70  
  
  
  
  
  
  
   
©RSB 2021  - All rights reserved                  26 
  
5.2.4  Normal cross slope  
5.2.4.1  Consistent with the type of road and amount of rainfall, normal cross slope vary between 3 % - 4 % 
for unpaved roads , 3% - 3.5 % for paved roads with chip seal and 2.5% for paved roads with asphalt concrete 
shall be adopted with due consideration of the d rainage system . Steeper slopes may be needed where curbs 
are needed to avoid ponding of water on the outside trough lane.  
NOTE: For unpaved roads, the range depends on the type of terrain  
5.2.5  5.2.4.2   Maximum superelevation rates  
5.2.5.1  Maximum superelev ation rates shall be controlled by four factors such 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 affect ed by high superelevation rates.  
5.2.6  5.2.5.2  For feeder roads, super ‐elevation should be limited to a maximum of 8%. Where 
terrain conditions are favourable, only the normal crown shall be provided.   Minimum radius  
5.2.6.1  For a given speed, minimum curve r adius 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 simp lified curve equation as:  
𝑅𝑚𝑖𝑛 =𝑉2
127 (0.01𝑒𝑚𝑎𝑥 +𝑓𝑚𝑎𝑥) 
5.2.6.2   The limiting values of the side friction are shown in table11.  
Table 11 — Limiting values of the side friction  
Design speed  
(km/h)  Limiting value of ‘f’  
30  0.17  
40  0.17  
50  0.16  
60  0.15  
70  0.14  
80  0.14  
5.2.6.3    DRS 267-1: 20 21 
27                ©RSB 2021  - All rights reserved  
 5.2.7  The straights  
Straight sections provide better visibility and more passing opportunities and hence enhance safety. The length 
of straight sections should not be greater than 20 x V where V is the design speed (km/h) to reduce the danger 
of glare and excessive speeding. A minimum of 6 x V should be also adopted between circular curves following 
the same direction.  
5.2.8  Minimum and maximum length of curves  
For small deflection angles, curves should be long enough to avoid the appearance of a kink. The length of a 
curve should  be at minimum 300 m, but should not exceed 1000 m.  
5.2.9  Turning roadways  
5.2.9.1 Compound curves  
Curves that are compounded should not be too short or their effect in enabling a change in speed from the 
tangent or flat curve to the sharp curve is lost. In a series of curves with decreasing radii, each curve should be 
long enough to enable the driver to decelerate at a reasonable rate. The recommended deceleration rate in 
gear alone represents 1.5 km/h - 2.5 km/h. The minimum compound curves lengths computed  on base of these 
criteria should be developed on the premise that travel is in the direction of sharper curvature. Table 12 gives 
values of acceptable and recommended minimum length of circular arcs based on the compound curve radii.  
Table 12 — Length of  circular arcs for different compound curve radii  
Radius  
(m)  Minimum length of circular arc  
(m)  
Acceptable  Recommended  
30  12  20  
50  15  20  
60  20  30  
75  25  35  
100  30  45  
125  35  55  
150 or more  45  60  
5.2.10  Turning design control  
For appearance and comfort, the length of superelevation runoff should be based on a maximum acceptable 
difference between longitudinal grades of the axis of rotation and the edge of pavement. Current practice is to 
limit the grade difference referred to a s the relative gradient, to a maximum value of 0.5 % or a longitudinal 
slope of 1:200 at 80 km/h. A comfortable and aesthetically pleasing runoff design can be attained through the 
exclusive use of the maximum relative gradient criterion (See Table 13).   
©RSB 2021  - All rights reserved                  28 
 Table 13 — Maximum relative slope as a function of the relative gradient and the design speed  
Design speed  
(km/h)  Maximum relative gradient  
(%)  Equivalent maximum  
relative slope  
30  0.75  1.133  
40  0.70  1.143  
50  0.65  1.154  
60  0.60  1.167  
70  0.55  1.182  
80  0.50  1.200  
 
5.2.11  The minimum length of super -elevation runoff  
5.2.11.1  The minimum length of super -elevation runoff should be determined as follows:  
 
𝐿𝑟=(𝑤𝑛 1)𝑒𝑑(𝑏𝑤)
∆ 
where  
Lr  is minimum length of superelevation runoff, m  
w  is width of one traffic lane, m (  3.5m for district roads  class 1 and 3.0 m for district road class 2 );  
n1 is number of lane rotated ;  
ed  is design superelevation rate, %;  
bw is adjustment factor for number of lanes rotated ;  
∆ is maximum relative gradient, %.  
5.2.11.2  Table 14 gives adjustment factor for the number of lanes rotated. From the Figure 3, the Rwandan 
feeder road network should be equal to unity. Therefore, the minimum length for super ‐elevation runoff should 
be calcu lated from the equation given above with the value of b w = 1 and w = 3.0m.  DRS 267-1: 20 21 
29                ©RSB 2021  - All rights reserved  
 Table 14 — Adjustment factor for the number of lanes rotated  
Number of lanes rotated, 
n1  Adjustment factor,* bw  Length increase relative to 
one- lane rotated  
(= n1bw)  
1  1.00  1.0  
1.5  0.83  1.25  
2  0.75  1.5  
2.5  0.70  1.75  
3  0.67  2.0  
3.5  0.64  2.25  
 
Figure 3 — Lane rotations and determination of the adjustment factor  
 
5.2.12  Minimum length of tangent run -out  
5.2.12.1 The length of tangent run -out is determined  by the amount of adverse cross slope to be removed and 
the rate at which it is removed. To achieve a smooth edge of pavement profile, the rate of removal should equal 
the relative gradient used to define the super -elevation runoff length. Based on this ra tionale, the following 
equation is used to compute the minimum tangent runout length:  
𝐿𝑡=𝑒𝑁𝐶
𝑒𝑑𝐿𝑟 
where  
 
©RSB 2021  - All rights reserved                  30 
 Lt minimum length of tangent runout, m ;  
eNC nominal cross slope rate, %;  
ed design superlevation rate, %;  
 Lr  minimum length of superelevation runoff, m .  
5.2.12.2  The minimum length of tangent run ‐out shall be computed using the adopted values of cross slope 
rates, the design super ‐ elevation rates and the minimum length of super elevation length by means of the 
equation in 5.2.11 .  
5.2.13  Location of the superelevation runoff length with respect to the point of curvature  
To achieve the balance in lateral acceleration, it is recommended to locate the runoff length on the tangent prior 
to the curve. The portion of the runoff length placed  on the tangent should be 0.8 as shown on Table 15.  
Table 15 — Runoff locations that minimize the vehicle's lateral motion  
Design speed  
(km/h)  Portion of runoff located prior to the curve  
Number of lanes rotated  
1.0  
20-70  0.80  
80-130  0.70  
5.2.14  Minimum length of spiral transition curve  
5.2.14.1  The minimum length of spiral transition curve shall be calculated using the following equation:  
𝐿=0.0214 𝑉3
𝑅𝐶 
where  
 L  is minimum length of spiral, m ;  
 V  is speed, km/h ;  
 R  is curve radius, m ;  
C is rate of increase of lateral acceleration, m/s3. 
5.2.14.2  The factor C is an empirical value representing the comfort and safety levels provided by the spiral 
curve. The minimum length of the spiral transition curve should be equal to the length needed for superelevation 
runoff.  DRS 267-1: 20 21 
31                ©RSB 2021  - All rights reserved  
 5.2.15  Maximum radius for use of a spira l  
The maximum radius for use of a spiral should be based on a minimum lateral acceleration rate of 1.3 m/s2.Table 
16 gives the maximum radius for use of a spiral curve transition.  
Table 16 — Maximum radius for use of a spiral curve transition  
Design spe ed   Maximum radius  
 20  24  
 30  54  
 40  95  
 50  148  
 60  213  
 70  290  
 80  379  
5.2.16  Maximum length of spiral  
5.2.16.1  Two criteria shall be used, the first being based on the driver’s comfort is intended to provide a 
spiral length that allows for a comfortable increase in lateral acceleration as a vehicle enters a curve, the second 
is based on lateral shift and is intended to provide a spiral curve that is sufficiently long to result in a shift in a 
vehicle’s lateral position within its lane th at is consistent with that produced by the vehicle’s natural spiral path. 
The minimum spiral; length is thus computed as:  
Ls,min should be the larger of:  
𝐿𝑆,𝑚𝑖𝑛 =√24(𝑝𝑚𝑖𝑛)𝑅 
or 
𝐿𝑆,𝑚𝑖𝑛 =0.0214𝑉2
𝑅𝐶 
where  
Ls,min  is minimum length of spiral, m ;  
pmin is minimum lateral offset between the tangent and circular curve (0.20 m) ;  
R  is radius of circular curves, m ;  
V  is design speed, km/h ;   
©RSB 2021  - All rights reserved                  32 
 C  is maximum rate of change in lateral acceleration (1.2 m/s2).  
5.2.16.2  A value of 1.0 m is recommended fo r pmax. This value is consistent with the maximum lateral shift 
that occurs as a result of the natural steering behaviour of most drivers. It also provides a reasonable balance 
between spiral length and curve radius.  
5.2.16.3  The recommended minimum value  of the length of the spiral shall be given by the highest value 
obtained from the two equations for L s,min as provided  in 5.2.15 but shall not exceed the recommended length 
of spiral as given in Table 17. Widening of the travelled way shall be envisaged  to minimize the potential for 
encroachments into the adjacent lanes wherever lower values than those given in Table 17 or the preceding 
formulas are used.  
5.2.17  Recommended length of a spiral  
The most recommended operating conditions are when the spiral curve  length is approximately equal to the 
length of the natural spiral path adopted by drivers. Differences between these two lengths resulted in 
operational problems associated with large lateral velocities or shifts in lateral position at the end of the tran sition 
curve. Table 17 gives recommended length of spiral curve transition. If these values are higher than those given 
by equations in the preceding section, the minimum value shall be used.  
Table 17 — Recommended length of a spiral  
Design speed  
(km/h)  Spiral length  
(m)  
20  11  
30  17  
40  22  
50  28  
60  33  
70  39  
80  44  
5.2.18  Limiting superelevation rates  
Table 18 gives superelevation rates associated with large relative gradients. The values of superelevation  to be 
used in the design should not exceed those given in table18 so as to avoid an increase in the maximum relative 
gradient allowed for a tangent –to-curve design. Column 1 in Table18 shall be used in this standard .  
Table 18 — Limiting superelevation ra tes  
Design speed  
(km/h)  Number of lane rotated  
1  
20  3.7  DRS 267-1: 20 21 
33                ©RSB 2021  - All rights reserved  
 30  5.2  
40  6.5  
50  7.5  
60  8.3  
70  8.9  
80  9.3  
  
5.2.19  Length of tangent runout  
5.2.19.1  The tangent runout length for a spiral curve transition design  is based on the same approach used 
for the tangent -to-curve transition design. A smooth edge of pavement design is desired so that the common 
edge slope gradient is maintained throughout the superelevation runout and runoff sections. Table 19 gives the 
values of tangent runout length for spiral curve transition design for superelevation rates from 2 %  - 10%.  
5.2.19.2  The length of tangent runout is given by the following formula:  
𝐿𝑡=𝑒𝑁𝐶
𝑒𝑑𝐿𝑆 
where  
Lt is length of tangent runout, m ;   
 Ls is length of spiral, m ;        
ed  is design superelevation rate, %;   
eNC is normal cross slope rate, %.  
Table 19 — Tangent runout length for spiral curve transition design  
Design 
speed(km/h)  Tangent rounout length (m)  
Superlevation rate  
2  4  6  8  10  
20  11  -  -  -  -  
30  17  8  -  -  -  
40  22  11  7  -  -   
©RSB 2021  - All rights reserved                  34 
 50  28  14  9  -  -  
60  33  17  11  8  -  
70  39  19  13  10  -  
80  44  22  15  11  -  
 
 
 
5.2.20  Widths outside travelled way  
The width of the shoulders should be dictated by the established law on Right ‐Of‐Way (ROW). Table 20 
gives a range of usable shoulder widths or equivalent lateral clearances outside of turning roadways not on 
structures. Where roadway barriers are provided, the width indicated should be measured to the face of the 
barrier, and the graded width should be about 0.6 m greater.  
Table 20 — Range of usable shoulder widths or equivalent lateral clearances outside of turning 
roadways not on structures  
Turning roadway condition  shoulder width or lateral clearance outside of 
travelled -way edge  
(m)  
Left  Right  
Short  length,  usually 
 within channelized intersection  0.6 - 1.5  0.6 - 1.5  
Intermediate to long length or in cut 
or in fill   1.2 - 3.0  1.2 - 3.0  
 
5.2.21  Horizontal sight distance  
As illustrated on Figure 4, 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 5 is a 
design chart showing the h orizontal sight line offsets needed for clear sight areas that satisfy stopping sight 
distance criteria for horizontal curves of various radii on flat grades. Figure 5 includes radii for all superelevation 
rates to a maximum of 12 %.  DRS 267-1: 20 21 
35                ©RSB 2021  - All rights reserved  
  
Figure 4 — Diagram illustrating components for determining horizontal sight distance  
 
 
©RSB 2021  - All rights reserved                  36 
 Figure 5 — Design Controls for Stopping Sight Distance on Horizontal Curves  
The horizontal sightline offset is calculated using the following equation:  
𝐻𝑆𝑂 =𝑅[1−𝐶𝑜𝑠(28.65𝑆
𝑅)] 
where  
HSO  is horizontal sight line offset, m ;  
S is stopping sight distance, m ;  
R is radius of curve, m .  
The chart from Figure 5 shall be used for corresponding applicable to design speeds and superelvations as 
identified in previous sections .  
5.3 Vertic al alignment  
5.3.1  Crest vertical curve  
5.3.1.1  Parameters illustrated in Figure 6 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 term s 
of algebraic difference in grade and sight distance is given below:  
When S is less than L,  
𝐿 = 𝐴𝑆2
100 (√2ℎ1+√2ℎ2)2 
When S is greater than L,  
𝐿= 2𝑆−200 (√ℎ1+√ℎ2 )2
𝐴 
 
where  
 L  is length of vertical curve, m ;  DRS 267-1: 20 21 
37                ©RSB 2021  - All rights reserved  
 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 .  
5.3.1.2  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 shou ld be simplified to the following:  
When S is less than L,  
𝐿=𝐴𝑆2
658 
When S is greater than L,  
𝐿=2𝑆−658
𝐴  
©RSB 2021  - All rights reserved                  38 
  
Figure 6 — Parameters considered in determining the length of the crest vertical curve to provide 
sight distance  
5.3.2  Sag vertical curve  
5.3.2.1  Four different criteria for establishing lengths of sag vertical curves should be recognized to some 
extent. These are:  
A. headlight sight distance;  
B. passenger comfort;  
C. drainage control; and  
D. general appearance.  
5.3.2.2  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 
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39                ©RSB 2021  - All rights reserved  
 as a distance between the vehicle and the point where one -degree upward angle of the light bea m intercepts 
the surface of the roadway:   
When S is less than L,  
𝐿=𝐴𝑆2
200 [0.6+𝑆(tan1∗)] 
Or, 
𝐿=𝐴𝑆2
120 +3.5𝑆 
When S is greater than L,  
𝐿=2𝑆200 [0.6+𝑆(𝑡𝑎𝑛1∗)]
𝐴 
Or, 
𝐿=−120 +3.5𝑆
𝐴 
where  
 L  is length of sag vertical curve ;  
 A  is algebraic difference in grades, %;  
 S  is light beam distance, m .  
5.3.2.3  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  7 with solid lines using rounded values of K as was done for crest vertical curves.   
©RSB 2021  - All rights reserved                  40 
  
Figure 7 — Length of sag vertical curve as a function of algebraic difference in grade A (%)  
Table 21 — Stopping sight distance and rate of vertical curvature as a fun ction of the design Speed  
Design speed  
(Km/h)  Stopping site 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  
a 
Rate of vertical curvature, k, is the length of curve (m)percent difference intersecting grades  
(A),k=UA   
5.3.2.4  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.  
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41                ©RSB 2021  - All rights reserved  
 5.3.3  Sight distances at undercrossing  
5.3.3.1  In some conditions, it is recommended to check the available sight distance at undercrossing such 
as at an undercrossing without ramp where passin g sight distance need to be provided (Figure 8).  
 
Figure 8 — Schematic view of sight distance at undercrossing  
 5.3.3.2  General equations should be developed for two cases as follows:  
A. Case 1 Sight distance greater than length of vertical curve ( S > L )  
𝐿=2𝑆−800 [𝐶−(ℎ1+ℎ2
2)]
𝐴 
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 )  
 
©RSB 2021  - All rights reserved                  42 
 𝐿=𝐴𝑆2
800 [𝐶−(ℎ1+ ℎ2
2)] 
where  
L length of vertical curve, m ;  
A is algebraic difference in grades, %;   
S is sight distance, m ;       
h1 is height of eye, m ;       
h2 is height of object, m .  
5.3.3.3  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 lengt h of vertical curve ( S > L )  
𝐿=2𝑆−800 (𝐶−1.5)
𝐴 
B. Case 2 Sight distance less than length of vertical curve ( S< L )  
𝐿=𝐴𝑆2
800 (𝐶−1.5) 
5.3.3.4  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.   
5.3.3.5  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 consi dered.  DRS 267-1: 20 21 
43                ©RSB 2021  - All rights reserved  
 5.3.4  Gradients  
5.3.4.1  To avoid stagnant water, the minimum gradient should be of at least 0.5 %and the maxima shall not 
exceed the values given in Table 22 according to terrain categories .  
NOTE : The desirable maximum gradient should be less than 6% f or unpaved roads to avoid the erosion of 
materials. Where gradient is higher than the recommended, other alternative of paving the section should be 
proposed , 
Table 22 — Maximum gradient according to terrain categories  
Terrain  Maximum road gradient  
(%)  
Flat  4  
Rolling  4 - 6  
Mountainous  7 - 11  
Steep  12 - 18  
5.3.4.2  These gradients shall be used for feeder roads with due consideration of other factors like the 
drainage system.  
5.3.5  Climbing lanes  
Climbing lanes shall not be required for conditions of traffic flow on feeder roads except when safety 
considerations may require such addition of climbing lane, in which case it shall be done regardless of grade or 
traffic volumes.  
5.3.6  Other features affecti ng geometric design  
5.3.6.1  Erosion control and landscape development  
Erosion control and maintenance should be minimized largely by using specific design features such as flat side 
slopes, drainage channels designed with due regards to width, depth, slopes, ali gnment, and protective 
treatment, inlets located and spaced with erosion control in mind; prevention of erosion at culvert outlets; proper 
facilities for groundwater interception; dikes berms, and other protective devices to trap sediment at strategic 
locations; and protective ground coverage and planting. To the practical extent, these features should be 
designed and located to minimize the potential crash severity for motorists who unintentionally run of the 
roadway.  
5.3.6.2  Utilities  
5.3.6.2.1  Depending on th e location of a project, the utilities involved should include the following:  
 sanitary sewers;   
©RSB 2021  - All rights reserved                  44 
 
 water supply lines;  
 oil, gas, and petroleum product pipelines; overhead and underground power and communications 
lines including fibre optic cable;  
  cable te levision;   
 wireless communication towers;   
 drainage and irrigation lines; and 
 special tunnels for building connections.  
5.3.6.2.2 All utility installation on, over or under road or street right -of-way and attached structures should be of 
durable materials designed for long service -life expectancy, relatively free from routine servicing and 
maintenance, and meet or exceed the applicab le industry codes or specifications.  
5.3.6.2.3  For the  design and  installation of utilities, the designer  shall refer to the instructions /regulations  
issued by the relevant authority . 
5.4 Cross section elements  
5.4.1  Travelled way  
5.4.1.1  A typical cross section  showing different configurations of a normal crown road way is shown on 
Figure 9. The rate of cross slope shall be an important element in cross -section design. Cross slope or crown 
on tangents or on long radius curves are complicated by two contradictory  controls. On the one hand, a 
reasonably  steep lateral slope is recommended to minimize ponding of water on pavements with flat profile 
grades as a result of pavement imperfections or unequal settlement. Horizontal and vertical alignment should 
also be coo rdinated to avoid creating flat spots where crest vertical curves and super -elevation transition 
coincide. On the other hand, steep cross slopes are not recommended on tangents because of the tendency of 
the vehicle to drift towards the low edge of the tra velled way.  
5.4.1. 2 In areas of intense rainfall, a steeper cross slope may be needed to facilitate roadway drainage.   
5.4.1. 3 Because of the nature of the surfacing materials used and surface irregularities, unpaved surfaces 
as earth, gravel or crushed stone need an even greater cross slope on tangents to prevent the absorption of 
water into surface.  
 
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Figure 9 — Typical Cross -Section, Normal Crown  
5.4.2  Lane width  
The lane width of the roadway influences the comfort of driving, operational characteristi cs, and in some 
situations, the likelihood of crashes. The lane width  of carriageway  for feeder roads s hall be 3 m  to 3.5m . 
However, where the feeder road falls under the classification in the Law  Governing R oads in Rwanda, the 
recommended width shall prev ail.    
NOTE  In very difficult terrain where cut and fill operations might require substantial amount of earthwork, demarcation 
from the standard lane width might be considered. The same is done for need of enlargement especially in curves . However , 
the la nes width remain s in the range of 3 m - 3.5 m for feeder  roads. In some circumstances , it may be advisable to consider 
the construction of a single lane road, with periodic turn ‐outs to allow opposing vehicles to pass. Such circumstances are 
largely confined to existing 4 m ‐ 5 m wide roads in side hill cuts in mountainous terrain. In such locations, earthwork cost 
can be prohibitive to construct a two -lane width, and slope stabil ity can be affected. Engineering judgment should be 
employed in making the decision to use a single lane facility in low traffic volume areas.   
5.4.3  Curve widening  
The pavement should be widened at sharp curves to accommodate off -tracking, whereby the rear whe els, 
particularly of larger vehicles, do not follow precisely the same path as the front wheels when the vehicle 
negotiates a horizontal curve or makes a turn. The amount of off -tracking, and therefore the amount of widening 
needed on a curve shall depend on the characteristics of the design vehicle and the radius of curvature 
negotiated.  
 
©RSB 2021  - All rights reserved                  46 
 5.4.4  shoulder widths  
5.4.4.1  A hard shoulder width should be in the range of 0.6 m -1.5 m where judged necessary to better 
accommodate pedestrian and bicycle traffic, and par ked vehicles, in built -up areas.  
5.4.4.2  When roadside barriers, walls, or other vertical elements are present, it is recommended to provide 
a graded shoulder wide enough that the vertical elements should be offset a minimum of 0.6  m from the outer 
edge o f the usable shoulder. On low -volume roads, roadside barriers may be placed at the outer edge of the 
shoulder however, a minimum clearance of 1.2 m should be provided from the travelled way to the barrier.  
 
Figure 10 — Graded and usable shoulders  
5.4.5  shoulders cross sections  
Shoulders for roads  class 1 shall  be 1.0 m ‐1.5 m wide while for road class  2, shoulders shall be 0.6 m - 1.0 m 
wide.   
5.4.6  Normal cross fall  
A cross slope is used on traffic lanes to promote drainage of surface water. For the Rwandan terrain, a cross 
slope  indicated in 5.2.4 .1  shall be considered .  
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 5.4.7  Slopes in shoulders  
Shoulders should be flush with the roadway surface and abut the edge of the travelled way. Any type of shoulder 
construction has a bearing on the cross slope an d the two should be determined jointly. Therefore, the shoulder 
should have the same slope as the carriageway.   
5.4.8  Side slope and back slope  
5.4.8.1  Side slopes and back slopes shall be designed to ensure roadway stability and provide reasonable 
opportunity for recovery for an out -of-control vehicle. Three main regions (Figure 11) of the side slopes are 
important to safety: the top of the slope (hinge point), the fore slope and the toe of the slope. Slope and soil 
data should be used in combination to approxi mate the stability of the slopes and the erosion potential.  Effective 
erosion control, low -cost maintenance, and adequate drainage of the subgrade are largely dependent upon 
proper shaping of the side slopes. Overall economy depends not only on initial con struction cost but also on 
the cost of maintenance, which is dependent on slope stability.  
 
Figure 11 — Designation of roadside regions  
5.4.8.2  Slopes of 1V: 3H to 1V:4H should be used, and for slopes less than 1V: 3H, road side barriers 
should be provid ed and slope stability as well as traffic safety should be evaluated. In such cases, fill slope 
should not be steeper than 1V:1H and cut slopes not steeper than 1V:1.5H with additional safety provisions 
(retaining walls, barriers etc…).  
5.4.9  Right -of-Way  
Right of way dimensions shall comply with the provisions of the Law Governing Roads in Rwanda.  
5.4.10  Clear zones  
Clear zone area shall be provided beyond the edge of the roadway for the recovery of errant vehicles. It may 
include any shoulders or auxiliary lanes an d is related to speed, volume, embankment slope and horizontal 
geometry. The need for clear zone increases with speed and curvature.   
Table 23 — Clear zones  
Speed limit  Desired (m)  Minimum (m)  
70  5  3  
 
©RSB 2021  - All rights reserved                  48 
 80  6  4  
5.4.11  Safety barriers  
5.4.11.1  The safety barriers are designated to redirect errant vehicles with a specified performance level 
and provide guidance for pedestrians or other road users.  
5.4.11.2  Safety barriers should be provided to protect vehicles from hitting dangerous obstacles tha t cannot 
be removed and also to protect vehicles from falling off the road down embankments  
5.4.11.3  Safety barriers such as concrete, guardrails, gabion walls, etc should be selected considering the 
road category, surfacing type and terrain conditions.  
5.4.12  Headroom and lateral clearance  
A minimum of 5.5 m headroom under a bridge structure, 7.0 m headroom under a high -power cables and 6.0 
m headroom under low -power cables shall be provided; The minimum lateral clearance width of 0.6 m on each 
side of a bridge for traffic volume of less than 400 VPD, 1.0 m -1.2 m on each side of a bridge for traffic volume 
of 400 VPD - 2000 VPD and increase the value by 1 m for bridges of more than 30 m span.   
5.5 Drainage design  
Proper drainage design shall be an essential feature  of overall design and planning. In drawing up a drainage 
plan information concerning the following factors shall apply:  
 hydrological consideration such as maximum rainfall and intensity rate of runoff and nature and 
amount of stream flow;  
 characteristic s of the drainage basin (area to be drained) such as size, shape, general slope, nature 
and type of vegetation and land use (existing and future; and  
 nature and type of basin soils including their permeability and tendency to erode.  
5.5.1  Longitudinal drainage  
Water should be drained from the carriageway and shoulders by virtue of the cross -fall or transverse slope and 
longitudinal grade. Such water should either be allowed to flow down the face of the side slope (for small 
embankments) or  collected at the edge of the shoulder by the use of kerbs, dykes or paved ditches and carried 
longitudinally for disposal at a convenient place. The water from the roadway and surrounding areas should be 
drained away by use of roadside ditches, mitre drai ns or cut -off drains. These usually carry the water for disposal 
at a convenient place or to a bridge or culvert inlets.  
5.5.2  Roadside ditches  
5.5.2.1  Drainage ditches shall be constructed  along the edge of the roadway to receive the runoff from the 
pavement surfaces and water from subsurface drains. Where the surrounding area is sloping toward the 
roadway, these ditches shall also serve to intercept and carry away water which would otherwi se reach the 
roadbed.   DRS 267-1: 20 21 
49                ©RSB 2021  - All rights reserved  
 5.5.2.2  With open drains, the slope next to the road should not be steeper than 1:4, so as to avoid the risk 
of severe damage and injury when errant vehicles fall into the drain.  
5.5.2.3  Generally trapezoidal shape ditches with sid e slope of 1 in 1 to 1 in 4 (depending upon soil type) 
and variable bottom width are used. In rolling to hilly terrain where space is limited, V -shaped drains can be 
used. The capacity of drainage ditch can be increased by widening or deepening the channel . Widening is 
preferred to limit potential scouring.  
5.5.2.4  The minimum depth of ditches should be 0.5 m measured from the bottom of the ditch to the 
formation level.  
5.5.2.5  Maximum velocity of the water in the ditch, which can cause erosion or scour d epends on the 
material of the ditch. An average value of 1 m/sec for loam or fine sand and 2 m/sec for coarse gravel cannot 
cause erosion. However, in cases when velocities are expected to exceed 2 m/ sec a lining shall be used.  
5.5.2.6  To assure flow, di tches should have minimum longitudinal slope of 0.5 % if unpaved and 0.3 % if 
paved. Key points to consider in the design of safe side drains shall be:  
 there should be sufficient discharge points and culverts to ensure that the drain never gets very 
deep;   
 with open drains, the slope next to the road should, as much as possible, be flat enough to reduce 
the risk of errant vehicles overturning;  
 in built -up areas channel drains deeper than 500 mm should be covered or under -drain system be 
used for the safet y and convenience of both pedestrians and vehicles;  
 the drain should terminate or discharge in a satisfactory manner without risk of causing erosion or 
other problems; and  
 the drain should be capable of being cleaned and maintained easily.  
5.5.3  Median drains   
Median drains not only drain the median, but also, in the case of a horizontal curve, prevent water from the 
higher carriageway flowing in a sheet across the lower carriageway. The transverse slopes should be in the 
range of 1:4 to 1:10. Unlike side drai ns, median drains, should be generally constructed with a shallow V -profile  
with the bottom gently rounded.  
5.5.4  Chutes drains  
5.5.4.1  Chutes are intended to convey a concentration of water down a slope which, without such 
protection, would be subject to scou r. They may vary in size from large structures to half -round precast concrete 
product, but they are all open channels. Flow velocities are high, so that stilling basins are required if down -
stream erosion is to be avoided. An example of the application of chutes is the discharge of water down a fill 
slope from an edge drain. The entrances to chutes require attention to ensure that water is deflected from the 
edge drain into the chute, particularly where the road is on a steep grade.   
©RSB 2021  - All rights reserved                  50 
 5.5.4.2  The chutes and stilling basins should be such that these drainage elements do not present an 
excessive risk to errant vehicles. Generally, they should be as shallow as is compatible with their function.  
5.5.4.3  Depths in excess of 150 mm should be viewed with caution .  
5.5.5  Mitre drains  
5.5.5.1  The water collected on side drains shall be disposed of by diverting the drains away from the road 
before it has become too long and collected too much water. If there is no stream or river into which it can be 
diverted, mitre drains  with small check bends should be constructed pointing away from the road and running 
downhill. Thus, putting up large size, culverts should be avoided. If it is not possible to construct mitre drains 
because the surrounding ground is sloping towards the r oad, then a culvert to take the water across the road 
away on the other side shall be provided.   
5.5.5.2  Where the terrain conditions permit the collected water should be drained freely, however when the 
conditions for free drain do not permit the mitre drains should be provided with the frequency of 20m.  
5.5.6  Catch -water drains  
5.5.6.1  Where the surrounding area consists of higher ground, as in a cut or where the highway runs along 
the side of a hill, additional drains known as catch water cut -off or interce ptor drains should be provided. These 
are effective in preventing erosion of the slope and consequent blocking of the side drain.   
5.5.6.2  Catch -water drains shall be constructed at the back of the top of the cut or on benches in the cut 
slope. The practi ce of providing catch -water drains along the top of the cutting may sometimes cause a slope 
failure. Therefore, when it is necessary, it should be provided between 3m -6m away  from the top of the cut with 
the draining slope/incline of 2% (Figure 12).   
 
Figure 12 — Catch -water drains  
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 5.5.7  Subsurface drainage  
The road base shall be designed either to exclude water completely or alternatively to permit egress of water 
which has entered. When impermeable bases such as stabilized soils or densely graded bituminou s concrete 
are used, drainage of base shall not be necessary. When permeable and porous base materials are used, 
particular attention shall be paid to the drainage of the base layer. The base and sub -base should extend the 
full width across the roadway and  the surface of the sub -base layer given adequate cross fall to assist drainage.  
5.5.8  Capillary cut -off  
Capillary cut off shall collect and lead to drains any water which may pass through the road surface (from top) 
or rising into the pavement from below by capillary action as shown in figure 13.Capillary cut -off can either be a 
layer of porous materials such as sand or gravel, or an impervious membrane such as layer of primer, tar felt 
or polythene. The cut -off should be located at least 0.6  m below top of s ubgrade. It should also be at least 0.15  
m above the general ground level or stagnant water level.  
 
 
Figure 13 — Capillary cut – off  
5.5.9  Control of seepage flow  
5.5.9.1  There are two methods of dealing with condition of seepage flow. If the seepage zone is narrow 
and within 0.6 m to 1.0  m from the surface then the procedure shall be to install an intercepting drain in the 
impermeable strata underlying the seepage zone as shown in figure 14.  
5.5.9. 2 If the seepage zone is wider or the impermeable strata are at a considerable depth below the 
surface, it is generally impracticable to construct the drainage trench sufficiently deep to intercept all the 
seepage water. In this case, the intercepting drain shall be well above the impervious strata, leading to a par tial 
interception of seepage zone.  
5.5.9. 3 Where a road is on sloping ground, longitudinal drains may not be capable of intercepting all the 
seepage water. In such cases it may be necessary to install transverse intercepting drains too.  
 
©RSB 2021  - All rights reserved                  52 
  
Figure 14 — Interception drain for the Seepage Zone  
5.5.10  Control of high water table   
A high water table can be lowered by the installation of a drainage system similar to the system displayed in 
Figure 14, above. It is recommended  that the water table should be maintained at a depth not less than 1.2  m 
below the formation level. The actual spacing and depth of drains to achieve this requirement shall depend on 
the soil conditions and the width of the road formation. In the case of dual carriageway, drains may be necessary 
under the median as well as under the edges of the formation.  
5.5.11  Cross drainage structures  
5.5.11.1  Cross drainage structures comprise a wide range of measures from major bridges to drifts and 
minor culverts. The major structures that are most commonly used for passing water from one side of a road to 
the other should be pipes, slabs, bridges, beams & wa lls, , box girders , box culverts , trusses, frames, arches 
and cable bridges.  
5.5.11.2  Collection of water from the roadway should be properly channelled to the bridges to avoid erosion 
of abutments. Culverts in various shapes and materials are used to con vey water from streams below the road 
and to carry water from one side ditch to the other. Culverts should have inlet headwalls on the upstream side 
and outlet headwalls on the downstream side. Wing walls on the upstream are intended to direct the flow int o 
the culvert and provide transition from the culvert to normal or regular channel on the downstream. Both help to 
protect the embankment from flood water. An example is shown in figure 15 below.  
 
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Figure 15 — Example of a pipe culvert  
5.5.12  Drainage design  
5.5.12.1  The rational method  
Runoff calculations shall be undertaken using the rational m ethod. The rational method is usually confined to 
smaller catchment areas, and other methods, such as unit hydrographs and stream gauges are used to 
determine peak flows for la rger catchment areas. However, as such data, with one exception, is not available 
for larger streams; the rational method shall be used for all catchment areas. Usually, this can result in flood 
values above the actual values, as peak rainfall intensities can vary locally within the larger catchment area.   
5.5.12.2  Design frequency  
The design flood value chosen varies depending on the type of a structure constructed. Recommended design 
storm frequencies shall be as follows:  
 longitudinal ditches: 1 in 2 years;  
 pipe culverts :1 in 10 years;  
 box culverts :1 in 25 years; and  
 bridges: 1 in 50 years  
5.5.12.3  Rational formula  
The Rational formula is:  
𝑄+0.00278 𝐶𝐼𝐴 (Formula 1)  
where  
Q is peak runoff in cubic meters per second (m3/s);  
C  is runoff coefficient representing a ratio of runoff to rainfall;  
I  is rainfall Intensity in mm/hr; and  
A  is catchment area in hectares .  
NOTE 1  The formula is modified through multiplication by a factor Cf , when design frequencies of greater than 10 years 
are specified. For a 25 -year frequency, Cf, = 1.1; for 50 -year, Cf, = 1.2.   
©RSB 2021  - All rights reserved                  54 
 NOTE 2  Most drainage areas will be found to be too small to be measured using a Geographic Information System 
1:20000 scale map s eries. For such areas, a pipe culvert of 100mm Ø should suffice to discharge the flood flow.  
5.5.12.4  Catchment area  
Catchment areas can be determined by application of the appropriate scale factor to the areas from the 
Geographic Information System 1:20000 Scale M aps, which include contours at 25 m increments. As most 
catchment areas are too small to be discernible at this map scale, only the larger catchments shall be shown.  
5.5.12.5  Catchment slope  
The first step should be to determine the change in elevation, ΔH, by subt racting the elevation at the structure, 
H1, from the highest elevation of the catchment, H2.  This is then divided by the length of the path of the runoff, 
L. For instance, when the elevation for a catchment drops from 973 m - 720 m (= 253 m) over a length of 2860 
m, this gives a slope of 253/2860 or 8.8 %.  
5.5.12.6  Hydrological soil group  
Soil properties influence the relationship between runoff and rainfall since soils have differing rates of infiltration. 
Permeability and infiltration shall be the principal data r equired to classify soils into Hydrologic Soils Groups 
(HSG). Based on infiltration rates, soils should be divided into four hydrologic soil groups as shown in Table 24.  
Hydrologic soil group D shall be used for clay soils, which are the most prevalent Hyd rological Soil Group. This 
soil group also gives the highest runoff rates, and shall be therefore the safest group to use to ensure that the 
runoff can be conveyed by the selected structure.  
Table 24 — Hydrological soil groups  
Group  Description  
Group A  Sand, loamy sand or sandy loam. Soils having a low runoff potential due to high infiltration rates. 
These soils primarily consist of deep, well -drained sands and gravels.  
Group B  Silt loam, or loam. Soils having a moderately low runoff potential due to moderate infiltration rates. 
These soils primarily consist of moderately deep to deep, moderately well to well drained soils with 
moderately fine to moderately coarse textures.  
Group C  Sandy clay loam. Soils having a moderately high runoff potential due to slow infiltration rates. These 
soils primarily consist of soils in which a layer exists near the surface that impedes the downward 
movement of water or soils with moderately fine  to fine texture.  
Group D  Clay loam, silty clay loam, sandy clay, silty clay or clay. Soils having a high runoff potential due to 
very slow infiltration rates. These soils primarily consist of clays with high swelling potential, soils 
with permanently -high water tables, soils with a claypan or clay layer at or near the surface, and 
shallow soils over nearly impervious parent material .  
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55                ©RSB 2021  - All rights reserved  
 5.5.12.7  Runoff coefficient  
This recommended Runoff Coefficient C for Pervious Surfaces by Selected Hydrologic Soil Groupings and 
Slope Ranges,” is shown in table 24. For instance, if terrain is mountainous (6 % – 15 %), and the Hydrologic 
Soil Group is Group D (see above), a clay lo am soil, and this table gives a “C” of between 0.28 and 0.38. A 
value between these extremes can be determined through interpolation using the actual cross terrain slope of 
the catchment area. For the example at 8.8 % mentioned in the above text, C= 0.31.  
Table 25 — Recommended runoff coefficient C for pervious surfaces by selected hydrologic soil 
groupings and slope ranges  
Terrain types  for 
hydrological design  Soil type  
A  B  C  D  
Flat < 2 %  0.04 - 0.09  0.07 - 0.12  0.11 - 0.16  0.15 - 0.20  
Rolling 2 % – 6  %  0.09 - 0.014  0.12 - 0.17  0.16 - 0.21  0.20 - 0.25  
Mountainous 6 % – 15 %  0.14 - 0.18  0.18 - 0.24  0.23 - 0.31  0.28 - 0.38  
Steep > 15 %  0.18 - 0.22  0.24 - 0.30  0.24 - 0.30  0.38 - 0.48  
5.5.12.8  Time of Concentration (TC)  
The determination of rainfall intensity first requires a determination of the Time of Concentration, TC.  
𝐿𝐶= (0.87 × 𝐿3
𝐻)0.385
 
where  
Tc is time of concentration (hr) ;  
 L  is length of catchment area (km) ;  
 H is height of catchment (m) .  
NOTE In the above example, a catchment area measuring 2860 m long with a drop of 253 m would result in Tc = 0.38 hrs 
or 23 min   
©RSB 2021  - All rights reserved                  56 
 5.5.12.9  Rainfall intensity, I.   
The rainfall intensity data shall be determined by using local references (National meteorological servi ce).The 
rainfall station closest to the road project should be chosen for hydrological design for that project to determine 
peak runoff Q.  
5.5.12.10  Minor runoff areas  
If the catchment area is too small to be measured on a 1:50000 scale map, then it will have a ru noff in ten years 
(Q10) flow of less than 3 m3/s, which means that a single barrelled 1000 mm Ø pipe culvert can handle the flow. 
Often such small catchments are already addressed by more than one cross drain, which further divides the 
flow into smaller ar eas, especially at intervals down a gradient.  
5.5.12.11  Spacing of cross drains on steep gradients  
For long steep road gradient segments, i.e. those of over 7 % gradient, it is recommended to transport the runoff 
out of the longitudinal side ditch, through cross d rains, at regular intervals. This shall prevent the side drains 
from overflowing, which otherwise would result in a diversion of a portion of the flow onto the gravel surfacing, 
and a loss of the surfacing. The side ditch does not only carry runoff flow fr om the roadway, but from the adjacent 
hillside. Rather than performing very tedious calculations to determine this flow, spacing can be selected at 
which the ditch flow is channelled through a cross drain culvert pipe  (relief culvert) . The spacing should b e 
located at every 100 m of distance .  
5.5.12.12  Verification of Stream Channel Peak Flow Using  Manning’s Equation  
5.5.12.12.1  Major catchment areas should be analysed to check the flood flow values obtained through the use 
of the Rational Method against flood flow values obtained from historic flood height observations. These are 
obtained through interviews with local residents. The claimed flood height is then checked against a topographic 
survey to determine the required par ameters such as flow area and petted perimeter, and used in the Manning’s 
Equation to obtain flood flows.This double check is especially valid given the uncertainty mentioned above in 
extending the intensity -duration -frequency curves beyond the range of av ailable data. Manning’s Equation to 
obtain flood flows, is described below.  
5.5.12.12.2 Manning’s Equation – For a given depth of flow in a channel with a steady, uniform flow, the mean 
velocity, V, can be computed with Manning’s equation:  
𝑉=(1/𝑛)𝑅2/3𝑆1/2 
where  
V  is velocity, m/s ;     
  n  is Manning’s roughness 
coefficient ;  
R is hydraulic radius ;  
S is slope of the energy gradeline, m/m (note: for steady uniform flow, S = channel slope, m/m) .  DRS 267-1: 20 21 
57                ©RSB 2021  - All rights reserved  
 Manning’s Equation can also be written as:  
𝑄=(1/𝑛)𝑅2/3𝑆1/2 
where       
 Q is discharge, m3/s;    
 A  is cross -sectional area of flow, 
m2; 
NOTE  The selection of Manning’s ‘n’ is generally based on observation.  
5.5.12.13  Pipe culvert design  
5.5.12.13.1  A pipe culvert shall comply with the requirements of RS 265.   
5.5.12.13.2  A pipe culvert should have a minimum cover from the top of the culvert to the finish grade line of 
600 mm, inclusive of embankment and wearing course.  
5.5.12.14  Side ditches (longitudinal drains)  
5.5.12.14.1 Side ditch flow should be accommodated  through a various range of ditch configurations, including 
rectangular U -ditches, trapezoidal ditches, and triangular v -ditches.  
5.5.12.14.2  Rectangular ditches are not recommended as they can be unsafe for vehicles. If an errant vehicle 
leaves the roadw ay and enters the ditch, damage may be done to the undercarriage of the vehicle that may 
likely render it inoperable. Because of this, drivers tend to stay away from the ditch or drive more towards the 
centreline of the road, creating an unsafe condition f or oncoming traffic. A vehicle entering the ditch can be 
“unrecoverable,” which means that it cannot be able to get out of the ditch and resume operations .  
NOTE Rectangular ditches are unsafe for animals and pedestrians. The vertical drop of more than 0.5  m would cause 
serious injuries. Rectangular ditches are disease vectors. Water and mud that collect in the flat bottom are a breeding groun d 
for mosquitoes.  
5.5.12.14.3  At places where a hard shoulder exists, drivers may tend not to use the hard shoulder  for its 
intended purposes of accommodation of stopped vehicles and emergency use, fearing that they may misjudge 
the distance to the ditch. Hence they may remain partially in the travel lane, creating further safety issues.  
5.5.12.14.4  It is recommended t o provide V -ditch configuration whereby the front ditch slope, i.e. the one facing 
the traffic lane, has a slope of 1.5:1 H:V. This should allow vehicles and pedestrians to resume travel if they 
enter the ditch. The ditch back slope should be made steeper in the interests of reducing the right -of-way take, 
and is set at a slope of 0.67:1 H:V. The ditch shall have a depth of 0.6 m. Ditches on gradients steeper than 
7 % shall be lined (mortared) ditches. 5.5.12.14.5  The drainage on the conglomeration of houses  should be 
lined and covered   
©RSB 2021  - All rights reserved                  58 
 NOTE  Debris and waste that collects in the flat bottom pose adverse health effects for someone who falls into the ditch. 
V-ditches, lacking a flat bottom, do not have the same magnitude of health issues.  
5.6 Site and m aterials inve stigation  
5.6.1   General  
5.6.1.1  Site investigation and surveys are a vital and integral part of selecting the road alignment and 
undertaking the design and construction of the road. Surveys provide essential information on the characteristics 
of the soils along  the possible alignments .  
5.6.1.2  Materials investigation  provide s the background for the general understanding of the approach to 
selecting and using materials for the construction of feeder roads  in an economic and sustainable manner, and 
to ensure that satisfactory levels of quality are attained.  
5.6.2  Subgrade strength  
5.6.2.1  The design of a road pavement dictates that the layer(s) of pavement and the sub -grade foundation 
should be capable of sustain ing the stresses applied to them by repeated vehicle loading without suffering 
damaging permanent deformation. When a vehicle wheel load is applied to a road surface, the resulting stresses 
are distributed through the pavement layers to the ground and the load is spread over an increasing area as it 
is transmitted downwards. The pavement structure shall be sufficiently thick over the subgrade soil to ensure 
that the soil is not overstressed. The greater the strength of the soil, the thinner the pavement lay ers need to 
be. Hence, the load carrying capacity of the subgrade is a principal factor in determining the required pavement 
thickness. 5.6.2.2  To complete the design process, material investigations of the existing and new road surface 
should be undertaken  to better determine the properties of the existing and new pavement and subgrade. A 
Dynamic Cone Penetrometer (DCP) complying with the requirements of RS ISO 22476 -2 shall be performed at 
least every 200m to rapidly obtain an insitu measurement of the str uctural properties of the gravel layers, and 
sub-grades after an excavation is made through the surface layers.  
NOTE  The DCP is a simple apparatus that can characterize the ground conditions in and beneath an unsealed road 
quickly and with accuracy appropr iate to the requirements of the design. In its use, a steel cone (20 mm diameter with a 60º 
angle) is driven into the ground under a fixed energy (an 8 kg mass falling through 575 mm).  
5.6.2.3  The rate of penetration into the gravel or soil material (DN mm /blow) should be a reasonably good 
predictor of the California Bearing Ratio (CBR) at the prevailing insitu moisture and density conditions as shown 
in table 25.  
5.6.2.4  The penetration should be measured up to a depth of 800 mm. On un -compacted soils (e.g ., 
investigations for new alignments, lane additions, or lane widening), do not do DCP measurements immediately 
after rainfall or if the ground surface appears wet as this will influence the rate of penetration and may result in 
weaker than actual soil str engths.  
Table 26 — Tabulated Correlation of Ca lifornia Bearing Ratio vs Dynamic Cone Penetrometer   
Dynamic Cone Penetrometer  
(DCP) Index  
Mm/blow  California Bearing Ratio (CBR)  
%  
< 3  100  
3  80  DRS 267-1: 20 21 
59                ©RSB 2021  - All rights reserved  
 4  60  
5  50  
6  40  
7  35  
8  30  
9  25  
10 - 11  20  
12  18  
13  16  
14  15  
15  14  
16  13  
17  12  
18 - 19  11  
20 - 21  10  
22 - 23  9  
24 - 26  8  
27 - 29  7  
30 - 34  6  
35 - 38  5  
39  4.8  
40  4.7  
41  4.6  
42  4.4  
42  4.3  
43  4.2  
44  4.1  
45  4.0  
46  3.9  
47  3.8  
48  3.7  
49 - 50  3.6  
51 - 52  3.5  
53 - 54  3.4  
55  3.3   
©RSB 2021  - All rights reserved                  60 
 56 - 57  3.2  
58  3.1  
59 - 60  3.0  
61 - 62  2.9  
Table 26 — Tabulated Correlation of Ca lifornia Bearing Ratio vs Dynamic Cone Penetrometer (Cont)  
Dynamic Cone Penetrometer (DCP) Index  
Mm/blow  California Bearing Ratio (CBR)  
%  
63 - 64  2.8  
65-66  2.7  
67-68  2.6  
69-71  2.5  
72-74  2.4  
75-77  2.3  
78-80  2.2  
81-83  2.1  
84-87  2.0  
88-91  1.9  
92-96  1.8  
97-101  1.7  
102-107  1.6  
108-114  1.5  
115-121  1.4  
122-130  1.3  
131-140  1.2  
141-152  1.1  
153-166  1.0  
166-183  0.9  
184-205  0.8  
208-233  0.7  
234-271  0.6  
272-324  0.5  
>324  <0.5  
5.6.3  Laterite wearing course  
5.6.3.1  Borrow pits should contain laterite materials (murram). Nearly all laterites are rusty -red because of 
iron oxides. They develop by intensive and long -lasting weathering of the underlying parent rock.  DRS 267-1: 20 21 
61                ©RSB 2021  - All rights reserved  
 5.6.3.2  Laterite should vary in gradation. It ranges from hard gravel to a softer earth emb edded with small 
stones. Not all laterite roads are therefore strictly gravel roads.  
5.6.3.3  Laterite which contains a significant portion of clay can become very slippery when wet and in the 
rainy season, it may be difficult even for four -wheel drive vehi cles to avoid slipping off highly cambered roads. 
As it dries out, laterite can become very hard.  
5.6.3.4  From 5.6.3.1 to 5.6.3.4 dictate that laterite should conform to an approved specification which 
addresses gradation and other performance indicators s uch as plasticity index, liquid limit, and CBRs.  
5.6.4  Material stabilization  
5.6.4.1  Stabilisation in a pavement construction (either as material delivered to site having been mixed in 
a pug mill plant or in -situ stabilised) or in the rehabilitation of an exist ing pavement (ie in -situ stabilisation) should 
be undertaken with the following purposes:   
 To correct any mechanical deficiencies (particles size distribution and/or plasticity) in unbound 
granular materials and sub -grades;   
 Increase the strength or bearin g capacity of pavement;   
 Reduce the permeability and or moisture sensitivity, which can result in a loss of strength of the 
material or seasonal volume change;   
 In-situ stabilisation in particular provides a means by which existing pavements can be recycle d;  
 Improve the strength of sub -grades so they are capable of accepting construction traffic as well as 
provide the support for more economical pavement configurations; and,   
 Enhance the compaction in unbound granular materials and in silty and clayey subg rades.   
5.6.4.2  The mechanical stabilisation consists of blending two or more materials to produce one with 
suitable grading and/or plasticity.  Mechanical can be possible by undertaking the following;  
 Mixing of materials from various parts of a deposit at  the source of supply.  
 Mixing of selected, imported material with in -situ materials.  
 Mixing two or more selected imported natural gravels, soils and/or quarry products on -site or in a 
mixing plant, and  
 The use of excessive compaction to consolidate a mater ial. 
5.6.4.3  Chemical stabilization consists of treating materials with sufficient amount of stabiliser complying  
with relevant standards requirements to increase the engineering parameters of materials to be used in the 
pavement design .  A chemical  stabil iser can be added to the sub -grade, sub -base and/or base layers to impart 
a necessary change to the engineering parameters of that layer. Base stabilization should not be considered as 
a wearing course however replacement to a bituminous seal.   
©RSB 2021  - All rights reserved                  62 
 NOTE: Commo n types of chemical stabilization materials include hydraulic stabilizing agents such as cement, lime and /or 
blended hydraulic materials  and bitumen . 
5.6.4.4  Where mechanical stabilisation methods cannot be applied or where naturally occurring materials 
that meet the specified requirements cannot be located within economical haul distance from the project site , 
chemical stabilisation should be considered.  
5.6.4.5  Selection of the correct stabiliser  requires an in -depth understanding of several input variab les. The 
following shall be taken into consideration:  
 Climate  
 Existing pavement conditions, such as variability of material, thickness of layers, etc.   
 Traffic data, especially loading   
 Engineering parameters of the material to be stabilised   
 Capacity and  experience   
 Distance to and quality of borrow pit materials  
 Availability and type of binder in terms of quantity required and transportation distance   
 Ease of application of binder in consideration of plant and equipment on site   
5.7  Pavement  
5.7.1  Pavement design  
5.7.1.1  The gravel road design process generally does not focus on cumulative axle loads, fatigue or a 
target life expectancy. This is because the gravel road seldom fails as a result of punching into the subgrade 
(i.e. shear failure of either the wearing  course or the subgrade material). Failure is typically either the result of 
continued slippage of the vehicle tires against the soil (gravel) when a high moisture content prevails (lack of 
frictional resistance leading to plastic failure) resulting in set tlement of the tire into the material, or shear failure 
of the upper portion of the wearing course with lateral displacement. Unlike paved roads, motorcycles can cause 
stress to a gravel road due to kick -out of gravel, and loss of fines.”  
5.7.1.2  A road p avement structure is usually composed of several layers of different materials overlying 
the (subgrade). Materials selection forpavement layers shall consider two major variables:  
 the bearing capacity and stiffness of foundation soil (subgrade) beneath the  pavement, which may 
be variable, changing with soil type, dry density and ground water conditions; and  
 The climate of the region;  DRS 267-1: 20 21 
63                ©RSB 2021  - All rights reserved  
 5.7.2  Traffic volume and axle loads  
5.7.2.1  Design life of pavements  
5.7.2.1.1   The “design life” does not mean that at the end of the pe riod the pavement will be completely worn 
out and in need of reconstruction; it means that towards the end of the period, the pavement shall need to be 
strengthened through the placement of additional surfacing materials so that it can continue to carry tr affic 
satisfactorily for a longer period.  
5.7.2.1.2   Design life provided in 5.1.5 for both paved and unpaved should be considered  
5.7.2.2  Estimation of traffic  
A traffic count study should be conducted on feeder  roads to provide an indication of what future co unts and 
traffic composition may be anticipated as the road develops over time.  
5.7.2.3  Traffic /Load Equivalence Factors (LEF/TEF)  
5.7.2.3.1  The commonly used equivalent load should be 80 kN equivalent single axle load, normally 
designated Equivalent Single Axe l Load (ESAL).  
5.7.2.3.2  Traffic Equivalence Factors (TEFs) shall be used for various sizes of trucks and buses to relate the 
fleet to the equivalent loads. Load assumptions are presented Table 27. 
Table 27 — Traffic Equivalence Factors (TEFs)  
Vehicle type  TEFs  
Light Vehicles   0.01  
Buses   0.50  
Heavy trucks   1.00  
 
5.7.2.3.3  As a rule -of-thumb, the damage caused by a particular load is roughly related to the load by a 
power of four (for reasonably strong pavement surfaces).For example:  
 for a 80 kN single axle, LEF =1.0    
 what is the TEF for a 133 kN single axle?  
 using the fourth power rule -of-thumb:    
©RSB 2021  - All rights reserved                  64 
 (133
80)=7.7 
5.7.2.3.4  Axle loads shall be estimated by taking an average of the present ADT and the future ADT by 
vehicle class, dividing by  two to account for the traffic volumes per direction or lane, and multiplying by the 
equivalence factor. An assumption is that the ratio of buses to trucks is 50/50.  
5.7.2.3.5  Feeder roads with light traffic and a low number of commercial vehicles shall be designed for gravel 
surfacing.” A gravel wearing course shall be placed at 150 mm thickness on a prepared subgrade where the 
subgrade CBR value is greater than 7 %. Where the CBR is between 4 and 7, the gravel wearing course shall 
be increased to 200 mm  in thickness. Instances where the CBR is less than 4 shall be considered special cases 
which may require an additional layer of 50mm of sub base material.  
5.7.2.3.6  Recommended thickness of gravel layers to the placed wearing coarse on the subgrade of th e 
gravel is shown in Table 27.  
Table 28 — Recommended thickness of gravel layers to the placed wearing coarse on the subgrade of 
the gravel  
Traffic (AADT) in 
both directions  Subgrade soil  Recommended total 
minimum thickness 
of gravel  
< 200   CBR > 7  150 mm   
3< CBR < 7   200 mm   
> 200   CBR > 7  200 mm  
3 < CBR < 7   250 mm  
5.7.2.4  Subgrades  
5.7.2.4.1  Design depth  
The subgrade depths of feeder roads varying from 0.3 m - 0.6 m shall be considered depending on the CBR. 
Soils with CBRs less or equal to 3  shall be improved or replaced.  
5.7.2.4.2  Centreline soil surveys  
Following the DCP tests, a  minimum of 2 CBR strength testing  shall be conducted at each uniform section but 
the spacing should not be more than  2 km ..  DRS 267-1: 20 21 
65                ©RSB 2021  - All rights reserved  
 5.7.2.4.3  Characterization of the subgrade strength design value  
The statistical approach of estimating subgrade CBR design value for a section which takes the 90 %ie CBR 
test value for a homogeneous section shall be used as the CBR design value.   
5.7.2.4.4  Subgrade classes  and CBR design values   
5.7.2.4.4.1  The following classification for classes of subgrades shall apply:  
 S3 is natural gravel/soil with nominal CBR value of minimum 3;  
 S7 is natural gravel/soil with nominal CBR value of minimum 7; and 
 S15 is natural gravel/soil with nominal CBR value of minimum 15.  
5.7.2.4.4.2  Improvement should be done on subgrades with strength values S3 and S7 only.  
5.7.2.4.4.3  For gravel roads, the materials for improved subgrade layers should meet the requiremen ts for 
class G15 and G7.  
5.7.2.4.4.4  Subgrades with CBR value less than 7 shall be used with care and good engineering judgment. 
Use of extra thickness shall be balanced against the option of upgrading to higher material strength.  
5.7.2.5  Gravel Wearing Course ( GWC)  
5.7.2.5.1  The following requirements shall be met by materials for gravel wearing course:  
 the need for a sufficient cohesion to bind the particles and prevent the surface from travelling and 
becoming corrugated in dry seasons; and  
 limiting the amou nt of fines and the plasticity so as to avoid the occurrence of a slippery surface in 
wet weather.  
5.7.2.5.2  The main material properties for fully engineered gravel wearing course should be CBR, gradation, 
shrinkage product, grading coefficient, and fiel d density and the gravel wearing course should also meet the 
requirements for the same properties.  
5.7.2.6  Earthworks compaction requirements  
5.7.2.6.1  Requirements for earthwork compaction are shown in the Table 29 . 
Table 29 — Compaction requirements for earth work  
Layer and typical material type specified  Minimum dry density  
(Nominal value)   
©RSB 2021  - All rights reserved                  66 
 Hard  material,  soft  material -waterlogged,  and 
swampy material  95 % MDD, 100 % MDD for 300 mm I cuttings, 
embarkments  
Upper improved subgrade layer, nominally G 15 material 
or better, for layers less than150 mm below the formation 
level  95 % of BS Heavy  
Lower improved subgrade layer, nominally G 7 material 
or better, for layers from 150mm to 300mm below the 
formation l evel  93 % of BS Heavy  
Fill, nominally G3 material or better, for layers more than 
300mm below the formation level  90 % of BS Heavy  
Fill or improved subgrade layers using rock fill(DR)  Compaction method specification shall apply  
Roadbed compaction to 150  mm depth after clearing, grubbing and removal of topsoil or other unsuitable 
material, where the roadbed level is:  
Less than 150 mm below the formation level  97 % of BS Heavy  
150 mm – 300 mm below the formation level  95 % of BS Heavy  
300 mm – 600 mm below the formation level  93 % of BS Heavy  
More than 600 mm below the formation level  100 % of BS Light  
  
5.7.2.6.2  The recommended requirements for fill and improved layers for earthwork are indicated in Table 
30.  
Table 30 — Requirements for fill and improved subgrade layers  
Material type  Requirements   
CBR(%)  CBR Swell (%)  Pl  
(%)  Maximum  
particle size  Maximum  
layer thickness  
G 15  Minimum 15 
after 4 days 
soaking  1.5, max.  25  
½  of  250  mm  DRS 267-1: 20 21 
67                ©RSB 2021  - All rights reserved  
     compacted layer 
thickness but not 
> 50 mm  compacted layer 
thickness placed 
in one operation  
G 7  Minimum  7 after 
4 days soaking  Max. 2  30    
G 3  3 after 4 days 
soaking,  
measured  at 
90 % of MDD of 
BS –Heavy 
compaction  Max. 2  -  
DR  -  -  -  Two  third of  
compacted  
layer  1 m placed in 
one operation  
 
5.7.2.7  Unbound materials  
5.7.2.7.1  Code designation for unbound materials is shown in Table 31 
Table 31— Code designation for unbound materials  
GW  Gravel wearing coarse for gravel roads or unpaved shoulders  
G 25  CBR minimum 25 %  
G 45  CBR minimum 45 %  
G 60  CBR minimum 60 %  
G 80  CBR minimum 80 % and include crushed materials with less than 50 % by mass of particles 
retained on the 5 mm sieve has a crushed face  
CRR  Crushed rock made by crushing and screening of fresh quarried rock or clean, unweathered  
boulders of minimum 0.3 m diameter. All particles should be crushed, no soil  
fines allowed  
CRS  Crushed stone made by crushing and screening of blasted rock, stones, boulders and 
oversize from natural gravel. Min 50 % by mass of particles larger than 5 mm shall have at 
least one crushed face. Max 30 % of material passing 5 mm can be soil fines.   
©RSB 2021  - All rights reserved                  68 
 5.7.2.7.2  For pavement layers of natural gravel materials, materials requirements for gravel wearing course, 
base and sub -base layers are indicated in Table 32.  
Table 32— Requirements for fill and improved subgrade layers  
Material 
type  Requirements      
Compaction  CBR  CBR  
swell (%)  Maximum  
LL  Passing 
37.5  
mm  
sieve (%)  Shrinka 
ge 
product  
(SP)  Gradient 
coefficie 
nt  
(GCO)  Max.  
Pl  Max. 
LS  TFV  
GW  95 % of BS  
Heavy  Min 25d      Min 95  120-400  16-34        
G 80a2  98 % of BS  
Heavy  Min 80d  0.5  303/354  83/104  43/54  80kN5  
G 60a2  98 % of BS  
Heavy  Min 60d  1.0  353/404  103/124  53/64  60%6  
G 45b2  95 % of BS  
Heavy  Min 45d  0.5  403/454  143/164  73/84  50kN5  
G 30d2  95 % of BS  
Heavy  Min 30d  1.0  453/454  163/184  83/94  60 %6  
a2 dmax is ½ of compacted layer thi ckness but not > 50mm and GM is 2.2  
b2  GM is 1.5  
c2  GM is 1.2  
d  After 4 days of soaking  
3 General requirements;  
4 Calcrete or other pedogenic materials  
5 Minimum TFV dry;  
6 Raio soaked to dry value of TFV      
5.7.2.7.3  The grading requirements for G 80 and G 60 materials shown in Table 33  shall apply.  
Table 33 — Grading requirements for G80 and G60 materials  
Sieve size 
(mm)  G 80 grading limits   
(% passing sieve)  G 60 grading limits   
(% passing sieve)  
63  100  100  
37.5  80 - 100  80 - 100  DRS 267-1: 20 21 
69                ©RSB 2021  - All rights reserved  
 20  60 - 95  60 - 95  
5  30 - 65  30 - 65  
2  20 - 50  20 - 50  
0.425  10 - 30  10 - 30  
0.075  5 - 15  5 - 15  
5.7.2.8  Crushed aggregate base course  
For a crushed rock to be viable as a pavement material, it shall be available, workable and give satisfactory field 
performance at the lowest possible cost. The final in -place cost should take into account supply and cartage 
costs, cost of repair to road d uring cartage, spreading and compaction costs and preparation of surface costs. 
Table 34 gives requirements for crushed aggregate base course materials.  
Table 34 — Crushed aggregate base course requirements  
Material 
type    Requirements  
Compaction  Max PI  Max. LS  TFV  Ratio TFV 
soaked to  
dry  Max. 
Flakiness 
index  
CRS   98 % of BS -  
Heavy  6   4  110kN  60 %  35 %  
CRR   102 % of BS -  
Heavy  Non plastic  3  110kN  75 %  35 %  
    
Sieve 
(mm)  size   Grading limits (% passing sieve)  
 CRS  CRR  
Coarse type   Fine type  Coarse type   Fine type  
50   100         
37.5   90 - 100   100  100    
28   75 - 95   90 - 100  87 - 97  100  
20   60 - 90   65 - 95  75 - 90  87 - 97   
©RSB 2021  - All rights reserved                  70 
 10   40 - 75   40 - 70  52 - 68  62 - 77  
5   29 - 65   29 - 52  38 - 55  44 - 62  
2   20 - 45   20 - 40  23 - 40  27 - 45  
1.18   17 - 40   15 - 33  18 - 33  22 - 38  
0.425   12 - 31   10 - 24  11 - 24  13 - 27  
0.075   5 - 12   4 - 12  4 - 12  5 - 12  
 
5.7.2.9  Design method  
5.7.2.9.1  The design process should include computation of the traffic volume, and conversion into 
Equivalent Single Axle Loads (ESALs) using a traffic growth rate. The design subgrade CBR values are 
determined based on estimated 90th percentile CBR values, and conv ersion into the Effective Modulus of 
Subgrade Reaction (K -value) as shown in Table 35.  
Table 35 — Recommended aggregate thickness Vs. roadbed soil quality and traffic  
Relative quality of roadbed soil  Traffic level*  Aggregate thickness  
Very good (K -value > 550 pci)  
High  
Medium  
Low  200 mm  
150 mm  
100 mm  
Good (K -value 400 - 550)  
High  
Medium  
Low  275 mm  
200 mm  
100 mm  
Fair (K -value 250 - 400)  
High  
Medium  
Low  325  
275  
150 mm  DRS 267-1: 20 21 
71                ©RSB 2021  - All rights reserved  
 Poor (K -value 150 - 250)  
High  
Medium  
Low  **  
**  
225 mm  
Very poor (K -value < 150pci)  High  
Medium  
Low  **  
**  
275 mm  
*High = 60 – 80000 ESLs; medium = 30 – 60000 ESLs; low = 10 – 30000 ESLs  
**higher type of pavement design recommended.     
5.7.2.9.2  The soil quality is fair as the traffic volume in ESALs is low (100000 ESALs/year), a thickness of 
150 mm is recommended. For any higher traffic volumes, the table indicates that “a higher ty pe of pavement 
design is recommended.”   
5.7.2.10  Design for annual gravel loss  
Regravelling should be considered whenever a gravel loss of 30 mm /year is encountered. This means that, an 
addition of 75 mm should be required (1/2 x 30 mm/yr x 5 yr)  is regravelling is planned in five years. Rather than 
having to add such thicknes s, methods of stabilizing the wearing course should be investigated.  
5.8  Design for climate resilience  
5.8.1  The projected climate change is likely to have several significant effects on the road infrastructure . To 
reduce this impact, new roads must be design ed incorporating the necessary climate adaptation  measures.  
5.8.2  To implement the necessary adaptations to make roads more climate resilient and assist with the 
prioritisation, it is necessary to carry out visual assessments of existing roads with particu lar attention being 
paid to the expected changing climatic conditions along those roads.  
5.8.3  To take account of future changes in climate, either in the design of new roads and structures or in  
improving the resilience of existing roads, the following sh ould be considered;  
1. In the design of storm water drains and the downspout, the hydraulic design should include the design 
of water catch basins / check dams/detention basins where possible after analysing the topography of 
the project area. This can reduce  the frequency and extent of downstream flooding, erosion, 
sedimentation and water pollution.  
2. In the design of the bridges and culverts, the rate of sedimentation should be analysed  
3. The design of high slopes in rural areas should consider the sealing opt ions to avoid erosion and 
slipperiness  
4. The bioengineering which is the use of grasses on cuts and embankment slopes, should be applied for 
protection against erosion. Grasses with long roots contribute to the stability of slopes  
5. The choice of the alignmen t of a new road, the section of the project prone to flooding, landsliding, 
erosion and scouring should be identified and analysed in particular. If found to be at high risk, an  
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 alternative alignment should be identified. With the topography conditions, th e geology and rainfall 
conditions, the proposition of relocation or change of the alignment can be supported.  
6. Clear indication of the height of the embankment against the level of water. It is recommended  that the 
height of the road structure be at a minimum 0.5 m from the highest estimated flooding level . 
7. Where there are underground water the analysis to determine the moisture level should be conducted 
to provide the underground soil drainage.   
8. Provision  of catch water drain at regular intervals in the road side drain to avoid the erosion of unpaved 
drainage. This is necessary for high gradient roads.  
9. Shape the earthen drain to allow water to run in the channel. If they are constructed, mitre drain should 
be provided at regular intervals and not distant (i.e. where possible at every 100 m) to avoid much water 
in the drain and then erosion.  
10. Provision should be made to discharge the runoff across the road at regular intervals to avoid such a 
concentrat ion of runoff at the low point and to maintain a balance of runoff. These intervals depend 
upon the locality and type of road and could be in excess of 500m in flat terrain.  
5.9 Seismic considerations  
Rwanda is located in an earthquake intensity zone identifie d as “Degree VII” on the Modified Mercalli and 
Ritchter Scales. Given Rwanda is relatively high rating on both scales, practioners shall be required to include 
seismic considerations in the project’s bridges and those locations where slope stability issues  warrant close 
examination.  
5.10 Demographics  
Appropriate design approaches shall be introduced in populous areas to mitigate the effects of dust and improve 
the safety of road users, that is, additional road widths, seal surfacing, parking, bus lay -bys, etc. a nd appropriate 
drainage systems.  
5.11 All-Weather Access  
5.11.1  General  
An essential consideration in the design of LVRs shall be to ensure all -weather access most of the time. This 
requirement places particular emphasis on the need for sufficient bearing capacity o f the pavement layer’s 
structure and the providing of drainage and sufficient earthworks protection in flood or problem soil areas.  
5.11.2  Surface Performance  
5.11.2.1 The performance of the LVR’s surface material primarily depends on its physical properties and 
qualities, location, and the volume of traffic. LVRs passing through populated areas in particular require 
materials that do not generate excessive dust in dry weather. Steep gradients place particular demands on LVR 
pavement layer materials, which shall n ot become slippery in wet weather or erode easily. Consideration should 
therefore be given to the type of LVR pavement layer materials to be used in particular locations such as towns 
or steep sections. Annual LVR surface loss rates of approximately 25 -mm thickness per year per 300 VPD is 
expected, depending on rainfall and materials properties, particularly its plasticity.  
5.11.2.2 Roughness should be maintained between 3.5 mm/m and 8.5 mm/m to satisfy road users and keep 
the vehicle operating costs low a t a maximum vehicle speed of 50 km/h on LVR pavement surface as shown 
below.  DRS 267-1: 20 21 
73                ©RSB 2021  - All rights reserved  
 5.11.2.3 In addition to the foregoing parameters , the LVR’s design engineering shall focus on eliminating 
the following deficiencies:  
 slick or slippery sections owing to the lac k of a gravel layer on the road surface;  
 water -logged or muddy sections due to the lack of adequate drainage;  
 water running down the road account faulty side ditches and the lack of adequate drainage facilities 
which contributes to the slippery conditions;  
 overly -steep sections that exceed the 12 % to 15 % maximum horizontal gradient, or more 
occasionally, which makes it difficult for a fully -laden truck to pass Poorly constructed culverts (pipe 
or box) and/or inadequate numbers of such drainage features pe r kilometre;  
 existing drainage structures that cause out flow erosion outside of and adjacent to the road reserve;  
 the overly -narrow sections of some existing LVRs that is exacerbated by the failure, that is, slippage 
of the natural, in situ materials excavated for inside slopes;  
 poor surfacing which slow vehicles or damage them as they pass;  
 failing or inadequately constructed bridges and causeways that traverse streams and wetlands;  
 the overall lack of signage or traffic control devices which adds t o wearing issues and traffic safety 
problems;  
 occasional sections where traffic speed and lateritic surfacing cause air pollution – dust –problems 
in populated areas; and  
 the overall lack of maintaining LVRs which, in general, exacerbates all of the fore going.  
5.12 Bridge design  
5.12.1  General  
5.12.1.1 Long bridges should typically not found on LVRs. Hence, for these guidelines, a small bridge or 
culvert should be a structure having individual or multiple spans of total length no greater than 12 m. Small 
bridges shal l be an essential part of all highway networks and are far more numerous than larger structures.  
5.12.1.2 The recommended load consists of 20 tons loading with four wheels, 8 tons in front axle and 12 
tons in rear axle in 4.27 m distance in the first type  of loading. In the second type of loading, in the same class 
of loading has 21.2 tons loading consisting of six wheels 4.2 tons in front axle, 8.5 tons in intermediate and rear 
axles.  
5.12.1.3 The distance between the front and intermediate axle should b e 3.96 m and the intermediate to the 
rear axle should be 1.22 m. The spacing between successive vehicles can be assumed to be 30 m.   
©RSB 2021  - All rights reserved                  74 
 5.12.2  Bridge planning  
5.12.2.1 Thorough planning shall be given to the list of site data to be collected. It is more time consum ing 
to skip the collection of site data until later in the design phase, i.e., having a complete set of data in hand for 
the planning phase ensures the most economical preliminary design. Having to make additional trips to the site 
to collect missing data during later stages simply slows down the entire process.  
5.12.2.2 Because the design of bridges is more involved than it is for geometrics and drainage, the bridge 
design engineer should be available to devote time during the planning process in order to  advise and assist 
the planners in this important phase of the project’s total design.  
5.12.3  Bridge design process  
The process of bridge design can be divided into the following phases:  
a) Bridge Type Selection and Layout Preparation;  
b) Foundation Investigation a nd Analysis;  
c) Detailed Design and Plan Development; and  
d) Checking , Reviews, and Approvals.  
5.13 Safety  
5.13.1 Throughout the process of designing safety elements, the designer shall keep in mind the relationship 
of vehicles and pedestrians. Neither should lose track of the presence of the other. The designer should consider 
the possibility that frequent reminders on both sides may be necessary in certain cases. Key Principles for 
designing safer roads are the following:  
 Design for all road users: This includes non -motorized vehicles, pedestrians, etc. and has 
implications for almost all aspects of road design including carriageway width, hard shoulder 
design, side slopes, and side drains;  
 Provide clear and consistent message to the driver: Roads should be easily “read” and 
understood by drivers and should not present them with any sudden surprises;  
 Encourage a ppropriate speeds and behaviour by design: Traffic speed can be influenced by 
altering the “look” of the road, for example, by providing clear visual clues such as changing the 
hard shoulder treatment or installing prominent signing;  
 Reduce conflicts: Conflicts cannot be avoided entirely, but can be reduced by design, i.e., by 
staggering junctions or by using guard rails to channel pedestrians to safer crossing points; and   
 Create a forgiving road environment: This requires the designer to establish an en vironment that 
forgives a driver’s mistakes or vehicle failure, to the extent possible without significantly increasing 
costs, which ensures that demands are not placed upon the driver which are beyond his or her 
ability to manage.  DRS 267-1: 20 21 
75                ©RSB 2021  - All rights reserved  
 5.12.2  Areas of safety h azards shall be identified, effective solutions shall be evaluated, and program me of 
available funds shall be made for their most effective safety use.  
5.14 Quality control  
This work shall consist of planning and implementing a construction quality process to e nsure work conforms to 
the feeder road design guidelines. It should include quality control inspection, documentation, process control, 
sampling and testing during study and construction/rehabilitation phases.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
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 DRS 267-1: 20 21 
77                ©RSB 2021  - All rights reserved  
 Bibliography  
[1] RS 267 -1: 2015, Feeder roads — Part 1: Guidelines for design  
[2] Design Manual for Low Level Roads , Part D, 2011  
[3] Indian Road Congress, Geometric design standards for rural (non -urban) highways   
[4] Tanzania Road Geometric Design Manual  
[5] Republic of Malawi, Low Vol ume Roads Manual, Volume 1: Pavement design  
[6] RS 267 -1:2011 Feeder roads -Part 1: Guidelines for design  
 DRS 267 -1: 2021  
 
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