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Lecture Note on Highway Engineering
2. Geometric Design of Highways
2.1 General

Geometric design is the stage of road design process where the dimension and layouts of
roads are related to the needs of drivers and vehicle operation. The safe, efficient and
economic operation of highway is largely determined by the geometric design.
The following are factors to be considered in geometric design:
 The road should aim at long service year
 Due consideration should be given to avoid faulty designs which may need large
costs of rectify.
 Design should be consistent with standard
 Design should include items such as road signs, lighting, intersection, etc.
 The design should also consider safety elements.
 The design should consider both initial construction costs and operation costs.
2.2 Design Controls and Criteria


A.
The choice of design controls and criteria is influenced by the following factors: the
functional classification of the road; the nature of the terrain; the traffic volumes expected
on the road ;the design vehicle; the design speed; the density and character of the
adjoining land use; and economic and environmental considerations.
As these factors usually vary along a route of some length, the design does not have to be
constant for the whole length of a road. On the contrary, changes in the design are usually
required in order to obtain proper correlation between the road layout and the above
factors, whilst maintaining construction costs at realistic levels.
ROAD FUNCTIONAL CLASSIFICATION AND NUMBERING
The functional classification in Ethiopia includes five functional classes. The following are
the functional classes with their description.
I. Trunk Roads (Class I)
 Centers of international importance and roads terminating at international
boundaries are linked with Addis Ababa by trunk roads. They are numbered with
an "A" prefix: an example is the Addis-Gondar Road (A3). Trunk roads have a
present AADT 1000, although they can have volumes as low as 100 AADT.
II. Link Roads (Class II)
 Centers of national or international importance, such as principal towns and urban
centers, must be linked between each other by link roads . A typical link road has
over 400 - 1000 first year AADT, although values can range between 50-10,000
AADT. They are numbered with a "B" prefix. An example of a typical link road is
the Woldiya- Debre Tabor- Woreta Road (B22), which links, for instance, Woldiya
on Road A2 with Bahir Dar of Road A3.
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Lecture Note on Highway Engineering
III. Main Access Roads (Class III)
 Centers of provincial importance must be linked between each other by main access
roads. First year AADTs are between 30-1,000. They are numbered with a "C"
prefix.
IV.

V.


Collector Roads (Class IV)
Roads linking locally important centers to each other, to a more important center, or
to higher class roads must be linked by a collector road. First year AADTs are
between 25-400. They are numbered with a "D" prefix.
Feeder Roads (Class V)
Any road link to a minor center such as market and local locations is served by a
feeder road. First year AADTs are between 0-100. They are numbered with an "E"
prefix.
Roads of the highest classes, trunk and link roads have, as their major function to
provide mobility, while the primary function of lower class roads is to provide
access. The roads of intermediate classes have, for all practical purposes, to provide
both mobility and access.
B. TERRAIN
 The geometric design elements of a road depend on the transverse terrain through
which the road passes. Transverse terrain properties are categorized into four
classes as follows:
o FLAT: Flat or gently rolling country, which offers few obstacles to the construction
of a road, having continuously unrestricted horizontal and vertical alignment
(transverse terrain slope up to 5 percent).
o ROLLING: Rolling, hilly or foothill country where the slopes generally rise and
fall moderately and where occasional steep slopes are encountered, resulting in
some restrictions in alignment (transverse terrain slope from 5 percent to 25
percent).
o MOUNTAINOUS:
Rugged, hilly and mountainous country and river gorges.
This class of terrain imposes definite restrictions on the standard of alignment
obtainable and often involves long steep grades and limited sight distance
(transverse terrain slope from 25 percent to 50 percent).
o ESCARPMENT:
In addition to the terrain classes given above, a fourth class
is added to cater to those situations whereby the standards associated with each of
the above terrain types cannot be met. We refer to escarpment situations inclusive of
switchback roadway sections, or side hill transverse sections where earthwork
quantities are considerable, with transverse terrain slope in excess of 50 percent).
 Topography plays important roles in the location and design of highways. Design
elements should be related to specific terrain or relief. For example, in mountain
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


Lecture Note on Highway Engineering
area the design speed is lower than the speed in the plain area. Topography also
affects alignment, gradient, sight distance and type of road cross sections.
In general, construction costs will be greater as the terrain becomes more difficult
and higher standards will become less justifiable or achievable in such situations
than for roads in either flat or rolling terrain. Drivers accept lower standards in such
conditions and therefore adjust their driving accordingly, so minimizing accident
risk. Design speed will therefore vary with transverse terrain.
It is often the case in Ethiopia that the roadway can be designed to a higher speed
than is indicated by the transverse terrain type. For instance, an alignment could be
chosen through rolling terrain that gives essentially a flat highway configuration.
Similarly, a narrow plateau should be chosen for an alignment in otherwise
mountainous terrain. The discrepancy arises from an ability to choose a roadway
longitudinal slope significantly superior to the transverse slope. Under such
circumstances, the Engineer should use his judgment in assigning a higher design
speed to the roadway segment.
Photographic representations of various terrain configurations are shown in Figures
4-1 through 4-8. Alignments that are superior to transverse terrain are those shown
in Figures 4-2, 4-5, and 4-7.
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Figure 4-1 Flat Terrain; Flat Roadway Alignment
Figure 4-2: Rolling Terrain; Flat Roadway Alignment
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Lecture Note on Highway Engineering
Figure 4-3: Rolling Terrain; Flat to Rolling Roadway Alignment
Figure 4-4: Rolling Terrain; Rolling Roadway Alignment
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Adama Science and Technology University
Lecture Note on Highway Engineering
Figure 4-5: Mountainous Terrain; Flat Roadway Alignment
Figure 4-6: Mountainous Terrain; Mountainous Roadway Alignment
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Figure 4-7: Escarpment Terrain; Mountainous Roadway Alignment
Figure 4-8: Escarpment Terrain; Escarpment Roadway Alignment
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Lecture Note on Highway Engineering
C. TRAFFIC






A further factor influencing the development of road design standards, and in
particular the design speed, is the volume and composition of traffic. The design of
a road should be based in part on factual traffic volumes. Traffic indicates the need
for improvement and directly affects features of design such as widths, alignments,
and gradients. Traffic data for a road or section of road, including traffic trends, is
generally available in terms of annual average daily traffic (AADT).
Using road functional classification selection and design traffic flow, a design class,
or standard, is selected from Table 4-1, with reference to the design parameters
associated with that class as given in Tables 4-2 through 4-12.
The functional hierarchy is such that traffic aggregates as it moves from feeder to
main collector to link and trunk roads. However the actual flows will vary from
region to region and it is important that the designation of a road by functional type
should not give rise to over-design for the traffic levels actually encountered.
Design classes DS1 to DS10 have associated bands of traffic flow as was shown in
Table 4-1. The range of flows extends from less than 20 to 15,000 motorized
vehicles per day (excluding motorcycles), and covers the design conditions for all
single and dual carriageway roads.
Although the levels of flow at which design standards change are based on the best
current evidence, the somewhat subjective boundaries should be treated as
approximate in the light of uncertainties inherent in traffic estimation and future
forecasting. Therefore, the Design Traffic Flow shall normally be limited to be no
more than one Design Class step higher than the average daily traffic (AADT) in
the first year of opening. For example, a road with a first year traffic flow of 190
vehicles per day rising to 1,100 vehicles per day in the last year of it’s design life,
should be constructed to Design Class DS4 rather than Design Class DS3 (see
Table 4.1)
The design traffic flow band in this case is therefore 200 – 1000 vehicles per day
(DS4) Design to the higher Design Class DS3 would result in an over-design of the
road during almost the whole of the life of the road and may provide a solution that
was less than economic.
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Lecture Note on Highway Engineering
Table 4-1: Design Standards vs. Road Classification and AADT
Road
Desig Design Traffic Surface
Width (m)
Design Speed (km/hr)
Urban
Functional
n
Flow
Type
/PeriClassification Stand
(AADT)*
Carriageway Shoulder Flat Rolli Mountain Escarpme Urban
ard
ng
ous
nt
DS1 10000–**15000
Paved
***
DS2
5000–10000
Paved
7.3
1000–5000
Paved
7.0
200–1000
Paved
6.7
100– 200
Unpave
d
7.0
DS6
50–100
Unpave
d
6.0
DS7
30–75
Unpave
d
4.0
DS8
25–50
Unpave
d
4.0
DS9
0–25
Unpave
d
4.0
DS10
0–15
Unpave
d
3.3
Dual 2 x
7.3
See T.4-2 120
100
85
70
50
120
100
85
70
50
100
85
70
60
50
85
70
60
50
50
70
60
50
40
50
60
50
40
30
50
60
50
40
30
50
60
50
40
30
50
60
40
30
20
40
60
40
30
20
40
See T.4-2
T
R
U DS3
N
L K
DS4
M I
A N
I K
DS5
N
C
O A
L C
E C
C E
T S
O S
F R
E S
E
D
E
R
See T.4-2
See T.4-2
See T.4-2
See T.4-2
See T.4-2
See T.4-2
See T.4-2
See T.4-2
* The design two-way traffic flow is recommended to be not more than one Design Standard step in excess of the first year AADT (excluding
DS7).
** For traffic volume more than 15000 a different design approach should be followed.
*** The width of each lane is 3.65m
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Lecture Note on Highway Engineering
D. DESIGN VEHICLE

Both the physical characteristics and turning capabilities of vehicles are controls in
geometric design. Vehicle characteristics and dimensions affecting design include
power to weight ratio, minimum turning radius and travel path during a turn, and
vehicle height and width. The road elements affected include the selection of
maximum gradient, lane width, horizontal curve widening, and junction design.

The present vehicle fleet in Ethiopia includes a high number of four-wheel drive
utility vehicles and overloaded trucks. Until more detailed information becomes
available regarding the makeup of the vehicle fleet in Ethiopia, the four design
vehicles indicated in Table 4-13 should be used in the control of geometric design:
Table 4-13: Design Vehicle Dimensions and Characteristics
Design Vehicle
Design
Vehicle
Height
Width
Length
Front
Rear
Wheelbas
e (m)
Utility DV1
1.3
2.1
5.8
0.9
1.5
3.4
Min.
Design
Turning
Radius
(m)
7.3
Single
Unit DV2
Truck
Single Unit Bus DV3
4.1
2.6
11.0
1.5
3.0
6.5
12.8
4.1
2.6
12.1
2.1
2.4
7.6
12.8
Semi-Trailer
Combination
4.1
2.6
15.2
1.2
1.8
4.8+8.4=
13.2
13.7
Designation
4x4
Vehicle
DV4
Overall (m)
Chapter Two: Geometric Design of Highways
Overhang
(m)
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Lecture Note on Highway Engineering
The maximum turning path for a single unit truck, a single unit bus, and a semi-trailer
combination are shown in Figures 4-9 through 4-11, respectively.
Figure 4-9: Dimensions and Turning Radius for a Single Unit Truck (DV2)
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Figure 4-10: Dimensions and Turning Radius Path for Single Unit Bus (DV3)
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Lecture Note on Highway Engineering
Figure 4-11: Dimensions and Turning Radius for a Semi-Trailer Combination (15m
overall) also Applicable for Truck (Tandem) Plus Trailer DV4

Roads conforming to Design Standards DS1 trough DS5 should be designed to
accommodate the most restrictive of the above design vehicle. Standards DS6 and DS7,
two lane roads should accommodate all but the semi-trailer combination DV4. Standards
DS8 and DS9, for single lane roads should be designed similarly to DS6 and DS7; and
Standard DS10 roads need only accommodate the requirements for utility vehicle and
passenger cars - DV1.
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Lecture Note on Highway Engineering
E. DESIGN SPEED
 The Design Speed is used as an index which links road function, traffic flow and terrain
to the design parameters of sight distance and curvature to ensure that a driver is
presented with a reasonably consistent speed environment. In practice, most roads will
only be constrained to minimum parameter values over short sections or on specific
geometric elements.
 Design elements such as lane and shoulder widths, horizontal radius, superelevation,
sight distance and gradient are directly related to, and vary, with design speed. Thus all
of the geometric design parameters of a road are directly related to the selected design
speed. The design speeds given in Table 4-1 have been determined in accordance with
the following guidelines:
(i)
Drivers on long-distance journeys are apt (likely) to travel at higher speeds than local
traffic.
(ii) On local roads whose major function is to provide access, high speeds are
undesirable.
(iii) Drivers usually adjust their speeds to physical limitations and prevailing traffic
conditions. Where a difficult location is obvious to the driver, he is more apt to
accept a lower speed of operation.
(iv) Economic considerations (road user savings vs. construction costs) may justify a
higher design speed for a road carrying large volumes of traffic than for a less
heavily trafficked road in similar topography.
(v)
Change in design speed, if required due to a change in terrain class, should not be
effected abruptly, but over sufficient distances to enable drivers to change speed
gradually. The change in design speed should not be greater than one design speed
step, and the section with the lower geometric standards should be long enough to be
clearly recognizable by drivers (not, for example, just one single curve).
(vi) It is often the case that the physical terrain changes two steps, i.e.- from mountainous
to flat terrain. Where possible in such circumstances, a transition section of road shall
be provided with limiting parameters equivalent to the rolling terrain type. Where
this is not possible, i.e.- a Departure from Standards, special attention shall be given
to the application of warning signs and/or rumble strips to alert the driver to the
changing conditions.

It is important to note that the design of a road in accordance with a chosen design speed
should ensure a safe design. The various design elements have to be combined in a
balanced way, avoiding the application of minimum values for one or a few of the
elements at a particular location when the other elements are considerably above the
minimum requirements.
F. DENSITY AND CHARACTER OF ADJOINING LAND USE

For urban or peri-urban conditions, the design speed selection is influenced by
other factors. In such areas, speed controls are frequently included. Traffic speeds
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Lecture Note on Highway Engineering
are in fact influenced by the presence of other vehicles traveling in and across the
through lanes, physical and right-of-way constraints, together with pedestrian and
safety considerations. However, of note is the fact that the present speed limit
through villages is 30 km/h. It is possible that this limit will be increased in the
future. A design speed through peri-urban or urban areas of 50 km/h shall be used,
although such segments are posted presently at 30 km/h. Legal speed limits should
not necessarily be used as design parameters.
2.3 GEOMETRIC DESIGN ELEMENTS
2.3.1 SIGHT DISTANCE
 Simply, sight distance is the distance visible to the driver of a passenger car. For
highway safety, the designer must provide sight distances of sufficient length that
drivers can control the operation of their vehicles. They must be able to avoid
striking an unexpected object on the traveled way. Two-lane highways should also
have sufficient sight distance to enable drivers to occupy the opposing traffic lane
for passing maneuvers, without risk of accident.
 There are different types of sight distances that are commonly of interest in
geometric design. These are
Stopping sight distance (SSD),
Decision sight distance (DSD),
Passing sight distance (PSD), and
Intersection sight distance (ISD)
STOPPING SIGHT DISTANCE
 Stopping sight distance is the distance traveled during a driver’s brake reaction time
plus the braking distance for the vehicle to come to a complete stop.
 SSD must be sufficiently long to enable a vehicle traveling at the design speed to
stop before reaching a stationary object in its path.
 The Stopping sight distance comprises two elements: d1 = the distance moved from
the instant the object is sighted to the moment the brakes are applied (the
perception and brake reaction time, referred to as the total reaction time) and d2 =
the distance traversed while braking (the braking distance).
 The distance traveled before the brakes are applied is:
Where:
d1 = total reaction distance in m;
V = initial vehicle speed in Km/h, and v in m/s
t = reaction time in sec (2.5 sec).
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The braking distance, d2, is dependent on vehicle condition and characteristics, the
coefficient of friction between tyre and road surface, the gradient of the road and
the initial vehicle speed.
Where:


d2 = breaking distance in meters;
V = initial vehicle speed in km/h; and v in m/s
f=
= coefficient of longitudinal friction; 9.81 m/s2 =g
G = gradient (in %; positive if uphill and negative if downhill)
AASHTO recommends that a =3.4 m/s2 be used in determining the
minimum stopping sight distance.
Then
the
total
stopping
sight
distance
can
be
The determination of design values of longitudinal friction, f, is complicated
because of the many factors involved. The design values for longitudinal friction
used in Overseas Road Note 6 are shown in table (*).
Table (*) Coefficient of Longitudinal friction
Design speed (Km/h)
30
40
50
60
70
85
100
120
Friction, f
0.60
0.55
0.50
0.47
0.43
0.40
0.37
0.35
STOPPING SIGHT DISTANCE: SINGLE LANE ROADS
 Certain classes of roads only have a single lane, with passing pullouts. In these
circumstances, a stopping sight distance is required to enable both approaching drivers to
stop. This distance is the sum of the stopping sight distance for the two vehicles, plus a
30-meter safety distance.
DECISION SIGHT DISTANCE
 Decision sight distance is the distance needed for a driver to detect and perceive an
obstacle or information, and select an appropriate maneuver. This is important
when a driver is approaching a traffic control device, or posted information signs.
Because decision sight distance is for drivers to a maneuver or evasive action rather
than just to stop, it is greater than stopping sight distance. The decision sight
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distance for change in speed, path or direction on rural, suburban, and urban road
may be calculated from
DSD = V*t
AASHTO (2001) recommends a range of 10.2 t 14.5 sec.
PASSING SIGHT DISTANCE
 Passing Sight Distance is the minimum sight distance on two-way single roadway roads
that must be available to enable the driver of one vehicle to pass another vehicle safely
without interfering with the speed of an oncoming vehicle traveling at the design speed
 The passing sight distance should be long enough to permit a vehicle to safely pass or
overtake vehicles and return to the right lane with reasonable clearance before meeting on
coming vehicles in the opposite lane. In computing safe passing sight distance the
following assumptions are made:
 The overtaken vehicle travels at a uniform speed.
 The passing vehicle is required to follow at the same speed until there is an
opportunity to pass.
 The driver of the passing vehicle requires a certain period of time to start his
maneuver.
 The passing vehicle accelerates during the passing maneuver and its average
speed during its occupancy of the left lane is greater than that of the overtaken
vehicle.
(1)
(1)
(1)
d1

(2)
(2)
(3)
(2)
d2
(3)
(1)
d3
d4
d1 is the distance traveled during preliminary delay time; d2 is the distance traveled
by passing vehicle on the left lane; d3 is the distance b/n passing vehicle at the end
of the maneuver and the opposing vehicle; d4 the distance covered by the opposing
vehicle.
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Safe or minimum passing sight distance(PSD) is d = d 1 + d2 + d3 + d4
 d1 = 0.278 t1 (V1 – m + at1 /2 ), Where t1 = time of initial maneuver in
seconds, a is average acceleration in km/h/s ,V1 is the average speed of
passing vehicle in km/h, m is difference in speed of passed vehicle and
passing vehicle in km/h
 d2 = 0.278 V2t2 Where V2 is average speed of passing vehicle, km/h,
 .t2 is time passing vehicle occupies left lane, in second
 d3 = safe clearance distance between vehicles at the end of the maneuver, is
dependent on ambient speeds as per Table 4-14:
Table 4-14: Clearance Distance (d3) vs. Ambient Speeds
Speed Group (km/h)
50-65
66-80
81-100
101-120
d3 (m)
30
55
80
100

d4 = distance traversed by the opposing vehicle, which is approximately equal
to d2 less the portion of d2 whereby the passing vehicle is entering the left
lane, estimated at: d4 = 2d2/3
Table 4-15: Elements of safe passing sight distance (AASHTO)
Speed group,V (km/hr)
Preliminary delay time t1(sec)
Average speed of passed vehicle,V1, (km/hr)
Average speed of passing vehicle,V2, (km/hr)
Average acceleration of passing vehicle, a, m/s2)
Time vehicle occupies the opposing lane, t2(sec)
Safety distance d3 in (m)

48-64
3.6
40
56.1
0.63
9.3
30
64-80
4
54.3
70.5
0.64
10
55
80-96
4.3
68.4
84.5
0.66
10.7
76
Resulting stopping and passing sight distances, according to the above relations
are given in table 4-16.
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Table 4-16: Sight Distances
Design
Speed
(km/h)
Coefficient of Stopping
Passing Sight
Friction (f) Sight Distance
Distance (m)
(m)
from formulae
Reduced
Passing Sight
Distance for
design (m)
20
0.42
20
160
50
30
0.40
30
217
75
40
0.38
45
285
125
50
0.35
55
345
175
60
0.33
85
407
225
70
0.31
110
482
275
85
0.30
155
573
340
100
0.29
205
670
375
120
0.28
285
792
425
INTERSECTION SIGHT DISTANCE
A motorist attempting to enter or cross a highway from a stopped condition should be able to
observe traffic at a distance that will allow safe movement. In cases where traffic is intermittent
or moderate in flow, the motorist will wait to find an acceptable “gap.” The driver approaching
the intersection on the through road should have a clear view of the intersection including any
vehicles stopped, waiting to cross, or turning. The methods described in the following paragraphs
produce distances that provide sufficient sight distance for the stopped driver to make a safe
crossing or turning maneuver. If these distances cannot be obtained, the minimum sight distance
provided should not be less than the stopping sight distance for the through roadway. This would
allow a driver on the through roadway adequate time to bring the vehicle to a stop if the waiting
vehicle started to cross the intersection and suddenly stopped or stalled. If this distance cannot be
provided, additional safety measures must be provided. These could include, but are not limited
to, advance warning signals and flashers and/or reduced speed limit zones in the vicinity of the
intersection.
There are three possible maneuvers for a motorist stopped at an intersection to make. The
motorist can (1) cross the intersection by clearing oncoming traffic on both the left and right of
the crossing vehicle, (2) turn left into the crossing roadway after first clearing the traffic on the
left and then making a safe entry into the traffic stream from the right, or (3) turn right into the
crossing roadway by making a safe entry into the traffic stream from the left.
In order to evaluate the amount of sight distance available to a stopped vehicle waiting to make a
crossing or turning maneuver, the American Association of State Highway and Transportation
Officials (AASHTO) adopted the concept of using “sight triangles”.
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CONTROL OF SIGHT DISTANCE
 Sight distances should be checked during design, and adjustments made to meet
the minimum requirements. The following values should be used for the
determination of sight lines (see Figures 4-12 and 4-13):
Driver's eye height:
1.07 meters
Object height for stopping sight distance:
0.15 meters
Object height for passing sight distance:
1.30 meters
Figure 4-12: Stopping Sight Distance at Sag
Figure 4-13: Stopping Sight Distance at Crest
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Lecture Note on Highway Engineering
On the inside of horizontal curves, it may be necessary to remove
buildings, trees or other sight obstructions or widen cuts on the insides of
curves to obtain the required sight distance (see figure). 4-14).
Figure 4-14: Sight Distance for Horizontal Curves
Table 4-2:
Design
Standard
Flat
DS1
3.0
Shoulder Widths
Rural Terrain/Shoulder Width (m)
Rolling
Mountainous Escarpment
3.0
0.5 – 2.5
0.5 – 2.5
Town Section Widths (m)
Shoulder Parking Foot
Lane*** Way
Median!
n/a
3.5
2.5
5.0
(min)
(min)
n/a
3.5
2.5
Barrier!
n/a
3.5
2.5
n/a
3.0
0.5 – 2.5
0.5 – 2.5
1.5 0.5 – 1.5
0.5 – 1.5
3.0++
1.5
1.5
0.5
0.5
n/a
3.5
2.5
DS4
+++
*
0.0
0.0
0.0
0.0
n/a
3.5
2.5
DS5
+++
0.0
0.0
0.0
0.0
n/a
3.5
2.5
DS6**
1.0 (earth) 1.0 (earth)
1.0 (earth)
1.0 (earth)
n/a
n/a +
n/a +
DS7
0.0
0.0
0.0
0.0
n/a
n/a +
n/a +
DS8**
0.0
0.0
0.0
0.0
n/a
n/a +
n/a +
DS9**
0.0
0.0
0.0
0.0
n/a
n/a +
n/a +
DS10**
* shoulders included in the carriageway width given in Table 2-1
** Shoulders included in the carriageway width given in Table 2-1
*** To be provided where urbanization requires this facility
+ Where these classes of roads pass through urban areas, the road shall be designed to Standard
DS6
DS2
DS3
3.0
1.5 - 3.0++
Chapter Two: Geometric Design of Highways
Page 21
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Lecture Note on Highway Engineering
++ The actual shoulder width provided shall be determined from an assessment of the total
traffic flow and level of non-motorized traffic for each road section
+++ Depending on the development of the town & Includes a shoulder
! Median with trees (DS1) is allowed for cross section shown in the table i.e. 2lane +parking
lane + Footway if otherwise the median should be a covered and an open one without trees or a
lower width of a median barrier shall be designed . Similarly for DS2 Roads in the town section,
i.e., one lane + parking lane +footway should have a covered median with no trees or otherwise a
lower width of a median barrier should be designed.
Table 4-3: Geometric Design Parameters for Design Standard DS1 (Paved Dual
Carriageway)
Design Element
Unit
Flat
Rollin Mountainou Escarpme Urban/Perig
s
nt
Urban
Design Speed
km/h
120
100
85
70
50
Min.
Stopping
Sight
m
285
205
155
110
55
Distance
Min.
Passing
Sight
m
425
375
340
275
175
Distance
Min. Horizontal Curve
m
630
395
270
175
85
Radius
Transition Curves Required
Yes
Yes
Yes
No
Yes
Max. Gradient (desirable)
%
3
4
6
6
6
Max. Gradient (absolute)
%
5
6
8
8
8
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Super elevation
%
8
8
8
8
4
Crest Vertical Curve
k
210
105
60
31
10
Sag Vertical Curve
k
74
51
36
25
12
Normal Cross fall
%
2.5
2.5
2.5
2.5
2.5
Shoulder Cross fall
%
4
4
4
4
4
Right of Way
m
50
50
50
50
50
Table 4-4:
Geometric Design Parameters for Design Standard DS2 (Paved)
Design Element
Unit
Flat
Rollin Mountainou Escarpme Urban/Perig
s
nt
Urban
Design Speed
km/h
120
100
85
70
50
Min.
Stopping
Sight
m
285
205
155
110
55
Distance
Min.
Passing
Sight
m
425
375
340
275
175
Distance
% Passing Opportunity
%
50
50
25
0
20
Min. Horizontal Curve
m
630
395
270
175
85
Radius
Chapter Two: Geometric Design of Highways
Page 22
Adama Science and Technology University
Design Element
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Super elevation
Crest Vertical Curve
Sag Vertical Curve
Normal Cross fall
Shoulder Cross fall
Right of Way
Table 4-5:
Unit
Flat
%
%
%
%
k
k
%
%
m
Yes
3
5
0.5
8
210
74
2.5
4
50
Unit
Flat
Design Speed
Min.
Stopping
Sight
Distance
Min.
Passing
Sight
Distance
% Passing Opportunity
Min. Horizontal Curve
Radius
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Superelevation
Crest Vertical Curve
Sag Vertical Curve
Normal Crossfall
Shoulder Crossfall
Right of Way
km/h
m
Mountainou
s
Yes
6
8
0.5
8
60
36
2.5
4
50
Escarpme
nt
No
6
8
0.5
8
31
25
2.5
4
50
Urban/PeriUrban
Yes
6
8
0.5
4
10
12
2.5
4
50
100
205
Rollin
g
85
155
Mountainou
s
70
110
Escarpme
nt
60
85
Urban/PeriUrban
50
55
m
375
340
275
225
175
%
m
50
395
33
270
25
175
0
125
20
85
%
%
%
%
k
k
%
%
m
Yes
3
5
0.5
8
105
51
2.5
4
50
Yes
4
6
0.5
8
60
36
2.5
4
50
No
6
8
0.5
8
31
25
2.5
4
50
No
6
8
0.5
8
18
18
2.5
4
50
No
6
8
0.5
4
10
12
2.5
4
50
Geometric Design Parameters for Design Standard DS4 (Paved)
Design Element
Design Speed
Min.
Stopping
Distance
Rollin
g
Yes
4
6
0.5
8
105
51
2.5
4
50
Geometric Design Parameters for Design Standard DS3 (Paved)
Design Element
Table 4-6:
Lecture Note on Highway Engineering
Sight
Unit
Flat
km/h
m
85
155
Rollin
g
70
110
Chapter Two: Geometric Design of Highways
Mountainou
s
60
85
Escarpme
nt
50
55
Urban/PeriUrban
50
55
Page 23
Adama Science and Technology University
Min.
Passing
Sight
Distance
% Passing Opportunity
Min. Horizontal Curve
Radius
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Superelevation
Crest Vertical Curve
Sag Vertical Curve
Normal Crossfall
Shoulder Crossfall
Right of Way
Lecture Note on Highway Engineering
m
340
275
225
175
175
%
m
25
270
25
175
15
125
0
85
20
85
%
%
%
%
k
k
%
%
m
Yes
4
6
0.5
8
60
36
2.5
4
50
Yes
5
7
0.5
8
31
25
2.5
4
50
No
7
9
0.5
8
18
18
2.5
4
50
No
7
9
0.5
8
10
12
2.5
4
50
No
7
9
0.5
4
10
12
2.5
4
50
Table 4-7:
Geometric Design Parameters for Design Standard DS5 (Unpaved)
Design Element
Unit
Flat
Rollin Mountainou Escarpme Urban/Perig
s
nt
Urban
Design Speed
km/h
70
60
50
40
50
Min.
Stopping
Sight
m
110
85
55
45
55
Distance
Min.
Passing
Sight
m
275
225
175
125
175
Distance
% Passing Opportunity
%
25
25
15
0
20
Min. Horizontal Curve
m
175
125
85
50
85
Radius
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
4
5
7
7
7
Max. Gradient (absolute)
%
6
7
9
9
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
4
Crest Vertical Curve
k
31
18
10
5
10
Sag Vertical Curve
k
25
18
12
8
12
Normal Crossfall (Paved)
%
2.5
2.5
2.5
2.5
2.5
Shoulder Crossfall (Paved)
%
4
4
4
4
4
Normal and Shoulder
%
4
4
4
4
4
Crossfall (Unpaved)
Right of Way
m
50
50
50
50
50
Table 4-8:
Geometric Design Parameters for Design Standard DS6 (Unpaved)
Design Element
Unit
Flat
Rollin Mountainou Escarpme Urban/Peri
g
s
nt
- Urban
Design Speed
km/h
60
50
40
30
50
Chapter Two: Geometric Design of Highways
Page 24
Adama Science and Technology University
Min.
Stopping
Sight
Distance
Min.
Passing
Sight
Distance
% Passing Opportunity
Min. Horizontal Curve
Radius
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Superelevation
Crest Vertical Curve
Sag Vertical Curve
Normal and Shoulder
Crossfall (Unpaved)
Right of Way
Table 4-9:
Design Element
Lecture Note on Highway Engineering
m
85
55
45
30
55
m
225
175
125
75
175
%
m
20
125
20
85
15
50
0
30
20
85
%
%
%
%
k
k
%
No
6
8
0.5
8
18
18
4
No
7
9
0.5
8
10
12
4
No
10
12
0.5
8
5
8
4
No
10
12
0.5
8
3
4
4
No
7
9
0.5
4
10
12
4
m
30
30
30
30
40
Geometric Design Parameters for Design Standard DS7 (Unpaved)
Unit
Flat
Rollin
g
50
55
Mountainou
s
40
45
Escarpme
nt
30
30
Urban/Peri
- Urban
50
55
Rollin
g
50
Mountainou
s
40
Escarpme
nt
30
Urban/PeriUrban
50
Design Speed
km/h
60
Min.
Stopping
Sight
m
85
Distance
Min.
Passing
Sight
m
225
175
125
75
175
Distance
% Passing Opportunity
%
20
20
15
0
20
Min. Horizontal Curve
m
125
85
50
30
85
Radius
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
10
10
7
Max. Gradient (absolute)
%
8
9
12
12
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
4
Crest Vertical Curve
k
18
10
5
3
10
Sag Vertical Curve
k
18
12
8
4
12
Normal and Shoulder
%
4
4
4
4
4
Crossfall (Unpaved)
Right of Way
m
30
30
30
30
30
Table 4-10: Geometric Design Parameters for Design Standard DS8 (Unpaved)
Design Element
Design Speed
Unit
Flat
km/h
60
Chapter Two: Geometric Design of Highways
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Adama Science and Technology University
Min.
Stopping
Sight
Distance
Min.
Passing
Sight
Distance
Min. Horizontal Curve
Radius
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Superelevation
Crest Vertical Curve
Sag Vertical Curve
Normal and Shoulder
Crossfall (Unpaved)
Right of Way
Max. Spacing of Passing
Bays
Design Vehicle
Lecture Note on Highway Engineering
m
85
55
45
30
55
m
225
175
125
75
175
m
125
85
50
30
85
%
%
%
%
k
k
%
No
6
8
0.5
8
18
18
4
No
7
9
0.5
8
10
12
4
No
10
12
0.5
8
5
8
4
No
10
12
0.5
8
3
4
4
No
7
9
0.5
4
10
12
4
m
m
20
500
20
500
20
500
20
500
20
500
DV 2/3
Table 4-11: Geometric Design Parameters for Design Standard DS9 (Unpaved)
Design Element
Unit
Flat
Rollin Mountainou Escarpme Urban/Peri
g
s
nt
- Urban
Design Speed
km/h
60
40
30
20
40
Min.
Stopping
Sight
m
85
45
30
20
45
Distance
Min.
Passing
Sight
m
225
125
75
50
125
Distance
Min. Horizontal Curve
m
125
50
30
15
50
Radius
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
13
13
7
Max. Gradient (absolute)
%
8
9
15
15
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
8
Crest Vertical Curve
k
18
5
3
2
5
Sag Vertical Curve
k
18
8
4
2
8
Normal and Shoulder
%
4
4
4
4
4
Crossfall (Unpaved)
Right of Way
m
20
20
20
20
20
Max. Spacing of Passing
m
500
500
500
500
500
Bays
Design Vehicle
DV 2/3
Chapter Two: Geometric Design of Highways
Page 26
Adama Science and Technology University
Table 4-12:
Lecture Note on Highway Engineering
Geometric Design Parameters for Design Standard DS10 (Unpaved)
Design Element
Unit
Flat
Mountainou
s
30
30
Escarpme
nt
20
20
Urban
60
85
Rollin
g
40
45
Design Speed
Min.
Stopping
Sight
Distance
Min.
Passing
Sight
Distance
Min. Horizontal Curve
Radius
Transition Curves Required
Max. Gradient (desirable)
Max. Gradient (absolute)
Minimum Gradient
Maximum Superelevation
Crest Vertical Curve
Sag Vertical Curve
Normal and Shoulder
Crossfall (Unpaved)
Right of Way
Max. Spacing of Passing
Bays
Design Vehicle
km/h
m
m
225
125
75
50
125
m
125
50
30
15
50
%
%
%
%
k
k
%
No
6
8
0.5
8
18
18
4
No
7
9
0.5
8
5
8
4
No
14
16
0.5
8
3
4
4
No
14
16
0.5
8
2
2
4
No
7
9
0.5
8
5
8
4
m
m
20
500
20
500
20
500
20
500
20
500
Chapter Two: Geometric Design of Highways
40
45
DV 1
Page 27