Traffic Engineering
Vehicular characteristics:
The design of geometric elements of a highway is affected by vehicular 
characteristics :-
 Width of vehicles affects on width of roadway (lanes), shoulder 
and parking. 
 Length of vehicles affects on radius of horizontal curves, widening 
of curves, minimum turning, road capacity, sight distances, .. etc. 
 Height of vehicles affects on the clearance providing under 
bridges and tunnels.
 Weight of vehicles affects on the thickness of layers of the road.
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Vehicle 
dimension Particulars Maximum 
dimension
Width maximum of truck or bus 2.44 m
Height a)Single-deck vehicle 3.81 m
b)Double-deck vehicle 4.72 m
Length
a)Single-deck vehicle 10.67 m
b)single with more than two axles
12.19 m
c)Tractor and trailer combination 15.24 m
weight
a)Unit of axels-maximum axle load
8.165 T
b)Any vehicle or combination of vehicles of 
width less than 2.44 m
1.415 T
c)If width greater than 2.44 m then W=1525(L 
+ 7.3) -14.7 L2
 For each 
vehicle
Vehicular 
characteristics:
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EXAMPLE:
Determine the weight of a single vehicle of width 2.44 m.
Solution;
From table L = 12.19 m
Therefore,
W =1525 ( 12.19 + 7.3 ) - 14.7 (12.19)
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 = 27538 kg 
Level of service
• To classify the different conditions of traffic flow on a 
highway, a term called (level of service) 
There are 6 level of service which are affecting by the following 
factors:
a) Speed and travel time.
b) Freedom to maneuver.
c) Traffic restriction.
d)Driving comfort and convenience.
e)Safety and rate of accidents.
f)User cost.
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Cont. Level of service (It has nationally 
agreed that level of services can be classified as)
I. Level ( A ) There are free flow, high speed, low 
density, little or no delays and driver are free to 
maneuvers.
II. Level ( B ) Flow is stable, speed begin to be restricted 
but still high, and driver still have degree of freedom 
but less than in level (A).
III.Level ( C ) Flow still in the stable zone, most of the 
drivers are restricted and satisfactory speed are 
possible to obtained. This level is reasonable for urban 
design
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Cont. Level of service
I. Level ( D ) Low speeds than in the above levels, flow 
approaches the unstable zone, little degree of freedom and 
little comfort and convenience.
II. Level ( E ) Low speeds than in the above levels& flow 
approaches the unstable.
III.Level ( F ) Forced flow , very low speeds and volume 
much than capacity. Flow in the form of queues.
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Cont. Level of service
level of 
service
Type of 
flow
Traffic 
volume/capacity 
for 3-lanes 2-
way highway
A Free ≤ 0.40
B Stable ≤ 0.58
C Stable ≤ 0.80
D Unstable ≤ 0.90
E Unstable ≤ 0.95
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F Forced ≤ 1.00
Cont. Level of service
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Factor Affecting Capacity and Volumes
1. Gradient:The longitudinal slopes of a roadway 
are effecting on:
a) Sight distance and hence efficiency.
b) Safe headways which are greater than 
calculated on down hills 
while on up hills it while be less. 
c) Speeds of vehicles and trucks which will be 
slower than the design speed on up grades.
2. Traffic composition: To avoid any reduction 
in speeds or volumes, trucks and buses are converted 
into passenger car units as follow.
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Vehic
le
Level 
of 
servi
ce
Equivalent 
Factor
Leve
l 
Rolli
ng
Mounta
in
Truck
s
A 3 4 7
B-C 2.5 5 10
Buse
s
D-F 2 5 12
All 
Level
s 
2 4 6
Factor Affecting Capacity and Volumes
3. Reduced lanes width: If the width of the lane is reduced than the 
standard width (3.65 m) .a reduction in capacity and service volume is 
anticipated.
4. Lateral clearance (width of shoulder): Any obstruction must be less 
than (1.83 m) at least apart from the edge of the roadway. If this 
distance is less, a reduction in volume and capacity is occurred.
Traffic control devices: which are usually employed for the control 
of traffic, consist of:
1) Pavement markings.
2) Traffic markers 
3)Traffic signs 
4) Traffic signals.
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1) Pavement markings
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2)Traffic markers
a. hazard markers
b. delineators
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3) Traffic signs.
a. Regulatory b. warning c. guide 
signs.
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4)Traffic signals