لخّصلي

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