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Ever been to a developing nation like India as well as a developed nation like US? You will see myriad kinds of vehicles on the roads of Indian cities. Whereas, in US, the roads are full of mostly cars and cars! Even there, some variety of vehicles can be seen like the occasional buses and bikes, trucks, vans, cars of abnormal sizes etc. Whereas, in cities of India countless buses, trucks of all sizes, auto rickshaws (tuck-tucks – a 3-wheeler), motor cycles, scooters, mopeds, bicycles, rickshaws, push carts etc., would grace the roads. The resulting traffic is very complex (read chaos). Though, less and less likely these days, you may still find a stray cow here and an indolent bullock cart there! Enough for a traffic engineer, right, ah?
All these vehicles occupy different amounts of space on roads and move at different speeds and charge at different accelerations. Besides, the behaviour of drivers of the different vehicles also found to vary considerably. This poses a problem for designing roads, junctions and signals. A uniform measure of vehicles is necessary to estimate the traffic volume and capacity of roads under mixed traffic flow conditions. It is rather difficult to do, unless the different vehicle classes are stated in terms of a common standard vehicle unit.
Thus the concept of Passenger Car unit (PCU) is born. It is a common practice to convert the other vehicle classes into PCUs. So, irrespective of the country, the mixed traffic conditions of city roads call for traffic volume and road capacity in terms of PCUs. It is generally expressed as PCU per hour or PCU/lane/hour and PCUs per kilo metre length of lane.
A PCU can be regarded as a measure of the relative space requirement of a Vehicle compared to that of a passenger car under a specified set of roadway, traffic and other conditions. In other words, the PCU of a kind of vehicle can be regarded as the ratio of the capacity of a road when there are vehicles of that particular kind only to that of the same roadway when there are passenger cars only.
So, is a vehicle has a PCU value of 3.0, it needs three times more space than a car or every addition of this vehicle type is equivalent to addition of three cars to road! Conversely, it reduces the capacity of the road in terms of cars to a third.
The following are some of the many factors on which the PCU values of different vehicle classes depend upon.
Dimensions, power, speed, acceleration and braking characteristics of the vehicle.
Road characteristics such as geometrics including gradients, curves, access controls, type of road: rural or urban, presence and the type of intersections etc.
Transverse and longitudinal clearances between vehicles moving on road, which in turn depends upon the speeds, driver characteristics and the classes of other moving vehicles.
Traffic stream composition of different vehicle classes.
Other factors include environmental and climatic conditions, traffic control methods, speed limits and barriers, one way traffic etc.
From the above discussion, it can clearly be seen that the PCU value of a particular kind do not remain constant but changes in space and time.
The following are typical PCU values for different vehicle classes on urban roads:
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Sl.
No
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Vehicle
Class
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Urban
roads and mid-block
sections
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Signalised
intersection
|
Kerb
parking
(parallel
&
angle)
|
|
1.
|
|
|
|
Car
|
1.0
|
1.0
|
1.0
|
|
2.
|
|
|
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Bus
and truck
|
2.2
|
2.8
|
3.4
|
|
3.
|
|
|
|
Three-wheeler
Automobile
|
0.5
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0.4
|
0.4
|
|
4.
|
|
|
|
Two-wheeler
automobile
|
0.4
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0.3
|
0.2
|
|
5.
|
|
|
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Bicycle
|
0.7
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0.4
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0.1
|
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6.
|
|
|
|
Bullock
cart
|
4.6
|
3.2
|
1.2
|
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7.
|
|
|
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Push
cart
|
4.6
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3.2
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0.3
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