What is Traffic Engineering:
Traffic engineering is defined as that phase of highway engineering which deals with the planning and design of highways and with the traffic operation thereon for the safe convenient and economic transportation of persons and goods. This is achieved by systematic traffic studies scientific analysis and engineering applications.
Traffic studies are further divided into:
1. Traffic characteristics 2. Traffic studies and analysis 3. Traffic control regulation
The Function of Traffic Engineering:
The function of traffic engineering can be covered in the following five categories:
1. Traffic characteristics
(a) Vehicular limitations like the weight, size, and power of the vehicle
(b) Road user limitation
(i) Physical limitations like vision, hearing, fatigue, etc.
(ii) Mental limitations like intelligence, skill, the experience of drivers, etc.
(iii) Emotional limitations like attentiveness, impatience, etc.
2. Traffic operations which constitute the traffic regulations, traffic control devices like traffic signs,
signals, marking, etc.
3. Traffic geometric design involving the design of expressways, streets, interchanges, intersections,
4. Traffic planning like the program of construction, off-street parking, etc.
5. Traffic administration
In traffic characteristics, we study the road user characteristics and vehicular characteristics.
Road User Characteristics:
The human element is involved in all the actions of road users i.e. motorists, pedestrians,s, etc. Factors affecting
road user characteristics are:
(i) Physical: Vision, hearing, strength, and the general reaction to traffic situations.
NOTE: The temporary physical characteristics of the road users affecting their efficiency are fatigue, alcohol
or drugs, and illness.
(ii) Mental: Knowledge, skill, intelligence, experience, etc.
(iii) Psychological: Attentiveness, fear, anger, superstition, impatience, the general attitude towards traffic
(iv) Environmental: Traffic stream characteristics, facilities to the traffic, atmospheric conditions and the
It is quite important to study the various characteristics of vehicles because a road can be designed for
any vehicle but not for an indefinite vehicle. The various vehicular characteristics affecting the road design are
classified as static and dynamic characteristics of the vehicle.
Static Characteristics of vehicle:
Static characteristics of vehicles affecting road design are the dimensions, weight, and minimum turning
Maximum Dimensions of Road Vehicles:
(i) Maximum width of vehicle = 2.5 m
(ii) Maximum height: 11 affects the clearance of the overhead structures and visibility of the driver
(a) Single decked vehicle = 3.80 m
(b) Double decked vehicle = 4.75 m
(iii) Maximum Length: li affects the capacity, OSD, and movability of vehicle
(a) Single unit truck with two or more axles = 11.0m
(b) Single unit bus with two or more axles = 120m
(c) Semi-trailer tractor combinations = 16.0 m
(d) Tractor and trailer combinations = 18.0 m
NOTE: No vehicle is allowed to be of more than two units and no such combinations, laden or unladen is
allowed to have an overall length exceeding 18 m.
Weight of Vehicles:
(i) Maximum weight of loaded vehicles affects the design of pavement thickness and gradients.
(ii) No single axle load, as suggested by IRC should exceed 102 kN (10.2 tonnes), and for tandom axle
180 KN (18 tonnes)
(ii) The gross load of any vehicle or combination of vehicles should not exceed the weight worked out by
the following formula:
W = 1525 (L + 7.3) – 14.7 L²
W = The gross weight of the vehicle in kg
L = The distance in meter between the extreme axles measured parallel
to the axis of the vehicle
NOTE: The above relation holds good only when L’is greater than 2.50 m.
Power of Vehicle:
The power of the heaviest vehicles and their loaded weights govern the permissible and limiting values
of gradient on road. Limiting gradients are governed by both the weight and power of heavy vehicles. Stability
of the vehicle and its safe movement on horizontal curve depends upon the width of vehicle and height of center of
Minimum Turning Radius:
(i) It depends upon the length of the wheelbase and the features of the steering system
(ii) It affects the design of sharp curves for the movement of vehicles at a slow speed.
Dynamic Characteristics of Vehicles:
Speed, acceleration, and braking characteristics are the dynamic characteristics of a vehicle. The speed
and acceleration of a vehicle depend upon the power of the engine and the resistance to overcome. The
deceleration and braking characteristics of vehicles depend on the design and type of braking system and its
A braking test is conducted to measure the skid resistance of a pavement surface. At least two of the
following three measurements are needed during the braking test to find skid resistance.
(i) Braking Distance, L (Length of a skid mark on the road surface in m)
(ii) Initial Speed, u (in m/s)
(iii) Actual duration of brake application, t(in seconds)
Following cases involved in the braking test are discussed below:
Case-1: When initial velocity and braking length is known
After the application of the brake, work done by the frictional force to stop the vehicle will be equal to the
the kinetic energy of the vehicle.
where f= coefficient of friction
g= Acceleration due to gravity (in m/s²)
Case-2: When initial velocity and actual direction of brake application is measured
After the application of brakes, frictional force acting on the vehicle to stop it will be equal to the driving force
of the vehicle.
fmg = ma
f= a/g (where, a = Retardation of vehicle during skidding in m/s²)
Case-3: When the braking length and actual duration of brake application is measured
On the application of brakes, the vehicle comes to rest with the retardation of ‘a’