Traffic Engineering

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Through Champion Study Plan for GATE Civil Engineering (CE) 2022, we are providing Traffic Engineering study notes and other important materials on every topic of each subject.

These topic-wise study notes are useful for the preparation of various upcoming exams like GATE CivilIESBARCISROSSC-JEState Engineering Services examinations and other important upcoming competitive exams.

The article contains fundamental notes on the “Traffic Engineering”  topic of the “Highway Engineering” subject. 

Traffic Studies on Flow, Speed & Volume

Traffic Engineering

Traffic engineering is that phase of engineering which deals with planning and geometric design of streets, highways, abutting lands, and with traffic operation thereon, as their use is related to the safe, convenient and economical transportation of persons and goods.

Theoretical maximum capacity, (C)


where S = Minimum clear distance between two vehicles (m).

S = 0.2V + 6 where, v = Speed of vehicle in km/hr.


C = Theoretical maximum capacity in vehicle/hour

Ht = Time headway in ‘Sec’.


C = Traffic capacity or traffic volume in vehicle/hour

δ = Traffic density in vehicle/km

V = Traffic velocity in km/hr


Passenger Car Unit

The PCU may be considered as a measure of the relative space requirement of a vehicle class compared to that of passenger car under a specified set of roadway, traffic and other conditions.


Accident Studies

The problem of accident is very acute in highway transportation due to complex flow patterns of vehicular traffic presence of mixed traffic and pedestrians. Traffic accidents may involve property damages, personal injuries or even casualties.


Where, e = Coefficient of restitution

image007 Velocity of separation

image008 Velocity of approach.



VA = Velocity of vehicle ‘A’ of mass mA before collision.

VB = Velocity of vehicle ‘B’ of mass mB before collision.

image011 Velocity of vehicle A after collision

image012 Velocity of vehicle B after collision.

e = 1 for perfectly elastic collision.

e = 0 for perfectly inelastic collision or plastic collapse.

i.e., both vehicle move with same velocity after collision.

Momentum Equation


Types of Collision

Collision of moving vehicle with parked vehicle (assumption: collision is perfectly plastic)


Where, v1 = Initial velocity of moving vehicle in km/hr.

v2 = Velocity of moving vehicle after travelling distance ‘s1’ (in meter)

v3 = Common velocity of moving & parked vehicle at the time of collision.

s2 = Distance travelled by both vehicle till both vehicle comes in rest finally.

Two Vehicle Approaching from Right Angle Collide at an Intersection




image017 are skid distance just before collision. image018 are skid distance after collision.

image019 are speed of vehicle A & B respectively after the collision.

image020 are speed of vehicle A & B respectively after skidding a distance image021

image022 are speed of vehicle A & B respectively before skidding.

Design Cycle Metho

Trial Cycle Method


Where, XA = Number of vehicle accumulated in one cycle time on Road A.

XB = Number of vehicle accumulated in one cycle time on Road B.

T = Total cycle time in ‘sec’ (assumed)

GA = 2.5 XA ηA = Traffic count on road A in 15 minutes.

GB = 2.5 XB ηB = Traffic count on road B in 15 minutes.


Where, T’ = Total cycle time (Actual)

AA = Amber time on road A

AB = Amber time on road B

GA & GB are green time on road A & B respectively.

It T’ = T then O.K. otherwise repeat the process.

Approximate Method


Whey, RA = Red time on road A

RB = Red time on road B

GAP = Green time on road A for pedestrians

GBP = Green time on road B for pedestrians

WA = Width of road A

WB = Width of road B

1.2 m/s = Speed of pedestrians GA = RB-AA


Where, GA = Green time on road A

GB = Green time on road B

AA & AB are Amber time on road A & B respectively.

Websters Method


Where, CO = Optical cycle time

L = Total lost time

L = 2n+R

Where, n = number of phase

R = All red time


Where, qA = Normal flow on road A

qB = Normal flow on road B

SA = Saturation flow on road A

SB = Saturation flow on road B


Where, GA & GB are green time on road A & B respectively.

Annual Average Daily Traffic (AADT or ADT)


Space Mean Speed (Vs)


Where, Vs = Space mean speed in km/hr.

d = Length of road in meter

n = Number of individual vehicle observations

ti = Observed travel time (sec) for ith vehicle of travel distance ‘d’ meter.

Time mean speed (vt)


Where, Vt = Time mean speed (km/hr)

Vi = Observed instantaneous speed of ith vehicles (km/hr)

n = number of vehicles observed.

Speed & delay study by floating car method

Average journey time (t) in minute



q = Flow of vehicles (volume per minutes) in one direction of the stream.

na = Average number of vehicles counted in the direction of the stream when the test vehicle travel in the opposite direction.

ny = The average number of vehicles overtaking the test vehicle minus the number of vehicle overtaken when the test is in the direction of ‘q’.

tw = Average journey time when the test vehicle is travelling with the stream q.

ta = Average journey time, in minute when the test vehicle is running against the stream ‘q’.

Relationship between speed, travel time, volume, density & capacity

Travel time per unit length of road, 


Where, V = Speed in km/hr


Where, q = Average volume of vehicle passing a point during a specified period of time (vehicle per hour).

k = Average density or number of vehicle occupying a unit length of roadway at a give instant (vehicles/km).

Vs = Space-mean speed of vehicles in a unit roadway length (km/hr)



Capacity flow or maximum flow, (qmax)


where, VSF = Free mean speed i.e., maximum speed at zero density.

Kj = Jam density i.e., maximum density at zero speed


Where, s = Spacing between vehicles.

Rotary intersection

A rotary intersection or traffic rotary is an enlarged road intersection where all converging vehicles are forced to move round a large central island in one direction (clockwise direction) before they can weave out of traffic flow into their respective directions radiating from the central island.


Radius of rotary, (R)


Where, V = Design speed of vehicle (km/hr)

f = Coefficient of friction may be taken as 0.43 & 0.47 for the speed of 40 & 30 km/hr respectively after allowing a factor of safety of 1.5.

(Rmin)Central Island = 1.33 (R)Entry curve.

(Rmin)Central Island = Minimum radius of central Island.

(Rmin)Entry curve = Radius of entry curve.




Where, w = Width of weaving section.

e1 = Entry width,

e2 = Width of non-weaving section.

Capacity of Rotary (QP)



Where, QP = Practical capacity of weaving section of a rotary in PCU per hour.

w = Width of weaving section (6 to 18 m).

e = Average width of entry ‘e1’ & width of non-weaving section e2 for the range e/w = 0.4 to image044

L = Length of weaving section between the ends of channelizing islands in meter for the range of image045

p = Proportion of weaving traffic or weaving ratio.


where, a = Left turning traffic moving along left extreme lane.

d = right turning moving along right extreme lane.

b = Crossing/weaving traffic turning towards right while entering the rotary.

c = Crossing/weaving traffic turning towards left while leaving the rotary.

Parking Facilities

Number of spaces (N)

image047 for parallel parking with equal spacing facing the same direction.

image048 for parallel parking when two cars placed closely.

image049 for 30° angle parking

image050 for 45° angle parking

image051 for 60° angle parking

image052 for 90° angle parking

Out of various angles used in angle parking, 45 degree angle is considered the best from all considerations discussed above.

Highway Lighting

Spacing between lighting units =


Trip Distribution


Where, Tij = Number of trips from zone I to zone j.

Gi = Trips generated in zone i.

Ai = Trips attracted to zone j.

Fij = Empirically derived ‘Friction Factor’ calculated on area wise basis.

n = Number of zones in the urban area.


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