By

K.Pranuthi,

Urban Transportation Engineering,

Jawaharlal Nehru Technological University,

14011D9010

IntroductionDefinition and NotationsDesign ProcedureFixed Time SignalsAdvantages DisadvantagesDetermination Cycle length Saturation flowProblemConclusion

Traffic signals are one of the most effective and flexible

active control of traffic and is widely used in several

cities world wide.

The conflicts arising from movements of traffic in

different directions is addressed by time sharing

principle.

Cycle: A signal cycle is one complete rotation

through all of the indications provided.

Cycle length: Cycle length is the time in seconds

that it takes a signal to complete one full cycle of

indications. It indicates the time interval between

the starting of green for one approach till the next

time the green starts. It is denoted by C.

Interval: Thus it indicates the change from one

stage to another.

There are two types of intervals :

1. Change interval and

2. Clearance interval.

Change interval : It is also called the yellow time

indicates the interval between the green and red

signal indications for an approach.

Clearance interval : It is also called all red and is

provided after each yellow interval indicating a period

during which all signal faces show red and is used for

clearing off the vehicles in the intersection.

Green interval: It is the green indication for a

particular movement or set of movements and is

denoted by Gi. This is the actual duration the green light

of a traffic signal is turned on.

Red interval: It is the red indication for a particular

movement or set of movements and is denoted by Ri.

This is the actual duration the red light of a traffic signal

is turned on.

Phase: A phase is the green interval plus the change and

clearance intervals that follow it. It allows a set of

movements to flow and safely halt the flow before the

phase of another set of movements start.

• Lost time: It indicates the time during which the

intersection is not effectively utilized for any

movement. For example, when the signal for an

approach turns from red to green, the driver of the

vehicle which is in the front of the queue, will take

some time to perceive the signal (usually called as

reaction time) and some time will be lost before vehicle

actually moves and gains speed.

Types of Signals:

Fixed-time Signals : Green Periods and Cycle lengths

are pre-determined with fixed duration.

Vehicle actuated Signals : Green periods vary and

are related to actual demands made by traffic.

Semi actuated Signals : Detectors are located only

on the side roads

Type Advantages

Fixed time Signals • Simple Construction and

• Inexpensive

Vehicle-Actuated signals • Flexible to adjust accord to

traffic demand

•Delay is minimized to attain

maximum capacity

Semi Actuated Signals Useful for junctions of a low

traffic volume

Type Disadvantages

Fixed time Signals • Inflexible

•Cause Avoidable delay

Vehicle-Actuated signals •Require Costly Equipments

such as detectors

Semi Actuated Signals High traffic accidents at times

of light traffic

Signal faces we have are :

Red

Amber

Green

The Amber period is generally taken as

2 seconds.

The Determination of Cycle time is one of most

important steps in fixed-signal designing.

Objective: least delay should be caused to the traffic

passing through the intersection.

Therefore for each traffic flow there is an optimum

cycle time which results in the minimum delay to the

vehicles.

Optimum Cycle Length : (Webster Method)

C0 =Optimum Cycle time

L = Total lost time per cycle(in seconds)

C0=

Y = y1+y2+……….+yn

y1+y2+……….+yn are the maximum ratios of

flow to saturation flow for phases1,2,…..n

( i.e q/s where q is the flow and s is the saturation

flow)

The Y value according to Webster is taken as the

highest ratio of q/s

Saturation Flow :

S= 525 w PCU/hr

S = Saturation flow

W= lane width of approach road in meters measured kerb

to inside of centre-line

Width w in meters Saturation flow in (PCU/hr)

3 1850

3.5 1890

4 1950

4.5 2250

5 2250

5.5 2900

Type of Vehicle PCU Equivalent

Heavy or medium goods vehicles 1.75

Light goods vehicles 1.00

Bus 2.25

Motor cycle 0.33

Pedal Cycle 0.20

Loss time is also considered :

L = n l+ R

L = The Total lost time

l = lost time for the phase

R = The time during each cycle when all signals

display red simultaneously (including amber

time).

Effective Green Time :

Effective green time available in a cycle can be

divided into different phases so as to give the least

overall delay to the traffic using intersection.

g1:g2…….gn = y1: y2……….yn

y1,y2……….yn= maximum of y values

g1:g2…….gn = effective green times allotted to phases

1,2,3…..n respectively.

North South East West

Flow ,q 800 400 750 600

Saturati

on flow ,

S

2400 2000 3000 3000

y=q / s 0.33 0.2 0.25 0.2

y (max)

values

0.33

0.25

Loss of time = n l + R

No. of phases considered are 2

= (2 * 2) + 2 +2

= 8

Optimum Cycle Time =

C0 =

=

C0= 40 Seconds

Effective green time per cycle = C0 – L

= 40 – 8

= 32 sec

The effective green times for the phase can be

calculated as :

g N-S =

g E-W =

2 s 18 s 2s 18s

22s 2s 14s

2s

Controlling the traffic based on the traffic

demand is more productive and has less delay

time .Therefore Vehicle Actuated signals are

more advisable to avoid unnecessary waiting

time.

Thank You