Highway Engineering

cycle will result in all queuing vehicles being cleared in the early part of the green

period, with the only vehicles crossing the stop line in the latter part being those

that subsequently arrive, often quite widely spaced. This too is inefficient. (The

discharge of traffic through a junction is at its most efficient when there is a

waiting queue on the approach road.)

Webster (1958) developed a set of formulae for establishing the optimum

signal settings in order to minimise the total delay to all streams on the approach

roads. The optimum cycle time Co is obtained from:

Co=(1.5L +5) ∏(1 -Y) (5.34)

where

L =total lost time per cycle

Y =the sum of the maximum yvalues for all of the phases which make up the

cycle

(yis the ratio of actual flow to saturation flow on each approach)

There is a minimum cycle time of 25 seconds based on safety considerations.

A maximum cycle time of 120 seconds is considered good practice. Normally,

the cycle time will lie within the range of 30 to 90 seconds.

Lost time

Lost time per cycle consists of the time lost during the green period (generally

taken as 2 seconds per phase) plus the time lost during what is known as the

intergreen period. The intergreen period is defined as the period between one

phase losing right of way and the next phase gaining right of way, or the time

between the end of green on one phase and the start of green on the next, The

intergreen period provides a suitable time during which vehicles making right

turns can complete their manoeuvre safely having waited in the middle of the

intersection.

If the amber time during the intergreen period is 3 seconds and the total inter-

green period is 5 seconds, this gives a lost time of 2 seconds, as this is the period

of time for which all lights show red or red/amber, a time during which no

vehicle movement is permitted. The period of time lost to traffic flow is referred

to as lost time during the intergreen period. It should not be confused with lost

time due to starting delays at the commencement of each phase.

Table 5.7 shows an example of a two-phase system.

140 Highway Engineering

Phase 1 G A A A R R Red

Amber period (3 s) Lost time (2 s) ̈5 seconds intergreen period (3 s +2s)

Phase 2 R R R R R/A R/A Green

Table 5.7Typical intergreen period within a two-phase signal system