Highway Engineering

5.5.4 Effective green time

Effective green time is defined as the length of time that would be required to

get a given discharge rate over the stop line if the flow commenced and finished

simultaneously and instantaneously on the change of colour as displayed on the

signal head.

An analysis of the flow of vehicles across the stop line at an intersection

permits the effective green time to be estimated. The discharge of vehicles across

the stop line starts at the beginning of the green period and finishes at the end

of the amber period. The intervals of time between the start of actual green

time and the start of effective green and between the end of effective green time

and the end of the amber period are termed lost time.

At the start of any given cycle, when the light goes green and traffic begins

to move off, the flow across the stop line rises from zero, gradually increasing

until saturation flow is achieved. The flow level remains steady until the light

138 Highway Engineering

Example 5.5 Contd

For the non-nearside lane, the saturation flow can be calculated using

Equations 5.26 to 5.33:

f =1 (all vehicles turning)

X 0 =0.6

Ns=2 (storage spaces)

t 1 =12 (0.6)^2 /(1 +0.6(1 -1) ¥2)

=4.32

t 2 = 1 - (1 ¥0.6)^2

=0.64

T = 1 +(1.5/15) +(4.32/0.64)

= 1 +0.1 +6.75

=7.85

Sg=(2080 -230)/(1 +(7.85-1) ¥1)

=1850/7.85

=236 pcu/hr

P =0.9 ¥ 1 +(0.1 ¥2.3)

=1.13

Sc=1.13 ¥(1 +2) (1 ¥0.6)0.2¥3600/(0.5 ¥60)

=367 pcu/hr

Therefore:

S 2 = 236 + 367

=603 pcu/hr