Highway Engineering

5.5.6 Average vehicle delays at the approach to a signalised intersection

Webster (1958) derived the following equation for estimating the average delay

per vehicle at a signalised intersection:

(5.35)

where

d=average delay per vehicle

c=cycle length

l=effective green time divided by cycle time

d

c

x

x

qx

c

q

= x

()-

()-

+

()-

-¥ÊË ˆ ̄ ()+

1

21 2 1

065

(^22)
2
13
l 25
l

. l

142 Highway Engineering

Example 5.6 Contd

Stage 2

Approach 2A – east/west movement (straight ahead and left-turning)

Approach 2B – west/east movement (straight ahead and left-turning)

Stage 3

Approach 3A – east/west movement (right-turning)

Approach 3B – west/east movement (right-turning)

For each approach, the maximum yvalue is marked with an asterisk in

Table 5.8.

Stage 1:ymax=0.25

Stage 2:ymax=0.25

Stage 3:ymax=0.22

For each of the three phases, the lost time due to starting delays is 2 seconds,

as is the lost time during the intergreen period. The total lost time is there-

fore 12 seconds (4 ¥3 seconds).

The optimum cycle time is calculated as follows:

Co =(1.5L +5) ∏(1 -Y)

where

Y =Symax=0.25 +0.25 +0.22 =0.72

Therefore

Co=(1.5(12)+5) ¥(1 -0.72)

= 23 ∏0.28

=82 seconds