5.5.7 Average queue lengths at the approach to a signalised intersection
It is normal practice to estimate the queue length at the beginning of the green
period as this is the instant at which it will be greatest.
If the approach is assumed to be unsaturated, then whatever queue forms
during the red period will be completely discharged by the end of the green
period. Where this is the case, the maximum queue formed is equal to the
product of the actual flow and the effective red time for the approach. (The effec-
tive red time is the cycle time minus the effective green time, i.e. the length of
time for which the signal on the approach in question is effectively red.)
The formula for the unsaturated case is therefore:Nu=qr (5.38)
where
Nu=queue length at the commencement of the green period (assuming the
approach is unsaturated)
q =actual flow rate
r =effective red period (cycle time -effective green time)144 Highway Engineering
Example 5.7 ContdUsing the precise formula:Using the approximate formula:dcA
q=¥ +ÊË ˆ ̄
=¥+()
=
09
09 14 12
23
.
.
B
secondsdc
xx
qxc
q= x()-
()-
+
()-
-¥ÊË ˆ ̄
=¥()+∏()-
=+-
=
(^1) ()+
21 2 1
065
100 0 14 3 23 0 278 3 6
14 12 4
22
(^22)
2
13
l 25
l
. l
....secondsA
B
C
=
()-
()-¥()
=
=
()-
=
=¥ÊË ˆ ̄ =
1065
21 065 088
014
088
21 088
323
065
100
0 278
088 36
22213
525