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184
Traffic Signal Systems Operations and Design: Isolated Intersections
A
ctivity
30: C
onsidering
M
inimum
G
reen
T
ime
, P
assage
T
ime
,
and
D
etection
Z
one
L
ength
Percentile
1-Lane
2-Lane
Headway (sec)
Unoccupancy time (sec)
Headway (sec)
Unoccupancy time (sec)
99
3.1
2.0
2.1
1.0
95
2.4
1.3
1.8
0.7
90
2.2
1.1
1.7
0.6
Table 15.
Maximum allowable headways and unoccupancy times for 1-Lane and 2-Lane conditions
This example shows that the unoccupancy times (and thus the passage times) are from 0.5 to 1.0 second lower
for the 2-lane case than for the 1-lane case. The clear implication is that the passage time for a 2-lane approach
should be lower than for a 1-lane approach if we are to achieve the same efficiency in signal timing and meet
our objective of providing sufficient green time to serve a clearing queue but not vehicles that arrive after the
queue has cleared.
Conclusion
The minimum green time establishes the minimum time that the green will be displayed for a phase. The
passage time parameter determines how long the green will be extended after the minimum green timer has
expired and is directly related to the MAH and the length of the detection zone. The stochastic variation of
headways in the vehicle stream means that the challenge in selecting the MAH (and thus the passage time)
is to balance two risks. The first risk (if the MAH is too short) is that the phase will be terminated too early
if the queue is still clearing. The second risk (if the MAH is too long) is that the phase will be extended past
the time that the queue has cleared. Selecting the MAH, and then the passage time, is the balance in risks that
the transportation engineer must determine. Finally, we need to consider shorter passage times for a 2-lane
approach than the value we would consider for a one-lane approach. The activities to follow will give you
specific experiences in dealing with each of these issues.