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Traffic Signal Systems Operations and Design: Isolated Intersections
3. What is the result of a phase termination analysis?
4. What is the basis for your selection of the MAH? Use the results from your phase termination analysis
in explaining your answer.
The MAH is the largest headway that you will tolerate in a departing queue before the phase should
terminate. The choice of this headway involves balancing two conflicting and competing issues: if
the headway that you select is too small, then you run the risk of terminating the phase too soon and
not serving all of the vehicles in the queue that formed during red. However, if the headway that you
select is too large, then you run the risk of allowing the phase to extend too long, serving not just the
initial queue but also vehicles that arrive after the initial queue has cleared. One problem comes in
recognizing that some of the headways that you observe after the queue has cleared might be in the
same range as those that you observed during the queue clearance. Conversely, a slowly reacting driver
in a vehicle that is part of a departing queue will result in a headway that is longer than the normal
saturation headway. The choice that you make in the value of the MAH will have some risk of both
conditions: AType 1 termination, or cycle failure, when not all of the initial queue is served or a Type
2 termination, an inefficient extension of the green, resulting in longer delays on the other approaches.
While the ideal goal is to achieve a Type 3 termination (when the phase terminates just after the queue
has cleared) each time a phase terminates, your challenge is to find a MAH that balances the risks of
the Type 1 and Type 2 terminations.
So what is a phase termination analysis?Aphase termination analysis is a tool that looks at the headway
data generated by a simulation model from a stream of vehicles departing from an intersection and,
given a value of the MAH, classifies each phase termination into one the three types described above.
The first part of the analysis involves the stream of headways for the departing vehicles, with the
stream separated into vehicles that were a part of the queue (noted by Q) and those that arrived after
the queue had cleared (noted by NQ). An example of the headway data are shown in the first two
columns of Figure 119. Note that when you collect your headway data in this activity, the passage
time will be set to a very high value (5 seconds) so that you can observe a sufficient sample of both
queued and non-queued vehicles and their resulting headways departing after the beginning of green.
The high passage time provides you with enough vehicles (and resulting headways) to allow you to
study different values of MAH and their effects on phase termination.
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