Sub-problem 2d - Page 1 of 2
Effects of a Signal on an Existing Coordinated System
In sub-problem 2c, we
analyzed the operation of a portion of the arterial, of which the Styner-Lauder
intersection is a part, using the HCM methodology. But we must also
consider another factor in the decision to signalize this intersection. How
will the new signal at U.S. 95/Styner-Lauder Avenue affect signal
coordination along the U.S. 95 corridor? To answer this question, we must
examine the three coordinated intersections as a system (see
The first task is to
determine whether the existing system is coordinated (i.e. has a common cycle length).
If the signals currently operate in an uncoordinated mode, we will have to establish coordination between them by choosing a common cycle length. There are three important considerations that
should be taken into account when selecting an appropriate cycle length:
Individual intersection timing requirements. Intersection
phasing, pedestrian timing, and other factors dictate the lower bound for
cycle length. As an example, consider the U.S. 95/SH 8 intersection, which
currently operates under split phasing to accommodate the existing lane
configuration. Split phasing strategies typically require a higher cycle
length than if the left turns were protected or permitted.
The effect is to cause this intersection to have the highest cycle length
requirement of the intersections on the arterial. Therefore, the SH8 cycle
length establishes the lower bound of the common cycle length requirement.
Distance between intersections. Closely-spaced intersections such as
Sweet and SH 8 often benefit from lower cycle lengths, which allow for
better queue management characteristics. The distance between intersections and queue storage
considerations are key in the development of
signal timing plans. While lower cycle lengths may sacrifice progression
efficiency, they can still perform better, because queue spillback and system
delay (especially on the side street) will be minimized.
Potential for cycle failures.
When the cycle length is too short, cycle failures will occur, and the
responsive operation that is characteristic of low cycle lengths may be
offset by increased delay on movements where demand exceeds capacity. In
such cases, somewhat longer cycles may actually achieve better progression—for example, where arterial
green phases must be displayed more or less simultaneously.
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