Sub-problem 2d: 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 Exhibit 1-25).

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:

1. Individual intersection timing requirements. Intersection phasing, pedestrian timing, and other factors dictate the lower bound for any common 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.

2. 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.

3. 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.