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Subproblem 5b - Page 3 of 7

ID# C305B03

Sub-problem 5b: Three-phase Traffic Signal Control with a Protected Westbound Left Turn

The Equal v/c Strategy
The equal v/c strategy was explored to some extent in the last sub-problem as the basis for the QEM timing plan synthesis. The QEM results are repeated in one column of Exhibit 3-39. The next column shows what happens when the QEM timing plan is transferred directly into the operational procedure. The following observations are offered:


The critical phase v/c ratios computed by the operational procedure are nearly, but not quite, equal (0.75 vs. 0.79). In other words, the more detailed treatment of saturation flow rate, lost time, etc., by the operational procedure has produced minor differences in the results.

bullet Additional performance measures are provided by the operational procedure, including v/c ratios delays and level of service for each lane group. The QEM does not carry its computations to this level.

The default yellow plus all red time for the QEM is 4 seconds per cycle per phase. The unusually wide intersection and high speed approaches dictates a longer inter-green period. For purposes of this discussion, values of 5 sec yellow and 1 sec all red will be used as an approximation of the local agency practice. While a 4-second inter-green is generally a reasonable default value for planning level analysis, this is a case in which the QEM assumptions do not apply to the intersection in question.

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