Subproblem 5b - Page 4 of 7

ID# C305B04

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

This analysis was repeated with two modifications. First, the inter-green times were increased from 4 sec to 6 sec. Second, the green times were redistributed by trial and error to produce a closer agreement between the v/c ratios for the critical movements. The results are shown in the next column of Exhibit 3-39. The following observations are offered:

	The critical v/c ratio for the whole intersection was increased from 0.79 to 0.83 as a result of increasing the inter-green times and thereby reducing the effective green times.
	The increase in v/c ratios was, predictably, accompanied by an increase in delay; but the delay increase was not sufficient to change the level of service for any of the movements.
	The critical v/c ratios for each phase are in closer agreement (0.83, 0.83 and 0.84). This was the closest possible agreement that could be produced by trial and error with 0.1 sec resolution in the green times.
	The control delays and levels of service differ widely among the various movements. This observation makes it clear that equalizing the v/c ratios does not necessarily equalize the delays among competing movements. Note that the delays varied from 18 sec per vehicle to 96 sec per vehicle and the LOS varied from B to F.

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