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Subproblem 5b - Page 5 of 7

ID# C305B05

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

The Equal Delay Strategy
The equal delay strategy was implemented by redistributing the green times, again by trial and error, to produce a closer agreement between the delays for the critical movements. The results are shown in the next column of Exhibit 3-39. The following observations are offered:


The signal timing for this strategy is noticeably different than the corresponding timing for equalizing the v/c ratios.


The previously equal v/c ratios for the competing movements now vary from 0.49 to 1.0 as a result of redistributing the green times.


The critical v/c ratio for the whole intersection has not changed from its previous value of 0.83. The critical v/c ratio is not affected by the distribution of green times. The computations for this performance measure are always based on the assumption of equal v/c ratios. This is an important point. The critical v/c ratio is a measure of overall intersection sufficiency and does not reflect the actual distribution of green times.


The control delays are now in much closer agreement among the competing movements. The EB through movement still has a slightly lower delay (47 sec per vehicle vs. 52 sec per vehicle for the other two movements). Note that the v/c for this movement is 1.0. To fully equalize the delays for all movements, the EB through movement would have to be forced to operate beyond its capacity. It is common signal timing practice to halt the iterative distribution of green times when further redistribution would create an oversaturated movement.


In spite of the slight difference in the control delay values, LOS D now applies to all of the competing movements and to the overall intersection.

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