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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:
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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. |
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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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