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 Signal Timing Design: Professional Practice

 
Timing Adjustments

The following excerpt was taken from section 12-306.3 of the 1995 Idaho Transportation Department Traffic Manual.

(This is a continuation of the CYCLE LENGTH discussion that was visited in the "cycle length determination" professional practice page.)

To ensure that critical lane volumes are adequately served, a capacity check should be computed for each green interval. This can be done by making the following computations for each phase:

1. For each signal phase, determine the critical lane.
2. Then for each signal phase, determine in that critical lane the vehicles served per cycle.
3. That phase minimum green time would be as follows:

Phase Minimum
Green Interval = Vehicles per cycle x 1.1 x 2.1 sec + 3.7 sec
1.1 sec provides a 10% increase for capacity traffic fluctuations
2.1 sec is the average headway per vehicle
3.7 sec is the time delay to start a traffic queue

4. The total cycle length equals the sum of the phase minimum green intervals determined in item no. 3.

The minimum green interval should be less than green intervals determined above, under the Webster method. If not, the cycle length should be increased with additional time allocated to those phases not meeting the capacity criteria.

(The Webster method portion, as referred to, was not included on this page.)