Sub-problem 4b: Effect of Unit Extension

Remember that, at the outset of this problem, we assumed that the unit extension times would be set to 3 seconds for single lane operation and 2 seconds for multiple-lane operation. These are commonly-used values for the unit extension and are consistent with an expected saturation flow rate of about 1,900 vehicles per lane per hour of green. Under these conditions, the 2/3-second unit extension values can be expected to have the effect of extending the green phase beyond its minimum length only so long as vehicle spacing is not much greater than one would expect under saturation flow rate conditions.

As a further illustration of the effect of the traffic actuated settings, let us examine what would happen to the operation of the Styner-Lauder/U.S. 95 intersection if we increased the unit extension times for all phases to 5 seconds each. This would mean that a phase would not terminate until a five-second gap was observed between vehicles on any approach. In other words, the controller would be waiting for stragglers instead of passing control to the next phase after the queue of vehicles has been serviced.

Exhibit 1-40 summarizes the results of this exercise. Using the same Appendix B procedures that we used in sub-problem 4a, we can see that the average phase times have increased from 15 and 16 seconds to 19 and 24 seconds, respectively. The cycle length has increased from 31 seconds to 43 seconds. It is also interesting to note that the apportionment of green time now favors the arterial movement to a greater extent, because the longer north-south maximum green time has allowed this phase to be extended more than the east-west phase. This is also evident in the proportionately greater increase in the east-west delays that are now being predicted.

Based on the information shown in Exhibit 1-40, is it better to use the originally-estimated unit extension values of 2-3 seconds, or the 5-second unit extension value assumed for this sub-problem? The answer depends on what the analyst is trying to accomplish and the environment in which the signalized intersection is located. In the case of the Styner-Lauder/U.S. 95 intersection, it might be appropriate to use the longer unit extensions on the northbound and southbound approaches of the state highway in order to favor and benefit through traffic. The shorter unit extensions might be appropriate to use on the eastbound and westbound side street approaches in order to minimize overall intersection delay and cycle length. An alternative approach, however, could also be used with equal validity.