Problem 4: Actuated Control

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In our previous analysis of the proposed signal at U.S. 95/Styner-Lauder Avenue, we have considered the signal to be fixed time. In reality, most new traffic signals that are installed today are actuated, responding to changing traffic demands during the day. The HCM provides a mechanism to estimate green times for an actuated controller, based on the relative traffic volumes on each intersection approach, and to estimate the delay and level of service that would result from traffic actuated timing plans.

Traffic-actuated control will generally accommodate a given volume of traffic with lower delays than pretimed control, because of its ability to adapt to demand variations.

The effects of actuated control are reflected in the HCM analysis procedure in two ways:

1. The equivalent cycle length and green times produced by the Appendix B procedure are typically lower than their pretimed counterparts, yielding a lower computed value of uniform delay.

2. As illustrated in Exhibit 16-13 of the HCM 2000, the incremental delay factor, K, is given a lower value for traffic-actuated control, depending on the unit extension time and the v/c ratio. Lower K values also produce lower delay estimates.

As we discovered in sub-problem 2d, we must also be aware of whether the intersection is part of a coordinated system of intersections, where a fixed cycle length must be used. The constraint of a fixed cycle length can have a significant effect on the operating characteristics of an actuated controller. We will explore this further in sub-problems 4c and 4d.