Sub-problem 5c: Pre-timed vs. Traffic-Actuated Operation

The signal timing specification for traffic-actuated control has the same format on the HCM worksheets as pre-timed control. The phasing plan is required in both cases, along with the allocation of green times. In the case of pre-timed control, the green times are expected to be the actual green time settings for the controller. In the case of traffic-actuated control, the actual green times will vary from cycle to cycle. The values expected by the HCM procedure are the average green times that will be generated over the analysis period by the controller logic. Appendix B to HCM Chapter 16 gives guidelines for determining average green times.

What are the primary operation effects of using actuated control? The input data associated with traffic-actuated control (e.g., lower cycle lengths) will often produce lower delay estimates than those associated with pre-timed control. There is only one difference in the analytical treatment of pre-timed and traffic-actuated control at isolated intersections. That difference is found in the k parameter of the incremental delay equation. HCM Exhibit 16-3 gives a table of k values that account for traffic-actuated control as a function of the unit extension and the v/c ratio.

What additional information is needed beyond the data already used in Sub-problem 5b? Because the v/c ratio is already computed for pre-timed control, the only additional piece of information you need to apply the procedure to traffic-actuated control is the unit extension time setting. This setting represents the duration of the gap between vehicles on the approach being serviced that will cause the controller to terminate the phase. Smaller unit extensions will produce smaller k values, and smaller k values will produce lower delay estimates. For purposes of this discussion, we will adopt the default setting of 3.0 seconds given in HCM Chapter 10.