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

The results are presented on the next page in Exhibit 3-41. While there was a wide variation in the resulting cycle length, there was very little difference observed in the overall intersection delay. The shortest cycle, 109 sec, was obtained by using the pre-timed optimal greens as maximum greens. The longest cycle (140 sec) was obtained from 150 percent of the optimal greens. 

So, what have we learned? In this example, the only way that traffic-actuated control could beat optimized pre-timed control in this table is by assuming that the actuated controller would generate an optimal timing plan; consistent with the optimized pretimed average green times. Of course, this is all predicated on the completely unrealistic assumption that the approach traffic volumes will be identical for each day and for every cycle. On the other hand, there is nothing built into conventional actuated control logic that maintains average splits at an intersection without tight control of the maximum green times.

How can actuated control improve the safety of a signalized intersection? Someone might question whether traffic-actuated control at high-speed isolated intersections should be applied because of the operations analysis. The answer is an emphatic yes, because of the dilemma zone protection that is possible with the proper placement of detection zones on each approach and proper timing settings within the controller. Certainly, support for this answer will not always come from an HCM analysis, but it is always important to keep considerations such as this in mind when making implementation decisions.