Sub-problem 4c: Coordinated Operation With Actuated Control

In the first two sub-problems involving traffic-actuated control, we have assumed that the controller at the Styner-Lauder/U.S. 95 intersection will operate in an isolated mode, independent of any other intersections on U.S. 95. It is, however, more likely that the local agency would want to establish a coordinated system of actuated controllers on this route to take advantage of the improvements that could be derived from the progressive movement of traffic on U.S. 95. This would require a full timing plan design, including cycle length, phase split times, and offsets. Coordinated arterial system timing design is beyond the scope of the HCM arterial analysis procedure. It would therefore be necessary to use one of the several available arterial signal timing software products for this purpose.

We will consider this example in the context of a coordinated system operating on a 90 second cycle as was introduced in Problem 2d (coordination effects of a new signal). Coordinated traffic-actuated control systems are generally modeled as pretimed systems for purposes of timing plan design. This is because actuated controllers within a coordinated environment are forced to operate under a constant cycle length. As a coarse approximation of their internal logic, traffic actuated controllers are usually represented as devices that will assign enough time to the cross street to maintain a reasonable degree of saturation, with the remaining time given to the arterial movements. 

If we follow that logic here, we must first determine the maximum amount of green time that the side street approaches can be expected to require. The maximum green time must be at least long enough to accommodate pedestrian crossing time requirements, which we have previously determined to be 20 seconds. If approaching traffic volumes are very high on the side street, then the maximum green time may need to be even longer to assure vehicle needs are also being met.

Applying the HCM procedure under an initial presumption that pedestrian needs will control side street maximum green times (i.e., a maximum green+yellow+all-red time of 20 seconds in this case), we find that the v/c ratio for the most critical movement (WB through and right) is 71% for a 90-second cycle. Thus, vehicle needs are also being adequately met, so we can conclude that a timing apportionment of 70 seconds to north-south traffic and 20 seconds to east west traffic would be an appropriate estimate of the average green+yellow+all-red times for a coordinated traffic-actuated system. This apportionment would be implemented by coordination hardware that would impose a background cycle of 90 seconds. 

The presumption of a 70-sec/20-sec split of time is adequate for conducting an operational analysis. In reality, however, the east-west phase would be traffic-actuated with a maximum phase time somewhat longer than the 20 second pedestrian requirement to provide for the occasional cycles that inevitably occur with heavier-than-normal cross street demand.