Sub-problem 3a: Actuated Signal Detection and Signal Timing

Let's discuss each of the questions posed on the last page.

Why would a corridor with progression need to maintain a fixed cycle length? A fixed cycle length is frequently used to maintain coordination between adjacent intersections where the traffic engineer seeks to preserve the quality of progression. The arrival type is the primary input to the methodology that affects this calculation.

A cycle length is imposed upon an intersection by assigning a coordinated movement or group of movements to the intersection. This is typically the main street through phase. Once this is established, other phases at the intersection are actuated, meaning that the time allotted to each of these phases is dependent upon the signal timing parameters and demand, or as the HCM states: these phases "can be extended by reassigning unused green time from actuated phases with low demand." In our case, Museum Road is the coordinated movement and Reitz Union is actuated. Thus, if Reitz Union does not have any demand during a cycle, Museum Road will stay green throughout the cycle. Essentially, whatever time is unused on Reitz Union is received by Museum Road because the traffic controller will maintain progression by giving remaining green to the coordinated phase.

What is the difference between looking at an average hourly cycle versus individual cycles?  The difference between looking at average phase lengths and phase lengths throughout the hour depends on the variability of traffic volumes. Volumes vary cycle by cycle throughout the hour. Exhibit 5-X shows the phase length versus the number of occurrences.

What should be noted from this figure is that throughout the hour the phase length varies as arrivals vary.  The Museum Road phase length would show an inverse relationship due to the effects of coordination within the controller, since it receives the additional time. In other words, in the cases where the phase length is 45 seconds for Reitz Union (40 seconds of green time), Museum Road is 70 seconds of green, and when Reitz Union is 20 seconds long, Museum Road is 90 seconds.

Before we can answer this, we will take a closer look at the signal timing parameters.