In this exercise, we will convert the existing pretimed example from sub-problem 1b (see Exhibit 1-11) to a traffic actuated control to illustrate the differences between the timing and performance measures from the HCM computations. Using Appendix B, Chapter 16 of the HCM with the control parameters set as discussed above, we can estimate the average green times if the signal controller is actuated. Exhibit 1-38 shows the results of this estimation.
Note that the estimated average phase times for traffic-actuated control are shorter than pretimed control. Perhaps even more surprising is the fact that the estimated average phase time for the east-west movement is actually below the minimum phase time, which has been set to reflect the crossing time requirements of pedestrians. This does not reflect an unsafe situation because these are equivalent phase times for purposes of computing delay. They reflect the fact that, because of low vehicular volumes, the east-west phases will sometimes not be displayed. When the east-west phase is displayed, it will always be at least 20 seconds long (i.e., the minimum phase time). When it is not displayed, it will obviously be zero seconds long. In this particular case, the average of these various phase lengths is expected to be about 15 seconds under actuated control. The very short cycle (32 sec) is also an equivalent value, and should not be viewed in the pretimed sense.
Because the east-west phase is sometimes skipped, the average lost time associated with this phase will also be less, during the analysis time period, than would be the case if it were pretimed. Specifically, the average lost time for the east-west phase is equal to the average lost time that would occur if the phase were pretimed, multiplied by the proportion of cycles during the analysis period when the east-west phase is NOT skipped.
The green times shown in the table above for actuated control will now be used in the estimation of the control delay and level of service for the intersection.