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Sub-problem 4a - Page 3 of 3

ID# C104A03

Sub-problem 4a: Pretimed Control vs. Actuated Control

Exhibit 1-39 summarizes the level of service and delay at each approach for a pretimed and actuated control.

Exhibit 1-39. Comparison of Pretimed and Traffic-Actuated Operation (Datasets 2, 18)







Pretimed delay (sec/veh) 20.3 22.0 6.1 6.5 10.9
Actuated delay (sec/veh) 7.9 8.3 7.4 7.9 7.8
Pretimed LOS C C A A B
Actuated LOS A A A A A

Note that traffic-actuated control has produced a substantial reduction in delay for east-west traffic accompanied by a very slight increase for north-south traffic. Note also that the delays are now more or less equal on all approaches, whereas the distribution of times under the pretimed design favored the north-south (arterial) movements at the expense of the east-west (cross street) movements. In apportioning the times, the controller has clearly ignored the much longer maximum green times for the north-south phase. In other words, the north-south phase would never reach the maximum time, because the unit extension settings would cause the phase to terminate whenever the queue of vehicles on the approach has been serviced.

While the timing plan presented above produces lower delays than the previous pretimed plan, the local agency may still wish to favor the north/south movements because of the heavier movement on U.S. 95. The results of this exercise suggest that increasing the maximum green time would not accomplish this goal. The maximum green setting will have a definite effect on the green time distribution under high traffic volumes; but when demand is low, the minimum green time is the only parameter that will force a green time distribution that is not in keeping with the distribution of demand.

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