Sub-problem 4d: Coordinated Operation With Left-Turn Protection

The left turning volumes are very light at this intersection, and all of the previous problems have assumed a simple two-phase operation. On the other hand, some agencies prefer to provide left turn protection even when left-turning volumes are low with the idea that the left-turn phases will be skipped if there is no demand during a given cycle.

We will therefore consider the addition of protected phases for the arterial (north-south) left turns, retaining a 90-second cycle. We will keep the same settings for the cross street through phases and add a phase for the northbound and southbound left turns with a 12-second maximum phase time consisting of 8 seconds green and 4 seconds yellow plus all red. The minimum green will be 8 seconds, so the full 12-second phase time will be displayed on cycles during which any left-turning vehicles arrive on the approach served by the left-turn phase.

In summary, a pretimed representation of the proposed signal timing would be 12 seconds for NB and SB left turns, 58 seconds for NB and SB through, and 20 seconds for all EB and WB movements. Since the left turns are very light, the phase will be skipped on some cycles and the unused time will be added to the through movements. The objective of this exercise is to determine how much green time on an average cycle will be added to the arterial through traffic phase. The amount of added green time must be based on the probability of one or more left-turn arrivals during any given cycle.