Quantifying and Comparing Left-Turn Strategy Travel Time and Queuing Performance

Intersection performance is often impacted and even limited by the need to serve left turn movements. Left turns may be served in a number of ways, varying from permitted left turns to flashing yellow left turn arrow to protected left turns. Each method may be employed in a number of different strategies. For example, protected left turns may be employed in leading left turns, leading and lagging left turns, conditional left turn phase reservice, and fixed left turn phase reservice. To give practitioners and researchers more insight into left turn operations, this research implements a number of different left turn strategies under varying volume, volume differential and turning conditions while measuring the delay and queue lengths associated with each left turn strategy. The following findings are discussed in this paper. First, at low volumes, permitted only phasing offers the best performance. Second, protective-permitted phasing, including leading left turn with green ball, Leading left turn with Flashing Yellow Arrow (FYA), and FYA followed by lagging lefts, performs better as volume differentials increase as well as when turning rates increase. Phase reservice on left turn phases has a definite impact on queue length and delay experienced by the left turn phase, though it is also likely to be accompanied by a decrease in performance elsewhere in the intersection.