Serving protected left turn phases for one or two vehicles can often be an inefficient use of cycle green time when the opposing through movements are over capacity. This paper assesses the performance of an intersection based on the application of controller logic that delays the call for a protected left turn phase based on vehicle wait times. Four weeks of evaluation were carried out where the delay on left turn phase calls was varied in 25 second increments, from 0 to 75 seconds. The results indicate that delaying left turn phase initiation substantially increases the amount of green time for saturated through movements, while minimally increasing the travel delay for left-turning drivers. The paper concludes by recommending agencies consider using a delay in the range of 25 to 50 seconds for calling protected phases at intersections where the opposing through movement is oversaturated and could benefit from additional green time. This research presents one of the first quantitative studies in evaluating the potential intersection capacity and performance improvements with respect to left turn detector delay.
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