Integrating Left-Turn Lane Geometric Design with Signal Timing

Abstract

The available design guidelines indicate a lack of proper methodologies for determining the length of left-turn lanes at signalized intersections. This is largely attributable to the alternating nature of stopped and unstopped approaches as a result of cyclic signal phases. The existing design guidelines recommend the design length of left-turn lanes as a sum of two components: queue storage length and deceleration length. This is appropriate for unstopped approaches at unsignalized intersections, but it is inapplicable to signalized intersections, in which the approaches switch between stopped and unstopped by phase. This paper introduces a new methodology that coordinates the requirement of each component with signal timing for the proper design of left-turn lanes. With the underlying assumption of Poisson arrivals, the model developed is limited to uncoordinated intersections or approaches. Compared to the existing guidelines, the proposed methodology provides a rational approach for determining left-turn lane lengths and generally results in shorter left-turn lanes that necessarily meet both safety and operational needs.