All-Red Clearance Intervals and Left Turn Lanes

Funding Source: Safer Sim

Title: A Field and Simulator Evaluation of All-Red Clearance Intervals for use in Left Turn Applications


Status: Current

Summary: Permissive left-turn and right-turn intervals have historically been communicated to drivers using nearly every traffic signal indication available, including the circular green (CG), flashing circular red (FCR), flashing red arrow (FRA), flashing circular yellow (FCY), and flashing yellow arrow (FYA) indications. National Cooperative Highway Research Program (NCHRP) and other research demonstrated that, by most measures, the FYA indication is the most effective for communicating permissive left-turns (1, 2). Therefore, the FYA indication was included in the 2009 edition of the Manual on Uniform Traffic Control Devices (MUTCD) (3). Since that time, most agencies have embraced use of the FYA and, to some extent, flashing red arrow (FRA) indications. The permissive left turn movement communicated by the FYA indication requires drivers to yield to oncoming traffic, while the FRA indication requires them to stop before proceeding to make the left-turn. How traffic signals transition from a protected left-turn indication to the FYA and FRA is not well defined and varies across the country. Specifically, there is no clear national guidance on the need for and the extent of clearance and change intervals when transitioning from a protected left-turn phase to a permissive left-turn [FYA/FRA] phase. Furthermore, there is a specific need for guidance on the duration of any associated change/clearance intervals given their impact on safety. The primary objective of this research is to develop recommendations for the use and duration of solid yellow arrow change and solid red arrow clearance intervals after a leading left-turn solid green arrow transitioning to a permissive left-turn FYA or FRA. The evaluation will be based on safety (human factors) and operational efficiency considerations. The recommendations developed in this research will help agencies choose the most appropriate transition from a protected left-turn phase to a permissive left-turn phase.

Team: Michael Knodler, Eleni Christofa

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