Michael Knodler

Publication Date



Left-turns are the most complex maneuvers in a signalized intersection. Based on the flow of the traffic volume through a given intersection, the left-turn phasing may be controlled in various fashions such as: protected only, permissive only, and protected/permissive. Following the introduction of the flashing yellow arrow (FYA) to the 2009 Edition of the Manual on Uniform Traffic Control Devices (MUTCD), many state agencies have implemented these indications for their left-turn permissive movements. Similar to the existing circular green (CG) permissive indication, the FYA requires drivers to yield to oncoming vehicles before making their left-turn. However, given the novelty of these traffic control devices there is a lack of standardization when it comes to the transition between protected and permissive left-turn phasing. A need exists to evaluate the surrogate safety of their implementation through a means of microsimulation evaluation. This research endeavor aims to model various protected-permissive left-turn (PPLT) phase sequences in the FHWA Surrogate Safety Assessment Model (SSAM). Both the FYA and circular green permissive left-turn indications will be implemented in VISSIM microsimulation models. Further, the phase sequencing for each permissive indication will comprise of two sequence options upon transitioning between protected and permissive left-turns; transitioning with and without the all-red clearance interval. Ultimately, this investigation will yield results to develop guidance for practitioners in designing the signal sequencing with PPLT phasing, particularly with the newly introduced FYA traffic control device.