Traffic conflicts associated with protected/permitted left-turn signal displays

Abstract

Several different protected/permitted left-turn (PPLT) signal displays are used in the United States, varying in configuration and permitted indication. Questions remain as to the safety implications of using each type of display. Since left-turn crash data do not contain information related to the type of signal display and permitted indication at the intersection, conflict studies are often used as a surrogate measure. This paper describes a study of 24 intersections in eight U.S. cities to evaluate traffic conflicts and events (safety implications) associated with selected PPLT signal displays. Based on the results of this study, there was no difference in the conflict rates associated with the PPLT signal displays evaluated. Conflict rates varied from 0.0 to 1.4 conflicts/1,000 entering vehicles. Conflicts attributed to driver misunderstanding of the signal display or indication were primarily associated with the green ball permitted indication. Most left-turn events were related to hesitation at the onset of the protected green arrow indication. Evaluating each event type individually suggested that the five-section horizontal display with both the green arrow and red ball illuminated was associated with a significantly higher rate of Type 1 (hesitate on green arrow indication) events. Currently, the Manual on Uniform Traffic Control Devices (MUTCD) requires simultaneous illumination of the green arrow and adjacent through movement indication in the PPLT signal display during the protected left-turn phase. This result demonstrated the increase in signal display complexity and driver workload with the simultaneous illumination of the green arrow and red ball indications.