Safety and Operations Assessment of Various Left-Turn Phasing Strategies

Abstract

This research evaluated the safety and operational impacts of different left turn movement treatments at signalized intersections. The project (1) compared the safety and operational impacts of protected-only left-turn (POLT) phasing with those of protected-permissive left-turn (PPLT) phasing with a flashing yellow arrow (FYA) indication, (2) compared the safety and operational impacts of doghouse displays with those of four-section vertical displays for PPLT with an FYA, and (3) identified whether time-of-day (TOD) variable left-turn control mode with an FYA produces confusion among left-turning drivers. The literature showed that converting a POLT to a PPLT control mode is associated with an increase in crash rates while reducing intersection delay. Previous research has recommended selecting the control mode on the basis of traffic volumes, speed limit, sight distance, number of lanes, and crash history. Previous research has shown that doghouse displays are associated with larger crash modification factors, more confusion among drivers, and higher delays than a four-section vertical display with an FYA. The results of our driver comprehension survey showed that half of the drivers who had encountered intersections with a left-turn control mode variable by TOD felt confused by that phasing strategy. On the other hand, the simulation-based analysis showed that changing the left-turn control mode by TOD yields more efficient traffic operations and lower average delays.