Field Assessment of Variable Left-Turn Mode by Time-of-Day for Intersections Being Upgraded With Flashing Yellow Arrow Signal Heads and Offset Left-Turn Lanes

Abstract

The present study conducted a field assessment of variable left-turn (VLT) mode by time-of-day (TOD) plan for signalized intersections being upgraded with offset left-turn lanes and four-section vertical flashing yellow arrow (FYA) signal heads in Alabama. The VLT-TOD plan involved changing left-turn phasing from protected-permissive left-turn to permissive by TOD, based on left-turn and corresponding opposing traffic volume criteria at selected intersections. Consequently, the field assessment involved comparing surrogate safety and traffic operational measures for permissive left-turn and through traffic movements before and after the VLT-TOD implementation. Video and signal event data were collected in before and after conditions of the VLT-TOD implementation for the field assessment. The before–after comparison of safety and operational measures revealed that the VLT-TOD plan coupled with the FYA signal heads facilitated improved operational efficiency for the through traffic without compromising the safety and traffic operations of left-turning drivers at the study sites during the TOD hours. The results showed that surrogate measures, such as critical gap, postencroachment time, and follow-up time for permissive left-turns, remained unchanged before and after implementing the VLT-TOD plan, whereas a significant increase in the percent of green arrivals for through traffic was observed. The results are promising as they indicate that at signalized intersections with significant traffic volume fluctuation, the VLT by TOD plan utilizing the four-section FYA signal heads has the potential to significantly enhance overall traffic operations, particularly in situations in which drivers do not encounter sight-distance issues during the permissive left-turn phases.