Driver Understanding of Simultaneous Traffic Signal Indications in Protected Left Turns

Abstract

A comprehensive assessment of protected and permitted left-turn (PPLT) signal displays was performed considering safety, operational performance, and driver-understanding measures. The research focused on a study of driver understanding of protected left-turn indications. All currently used PPLT display arrangements and protected indication combinations were evaluated, including those with simultaneous green-arrow and red- or green-ball indications and those with the green-arrow indication only. Driver understanding was evaluated through a computer-based driver survey completed by 2,465 drivers. In total, 73,950 survey responses were received pertaining to the 200 different survey scenarios evaluated, 24,863 pertaining to protected left-turn indications. Findings show that the simultaneous illumination of the green-arrow and red-ball indications in a five-section PPLT signal display during a protected left-turn phase significantly reduces driver understanding and increases driver error. This finding is especially true for drivers over the age of 65. Simultaneous illumination of the green-arrow and green-ball indications also resulted in levels of driver understanding lower than the green-arrow-only indication; however, these differences were not statistically significant.