Drivers’ Understanding of Protected-Permitted Left-Turn Signal Displays

Abstract

A comprehensive assessment of protected–permitted left-turn (PPLT) signal displays was performed considering safety, operational performance, and driver understanding measures. The research focuses on a study of driver understanding of permitted left-turn indications. All currently used PPLT display arrangements and permitted indication combinations were evaluated, including those with flashing red and yellow permitted indications. Driver understanding was evaluated through a computer-based driver survey completed by 2,465 drivers. A total of 73,950 survey responses were received pertaining to the 200 different survey scenarios evaluated. The study results indicate that yellow or red flashing permitted indications may lead to higher levels of driver comprehension. Both the flashing red and yellow permitted indications had a significantly higher correct response rate than did the green ball permitted indication. Drivers over the age of 65 found the flashing ball permitted indications easier to comprehend and responded more quickly with fewer fail critical (turning left without the right-of-way) errors. Higher correct response rates with flashing permitted indications were also found in other important demographic groups, including inexperienced drivers and drivers with limited education.

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