United States Department of Transportation
i
Safety Effects of Protected and Protected/Permissive Left-Turn Phases [Research Brief]
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2019-10-01
[PDF-706.35 KB]
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Abstract:This study presents an approach-level evaluation of the safety performance of left-turn phases including permissive, protected-permissive, protected, and, in particular, flashing yellow arrow indications. Extensive field data collection and processing were used to create reliable approach level datasets. These efforts included manual identification of locations and extraction of exact dates of flashing yellow arrow (FYA) installations, extraction and processing of long-term high-resolution left-turn and opposing volumes, and manual verification of crashes to correct travel directions and crash assignments to specific approaches. Statistical models produced consistent trends among the three main groups being evaluated (permissive to FYA, permissive-protected to FYA, and protected to FYA) using an empirical Bayes (EB) before-after methodology. Safety performance functions were developed for permissive, permissive-protected, and FYA indications. Additional analysis included time-of-day, seasonal trends, and possible novelty effects soon after a FYA indication was installed. A risk metric using high-resolution data (5-minute counts) was also proposed to continuously monitor left-turn crash risks.
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