United States Department of Transportation
i
Safety Effects of Protected and Protected/Permissive Left-Turn Phases [MPC 19-377]
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2019-10-01
[PDF-3.12 MB]
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Abstract:This study evaluated the approach-level safety performance of left-turn phases at signalized intersections, including permissive, protected-permissive (PPLT), protected indications, and flashing yellow arrow (FYA). Data collection included exact dates of traffic signal changes, high-resolution datasets of left-turn and opposing volumes, and manual verification of crashes to correct vehicle travel directions and approaches. Analysis showed similar performance of the FYA indication compared with a permissive phase, and higher crash rates with the FYA indication compared with a PPLT phase (an increase of 0.28 crashes per approach per year for an average approach, and a CMF of 1.33 ± 0.12). An important caveat to direct and strict comparisons between FYA and PPLT arise from differences in their operational capabilities, not included in this evaluation. As expected, crashes with protected phases did not show valid systematic trends since crashes were not a result of permissive movements, but due to traffic violations. A time-of-day analysis revealed higher-than-expected concentration of crashes before the afternoon peak (2 pm to 4 pm), pointing at opportunities to further reduce crashes. Lastly, a new metric to estimate risk of left-turn crashes using high-resolution data (5-minute counts) was introduced to monitor intra-day risk fluctuations in real time.
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