United States Department of Transportation
i
Safety Effects of Protected and Protected/Permissive Left-Turn Phases
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2018-12-01
[PDF-3.22 MB]
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Edition:Final, Apr 2015 to Dec 2018
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Abstract:This study evaluated the approach-level safety performance of left-turn phases including permissive, protected-permissive (PPLT), and protected indications, and the more recent flashing yellow arrow (FYA). Extensive data collection efforts included identification of suitable locations changing from a traditional to a FYA phase, exact date of such change, long-term high-resolution datasets of left-turn and opposing volumes, and manual verification of crashes to correct travel directions and crash assignments to specific approaches. Analysis in terms of yearly crash rates using SPFs and the empirical Bayes before-after method showed overall similar performance of the FYA indication compared to a permissive phase (the exact magnitude of this CMF was found dependent on the range of values in the sample) and higher crash rates with the FYA indication compared to a PPLT phase (0.28 crashes per approach per year for the average approach in terms of conflicting volumes, and a CMF of 1.33 ± 0.12). As expected, crashes for protected phases did not show valid systematic trends since crash events are not a result of permissive movements but rather due to traffic violations and unexpected events. A time-of-day analysis of crashes revealed higher-than-expected concentration of crashes in the hours preceding afternoon peaks (2pm-4pm), pointing at opportunities to reduce crashes during these periods perhaps by extending operational strategies from peak hours into the off-peak hours whenever possible. Furthermore, a new metric to estimate risk of left-turn crashes using high-resolution data (5-minute counts) is introduced to monitor in real time the intra-day risk fluctuations and help fine tuning signal timing and phasing.
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