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Research Reports |
| Title: | Safety Effects of Protected and Protected/Permissive Left-Turn Phases |
| Authors: | Juan C. Medina, M. Scott Shea, and Nuzhat Azra |
| University: | University of Utah |
| Publication Date: | Oct 2019 |
| Report #: | MPC-19-377 |
| Project #: | MPC-495 |
| TRID #: | 01699576 |
| Keywords: | crash analysis, crash rates, exclusive permissive phasing, flashing traffic signals, left turn phase, signalized intersections, traffic crashes, traffic safety, yellow interval (traffic signal cycle) |
| Type: | Research Report – MPC Publications |
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.
Medina, Juan C., M. Scott Shea, and Nuzhat Azra. Safety Effects of Protected and Protected/Permissive Left-Turn Phases, MPC-19-377. North Dakota State University - Upper Great Plains Transportation Institute, Fargo: Mountain-Plains Consortium, 2019.
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