Development and Calibration of Safety Performance Functions for Intersections on Rural Divided Highways in Alabama
Type of DegreeMaster's Thesis
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Although median separated highways provide apparent benefits over undivided highways, sometimes median openings on rural divided highways (RDHs) provide some of the greatest opportunities for frequent and severe crashes on the highway system. Minor road drivers may fail to select a safe gap when they cross or turn left on to the farside of the intersection. Right-angle crashes are common at a two-way Stop-control intersection (TWSC) and the most problematic of these crashes tend to occur at the farside of the intersection. At-grade intersections with wide medians in rural settings have the potential for severe crashes due to numerous conflict points and high speeds and are therefore worthy of evaluation of the safety performance of these intersections. The locations that are the focus of the study are the unsignalized intersections on rural divided highways with medians wider than 30 feet. Crash prediction models in the Highway Safety Manual (HSM), published by AASHTO in 2010, are statistical tools that can be used to predict the number of crashes and evaluate road safety. This thesis supports the safety analyses by developing a local calibration factor for the HSM-provided safety performance function (SPF) for 3-legged (3ST) and 4-legged (4ST) stop-controlled intersections on rural divided highways in Alabama. This study also calibrated state-specific (SPFs) for unsignalized intersections with wider medians greater than 30 feet on RDHs in Alabama. It also documents the selection of appropriate crash modification factors for a specific countermeasure deployed at a treated location. The calibration factors obtained from the analysis for 3ST and 4ST intersections are 0.61 and 0.57 respectively which implies that the HSM crash prediction methodology over-predicts the crashes at the intersections on RDHs in Alabama. The results of the analysis support the safety analyses needed for the larger research project in planning, design, operations, and maintenance.