A large percentage of fatal and injury crashes on roadways occur at intersections. To aid in reducing these severe crashes traffic signals are often implemented. To truly understand whether the signal installations are helping to reduce severe crashes, there is a need to evaluate the effectiveness of the traffic signal improvements through the development of Crash Modification Factors (CMFs). Recent research has shown that traditional safety evaluation methods have been inadequate in developing CMFs. In recent years, Bayesian statistical methods have been utilized in traffic safety studies to more accurately analyze the effectiveness of safety improvements. The hierarchical Bayesian method is an advanced statistical technique that has the capability to account for the shortcomings of traditional methods and to more fully reflect the effectiveness of safety improvements. This report uses a hierarchical Bayesian model to analyze the effectiveness of new traffic signal installations and modifications to existing traffic signals from permissive only phasing to protected/permissive left turn phasing through the development of CMFs for multiple scenarios. A benefit-to-cost (B/C) analysis was also performed for each improvement to determine how long it would take to recover the cost of installation. The results showed that there was a slight increase in overall and non-severe crashes and a decrease in severe crashes for both new signal installations and modifications to existing signals. The B/C analysis indicated that there is a benefit to both improvements and that new signal installation costs can be recovered in approximately 5 years, while the installation of a left-turn signal modification can be recovered in approximately 9 weeks. .
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