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Evaluating Safety Impacts of Access Management Alternatives with the Surrogate Safety Assessment Model

In a traditional safety impact analysis, it is necessary to have crash data on existing roadway conditions and a few years must pass before accumulating additional crash data to evaluate the safety impact of an improvement. This is a time-consuming approach and there remains uncertainty in the crash data integrity. The surrogate safety assessment model (SSAM) was developed for resolving these issues. With SSAM, a conflict analysis is performed in a simulated environment. A planned improvement alternative is modeled and no physical installation of the alternative is needed. This study evaluated if SSAM can be used to assess the safety of a highway segment in terms of the number and type of conflicts and to compare the safety effects of multiple access management alternatives. An evaluation of the effect of converting a two-way left-turn lane (TWLTL) into a raised median on a section of an urban street was performed using SSAM working on VISSIM simulation’s trajectory files. The analysis showed that a raised median would be much safer than a TWLTL median for the same level of traffic volume, with approximately 32 to 50 percent reduction in the number of crossing conflicts. The analysis showed that about 34,000 to 38,000 veh/day would be the demand level where the median conversion is recommended for the four-lane study section. The study concluded that the combination of a simulation software program with SSAM could be a viable surrogate analysis approach for evaluating and comparing the safety effects of multiple access management alternatives.