Evaluation Of The Highway Safety Manual

1606 Words Mar 29th, 2015 7 Pages
The American Association of State Highway and Transportation Officials (AASHTO) defines safety performance functions (SPFs) as statistical models used to estimate the average accident frequency for a specific site type with specific base conditions, based on traffic volume, roadway segment length, and other site characteristics such as lane width, shoulder width, and radius and degree of horizontal curvature. In essence, SPFs are mathematical equations developed through statistical regression modeling of historical data, and are used to predict accident occurrence at sites comparable to those where the historical data was obtained.
The Highway Safety Manual (HSM) published by AASHTO provides guidance on how to utilize SPFs to make better safety evaluations of sites in three ways. The first application is towards the screening of a network to identify sections that may have the best potential for improvements. The second application is to use SPFs to determine safety impacts of design changes at a project level, while the third suggestion is to use SPFs to evaluate the safety effects of engineering treatments in the form of before-after studies. Given the nature of the applications of SPFs, a significant level of accuracy in predictions is desirable. The predominant form of SPFs in use, utilize log-normal forms of count regressions with parameters fixed across all observations. These are not always the best models to use for predictions as they ignore the effects of…
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