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Evaluation Of The Highway Safety Manual

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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…show more content…
As a solution to this issue, research has been done on using random effect and random parameter (RPM) regression models, with random intercept terms, and varying parameter estimates for each observation respectively, but these methods are not without their own assumptions and prediction issues. Advanced safety performance functions in the form of random parameter models have tended to treat parameter heterogeneity as a representation of unobserved effects not accounted for in fixed parameter models (For example, Anastasopoulos 2009; Venkataraman et al 2011; 2013). Even after accounting for overdispersion of data, interactions due to segment-to-segment variations, driver behavior, environmental factors and roadway geometry can have a substantial effect on heterogeneity. Random parameter models are structured on the assumption that unobserved effects contributing to parameter heterogeneity are in the form of stochastic continuous representations across segments that can be drawn from known distributions. An alternate view might be that a significant portion of the unobserved effects and its variation across segments may be due to the lack of proper functional form. Therefore, an investigation of functional form classes is proposed to determine which roadway segments, or combinations have similar functional forms, and how many classes
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