Evaluating the Effects of Freeway Design Elements on Safety

Increased emphasis has been placed on improving the explicit role of highway safety in making decisions on highway planning, design, and operations. This end can be achieved by quantifying the safety effects of geometric design elements for various highway facilities. The objective of this study was to investigate the safety effects of two important design elements for freeways: ramp density and horizontal curve. Data available for use in the evaluation included 324.2 centerline mi of freeways collected in Texas. Five years (1997–2001) of freeway crashes were examined. Negative binomial regression models were used to estimate the effects of independent variables on crashes. The final model for evaluation revealed that crashes on freeway segments were associated with average daily traffic, on-ramp density, degree of curvature, median width with inside shoulder, the number of lanes (for urban freeways), and whether the freeway is in an urban or rural area. Off-ramp density was not statistically significant in the model. Furthermore, the effect of on-ramp density on freeway crashes was significant for horizontal curves but not for tangent sections and indicates that freeway designers should eliminate or minimize the number of on-ramps within the horizontal curves. The statistical modeling results were geared into the development of accident modification factors for on-ramp density and horizontal curves that can be used for safety prediction of freeways.