Evaluating Safety Effects of Ramp Density and Horizontal Curve for Freeways Using Texas Data

Increased emphasis has been placed on improving the explicit role of highway safety in making decisions on highway planning, design, and operations. This can be achieved by quantifying the safety effects of geometric design elements for various highway facilities. While great attention has been directed primarily on rural two-lane highways and to a lesser degree, on urban arterials and rural multilane highways, freeway is a highway facility where additional efforts are needed. Despite one of the most critical design elements for freeways, the safety effect of on-ramps or off-ramps has yet to be quantified through research so far. The crash risk is likely to increase when they are combined with sharp horizontal curves. In this context, this study investigated the effect of ramp density on freeway segment crashes both in urban and rural areas, and whether that effect is different for tangent segments than horizontal curves. In addition, the effect of horizontal curves on freeway crashes was also identified using the degree-of-curvature variable. Data available for use in the evaluation included 324.3 centerline miles of freeways in Texas. Negative binomial regression models were used to estimate the effects of independent variables on crashes. The statistical modeling results were geared into the development of the corresponding accident modification factors (AMFs) that can be used for safety prediction for freeways. Fitzpatrick, Lord & Park 3