Safety Data Analysis to Evaluate Highway Alignment Consistency

Road safety has become a priority field worldwide and one of the major factors describing the state of the transport system with its positive and negative changes. Many studies on driver speed behavior can be found in the scientific literature, and researchers have addressed roadway alignment consistency for travel safety in the context of real operating speeds. This study illustrates an experimental analysis conducted on the Tirrenia Inferiore State Highway in southern Italy without spiral transition curves between geometric tangent and circular elements on the horizontal alignment in order to check a new prediction consistency model. Two consistency measures were developed and compared with the results available in the literature: the first was the relative area bounded by the speed profile and the average weighted speed, and the second was the standard deviation of operating speeds in each design element along the entire road examined. With a combination of these two previous measures and according to an extensive sensitivity analysis, a consistency model was developed and thresholds for good, acceptable, and poor road consistency can now be proposed. The consistency prediction model was related to the number of crashes occurring between 2003 and 2010. It was found that as design consistency increased, the number of crashes decreased significantly. The consistency model can be used for this purpose during the geometric design process or during the evaluation process for two-lane rural highways.