Identification of the Impact of Radius Ratio of Horizontal to Vertical Curves on Highway Safety

This paper is focused on the identification of the impact of radius ratio of vertical to horizontal curves (R_V/H) on highway safety. Highway alignment facilities and statistics of traffic accidents of six highways from Washington State have been analyzed. Our research shows that accident rate decreases with the increasing of R_V/H as a whole and begins to keep a low level when the R_V/H large enough (more than 14). And for safety reasons, R_V/H should be at least larger than 10 in the segment of the coordination of the crest and horizontal curve. Our research results also show optimum R_V/H should be between 14 and 22 if radius of horizontal curves (RHC) are less than 3,000 ft, and the optimum R_V/H should be larger than 14. Research results reveal that R_V/H has important design guidance function. R_V/H can be selected as reasonable index in horizontal and vertical alignment coordination design.