Is Horizontal Curvature a Significant Factor of Safety in Rural Multilane Highways?

The Highway Safety Manual (HSM) crash prediction models estimate the expected number of crashes for different facility types. Models in Part C of the first edition of the HSM include crash prediction models for divided and undivided rural multilane highway segments. Each of the HSM crash prediction models for highway segments is composed of a safety performance function (SPF) that is a function of annual average daily traffic and segment length plus a series of crash modification factors (CMFs). The SPF estimates the number of crashes for a site if the site features are of base condition. The effects of other features of the site, if their values are different from the base condition, are carried out through use of CMFs. Existing models for rural multilane segments do not have any CMFs for horizontal curvature. The goal of this research was to investigate whether horizontal alignment has any significant effect on crashes on these types of facilities and if so, to develop a CMF for this feature. Washington State cross-sectional data from the Highway Safety Information System Laboratory were used in this research. Data from 2007 to 2009 were used to conduct the investigation and 2010 and 2011 data were used to validate the results. As the results showed, horizontal curvature has a significant safety effect on rural multilane highways, and using a CMF for horizontal curvature improves the prediction of crashes significantly for tangent and curve segments.