Horizontal curves are relatively dangerous features, with collision rates at least 1.5 times that of comparable tangent sections on average. To help make these segments safer, this research developed consistent study methods with which field personnel could examine curves. This research included an examination of curve crash characteristics, the development of a manual field investigation procedure, the development of calibration factors for the relevant collision prediction equations in Highway Safety Manual, and GIS methods for finding key curve parameters. Based on this work, the research team recommends a systematic approach for identifying, investigating, analyzing, and evaluating horizontal curves. This systematic approach can lead to the selection and evaluation of promising curves, assessment of funding sources, and a recommendation of appropriate countermeasures. The approach should sometimes include a statistical analysis of horizontal curve collisions taking into account various road, crash, weather, and temporal attributes to help identify any unique circumstances that create an overrepresentation of certain types or characteristics of collisions. The research team found a calibration factor of 1.33 for the collision prediction equation for two-lane roads in the Highway Safety Manual. The research team also recommends several methodologies to identify and measure the key characteristics of horizontal curves using data already stored in GIS files.
