As the number and prevalence of intersection forms and configurations increase, it has become more challenging for practitioners to quantify the safety effects of constructing these designs. Currently the Highway Safety Manual (HSM) provides safety performance functions (SPFs) for a few conventional intersections based on empirical research, but any deviation from the basic assumptions in the HSM requires the application of one or more crash modification factors (CMFs). Although there are a high number of CMFs from various sources, some apply to alternative intersection forms and fewer apply to complex traditional intersection that very from the basic intersections in the HSM. Intersection control evaluation (ICE) policies and efforts typically promote the consideration of alternative intersections not covered in the HSM, which has limited the ability of practitioners to compare these alternative intersections with conventional designs.
One measure of safety common to all at-grade intersections is the conflict point, or the point where two or more vehicle paths may cross. Conflicts may be classified as crossing, merging, or diverging based on the angle between vehicle paths. Previous research has demonstrated the relationship between the number of conflict points at an intersection and the number of resulting crashes over a given time-period. Similarly, crash severity can be roughly correlated to conflict type/angle/speed differential. Alternative intersections have been put forward as an improvement in safety over conventional forms due to a reduction in total conflict points, conversion of angle conflicts to merge/diverge conflict points and/or reducing speeds at conflict points.
The product of the research will be a series of crash prediction models with not only the typical rural, urban/suburban setting types, but conflict point frequency, type (merge, diverge, crossing), and speed differential as independent variables, applicable to all intersection forms, and implementable within the HSM Part C methodology.
The objective of the research is to identify and quantify the relationships between each basic conflict point and type of conflict within an intersection to the overall crash frequency of the intersection. The research will result in a methodology to develop new SPFs to supplement those in the HSM Part C models.