A safety performance function is a mathematical relationship between the frequency of crashes by severity and causal factors on specific roads.  For example, crashes for a given time period can be estimated based on vehicle exposure.  For a highway segment, exposure can be measured by the number of vehicles exposed and the distance for which they are exposed. 

By comparing actual crashes on a segment of road to that obtained from the safety performance function, the potential for safety improvement can be determined. 

The purpose of this study is to develop a set of safety performance functions for various conditions on two-lane highway segments and intersections that can be used to prioritize of the need for safety improvement projects in Virginia.