Identifying Intersection-Related Traffic Crashes for Accurate Safety Representation

Traffic collisions can occur within the limits of an intersection, but also on the approach to or exit from the due to activity or behavior related to the movement of traffic through the intersection. This study investigates the so-called safety influence area in order to achieve a more reliable way of identifying crashes and hence accurately estimating an intersection's safety. A nationwide survey was conducted to review how safety engineers currently identify crashes. Data for a sample of 177 four-legged signalized intersections were collected. The survey shows that many states apply a default distance as a radius to classify crashes that occur within this area as crashes. However, the values of the influence zones were mostly defined by convention or judgment and not based on a specific function of other characteristics. A decision tree method was used to identify the significant factors of the safety influence area. Results show that the inside area is mainly influenced by the intersecting roadways, but that the approach's upstream safety influence is mainly affected by the attributes of that approach. Since the factor affecting the inside area and the area are different, the findings suggest that it is better to define at intersection and intersection-related safety influence areas for each approach separately and then to determine the size of the safety influence areas based on the identified significant variables. Varied influence areas should be applied when analyzing heterogeneous groups of intersections.