Safety of Intersections on High-Speed Road Segments with Superelevation

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Indiana Department of Transportation or the Federal Highway Administration at the time of publication. The report does not constitute a standard, specification, or regulation. Introduction In recent years, the State of Indiana has built a number of intersections where one or both of the roads are located on curves. The AASHTO Policy on Geometric Design of Highways and Streets recommends that in such cases " the alignment should be as straight and the gradient as flat as practical. " This wording, consistent with Part V of the Indiana Design Manual, allows for the design of intersections on curves if other solutions prove to be too expensive. Several of these intersections have raised safety concerns and led to expensive corrective measures. Due to these safety and economic issues, INDOT currently avoids designing intersections on segments with steep superelevation. The focus of this research is to determine what effect curvature and superelevation have on intersection safety. Based on the results, the goal is to provide guidelines for improvement of existing intersections and design of new intersections where the major road is a superelevated curve. Findings The safety analysis of intersections where both routes are two-lane roads did not show curvature to have a significant impact on safety in terms of crash frequency or severity. However, this result is unclear and may be partly due to randomness as the sample was relatively small. Curvature does appear to be a significant factor in the case where the major road is a four-lane divided highway. Full curvature and superelevation was found to increase crashes by 300% in comparison to tangent intersections. Through consultation with INDOT, these results were used to propose maximum recommended and allowable design values for superelevation and curve radius. The four-lane case provided additional insight into driver behavior at intersection on curves. Crashes tended to be overrepresented at the sample intersections during nighttime conditions, indicating lighting should be a primary concern at such intersections. During adverse weather conditions, crashes in the sample were underrepresented for the intersections on curves. It is possible that drivers travel more cautiously during severe weather because they perceive a greater risk. A number of findings from this study are …