U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
NCHRP Report 500 / Volume 5: A Guide for Addressing Unsignalized Intersection Collisions
Unsignalized intersections with a high frequency of crashes resulting from insufficient intersection sight distance and awkward sight lines at a skewed intersection.
Photo by: FHWA
When roadways intersect at skewed angles, the intersections may experience one or more of the following problems:
Realignment of intersection approaches to reduce or eliminate intersection skew may be desirable to improve safety at a skewed intersection.
Identify candidate locations where there exist crash patterns related to the intersection angle. Any intersection with a pattern of right-angle or turning crashes should be checked to determine whether the skew angle of the intersection is contributing to these crashes.
When realigning a skewed intersection approach, it is possible to create such a sharp horizontal curve that the curve itself becomes a safety concern. Thus, the designer should be alert to avoid trading one safety concern for another. Realignment may negatively affect adjacent properties.
This strategy requires an implementation time of 1 to 4 years. At least 1 year is necessary to work out the details of intersection approach realignment and to communicate the plan to affected business owners and residents. Where relocation requires right-of-way acquisition and/or demolition of existing structures, an extensive project development process up to 4 years long may be required.
Reducing or eliminating the skew angle of an intersection involves the realignment of at least one intersection approach. The cost of this type of construction project is usually high. Furthermore, additional right-of-way will generally need to be acquired.
PROVEN: A recent study concluded from a review of the literature that realigning intersection approaches to reduce or eliminate intersection skew improves safety at unsignalized intersections. The study concluded the safety effectiveness of realignment to be as follows:
AMF = e0.0040xSKEW for three-legged intersections and AMF = e0.0054xSKEW for four-legged intersections
AMF = Accident modification factor
SKEW = Intersection skew angle (degrees), expressed as the absolute value of the difference between 90 degrees and the actual intersection angle.
Example: Three-leg intersection with a 15 degree skew on the approach.
AMF = e0.004x15 = 1.06 (6% more crashes than an approach with no skew)
Reducing or eliminating the skew angle of an intersection may be done in conjunction with most other strategies for improving safety. Indeed, in many cases, the purpose of realigning an intersection approach may be to make those other strategies feasible.
For more details on this and other countermeasures: http://safety.transportation.org
For more information contact:
FHWA Office of Safety Design
E71, 1200 New Jersey Avenue SE
Washington, D.C. 20590
FHWA Resource Center – Safety and Design Team
19900 Governor's Drive, Suite 301
Olympia Fields, IL 60461