U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
NCHRP Report 500 / Volume 12: A Guide for Reducing Collisions at Signalized Intersections
Signalized intersections with high frequencies of crashes that are not reduced through other lower-cost solutions.
Photo by: FHWA
Signalized intersections may have such a significant crash problem that the only alternative is to change the nature of the intersection itself. Thus, low-cost, short-term solutions will often not be available. Implementing these strategies will necessitate significant public involvement and stakeholder activity.
Safety problems associated with left turns at signalized intersections are magnified at intersections with high volumes of left turns. Indirect left-turn treatments, such as jughandles before the crossroad, directional median crossovers, and loop roadways beyond the crossroad, can address both safety and operational problems related to left turns. These treatments remove the left-turning vehicles from the traffic stream without causing slow down or stoppage in a through-traffic lane. Right-angle crashes are also likely to decrease after implementation. Alternative left-turn designs are discussed in various publications and included in the Federal Highway Administration (FHWA) Signalized Intersection Guide.
A roundabout can have a better crash experience than a conventional signalized intersection (for low- to medium-volume roads). Consult the FHWA's Roundabouts: An Informational Guide for the current state of the practice on the design, operation, and safety of roundabouts. Refer to Unsignalized Strategy F3 for more detailed information.
When two-way streets are converted to one-way streets (typically in a central business district environment), it is generally to increase capacity, but removing opposing traffic flows can improve safety as well. Removing one direction of traffic from a two-way street allows better signal synchronization and progression of platoons. Smooth progression and reduced congestion can reduce rear-end crashes. Removing one direction of traffic can improve safety by:
Safety-related drawbacks to conversion to one-way streets may include the following:
Consider providing a grade separation or interchange for signalized intersection locations with extremely high volumes, extremely poor crash histories, or other mitigating factor(s). Other solutions may include quadrant design, superstreet, and diverging diamond designs, and continuous flow intersections.
Major construction projects have a greater chance of success when all key stakeholders-including owners of nearby businesses, transit agencies, neighborhood/resident groups, and other primary users of the intersection—get involved.
In general, the time frame for most projects of this magnitude is lengthy.
Costs will generally be high when constructing special solutions.
TRIED: It is expected that these strategies will reduce both rear-end collisions resulting from the conflict between vehicles waiting to turn left and following vehicles and right-angle collisions resulting from the conflict between vehicles turning left and oncoming through vehicles.
With major changes to an intersection, other solutions are not likely to be appropriate and not necessarily compatible.
Refer to the FHWA Signalized Intersections: Informational Guide for more information on this strategy.
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