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 the conflict between (1) vehicles turning left and following vehicles and (2) vehicles turning left and opposing through vehicles.
Photo by: FHWA
Many crashes at unsignalized intersections are related to left-turn maneuvers. Left-turn lanes remove vehicles waiting to turn left from the through-traffic stream, thus reducing the potential for rear-end crashes. Because they provide a sheltered location for drivers to wait for a gap in opposing traffic, left-turn lanes may encourage drivers to be more selective in choosing a gap to complete the left turn maneuver. This may reduce the potential for crashes between left-turn and opposing through vehicles.
Make sure that any left-turn lane considered is operationally warranted based on traffic volumes or justified on the basis of an existing pattern of left-turn collisions.
Sight Distance. In providing left-turn lanes, vehicles in opposing left-turn lanes may block their respective driver's view of approaching vehicles in the through lanes. This potential problem can be resolved by offsetting the left-turn lanes (see Strategy B3 fact sheet).
Roadway Design. If the shoulder and through lanes are restriped to make provision for a left-turn lane, part of the safety benefits may be lost due to the loss of shoulder, the greater proximity of traffic to roadside objects, and, possibly, a reduction in intersection sight distance.
Turn Restrictions. Provision of a left-turn lane on an intersection approach may involve restricting left turns in and out of driveways on that intersection approach. Such restrictions may be implemented by signing or provision of a median adjacent to the left-turn lane.
Pedestrians. When installation of left-turn lanes increases the overall width of the intersection, the additional width may cause problems for pedestrians crossing the intersection. One possible solution to this problem is to provide a pedestrian refuge island in the median.
Implementation time may vary from 3 months to 4 years. At some locations, left-turn lanes can be quickly installed simply by restriping the roadway. At other locations, widening the roadway, installing a median, or acquiring additional right-of-way may be needed. Where right-of-way is required or where the environmental process requires analysis and documentation, project development and implementation may require as long as 4 years.
Costs are highly variable. Where restriping within an existing roadway is possible, the costs are nominal. Where widening and/or reconstruction are necessary, costs will be significantly higher.
PROVEN: Research has determined that installation of a single left-turn lane on a major road approach would be expected to reduce total intersection crashes at rural unsignalized intersections by 28% for four-legged intersections and by 44 % for three-legged intersections. At urban unsignalized intersections, installation of a left-turn lane on one approach would be expected to reduce total crashes by 27 % for four-legged intersections and by 33 % for three-legged intersections.
Installation of left-turn lanes on both major road approaches to a four-legged intersection would be expected to increase, but not quite double, the resulting effectiveness measures for total intersection crashes.
This strategy can be used in conjunction with most other strategies for improving safety at unsignalized intersections.
Optimal operation and safety of left-turn lanes require appropriate design. This includes sufficient length of lane and taper (see Strategy B2 fact sheet).
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