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Strategy B17. Use Indirect Left-Turn Treatments to Minimize Conflicts at Divided Highway IntersectionsNCHRP Report 500 / Volume 5: A Guide for Addressing Unsignalized Intersection Collisions WHERE TO USEUnsignalized intersections with operational and safety problems that can be traced to difficulties of accommodating left-turn demand.
DETAILSMany operational and safety problems at two-lane and divided highways can be traced to difficulties of accommodating left-turn demand. Such difficulties involve both demand volume and the frequency of demand along a corridor. Furthermore, vehicles that slow down or stop to turn left in a lane used primarily by through traffic increase the potential for rear-end crashes. One way to address the impacts of such left-turn movements is the use of indirect left-turn treatments. Indirect left-turn treatments include the use of jug-handle roadways before the crossroad, loop roadways beyond the crossroad, and directional median crossovers beyond the crossroad. Indirect left-turn treatments enable drivers to make left turns efficiently on divided highways, including highways with relatively narrow medians. KEY TO SUCCESSMake sure that this strategy is justified on the basis of high left-turn demand or an existing pattern of left-turn collisions. Also, involve the affected owners of adjacent property and residents in the decision-making process to develop and maintain support for the project. ISSUESDiverted traffic may contribute to safety problems at adjacent intersections or on alternative routes, resulting in no net benefit. Owners of properties where access may be reduced, especially those with commercial operations, may oppose this strategy. Thus, careful evaluation of the potential impacts of proposed improvements is needed to avoid or minimize such problems. A temporary hazard may exist during the transition period after the changed condition is opened to traffic. Advance notification of drivers is important, both in terms of notification prior to instituting the change and in signing that provides the appropriate notice of a change. TIME FRAME: MediumImplementation time can vary from 3 months to 4 years. At some locations, indirect left turns can be implemented simply by appropriate signing. Other locations may require major reconstruction. Such projects require a substantial time for development and construction. Where right-of-way is required or where the environmental process requires analysis and documentation, the time will be longer. COSTS: ModerateCosts are highly variable. Where an improvement can be implemented by signing an existing roadway, the costs are nominal. Where reconstruction is necessary, costs over $100,000 per intersection approach may be incurred. EFFECTIVENESSTRIED: It is expected that this strategy will reduce (1) rear-end crashes resulting from the conflict between vehicles waiting to turn left and following vehicles, and (2) right-angle crashes resulting from the conflict between vehicles turning left and oncoming through vehicles. One study concluded that installing indirect left-turn treatments may result in an 18-26% decrease in all crashes depending on the number of lanes. COMPATIBILITYThis strategy can be used in conjunction with other strategies for improving safety at unsignalized intersections. SUPPLEMENTAL INFORMATIONOptimal operation and safety of indirect left turns requires appropriate design and signing. For more details on this and other countermeasures: http://safety.transportation.org For more information contact: FHWA Office of Safety Design FHWA Resource Center – Safety and Design Team 
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Program ContactEd Rice Jeffrey Shaw What’s NewRoundabout Outreach and Education Toolbox Stop-Controlled Intersection Safety: Through Route Activated Warning Systems Roundabouts: An Informational Guide, Second Edition (NCHRP Report 672) Roundabouts Peer-to-Peer Assistance How to drive a roundabout (WSDOT) Modern Roundabouts: A Safer Choice FHWA's Intersection Resources Library CD-ROM HighlightsMini-Roundabouts Technical Summary Access Management in the Vicinity of Intersections Technical Summary Intersection Safety Case Studies Intersection Safety Technologies Presentation: Intersection Safety Example Intersection Safety Implementation Plan |
