Strategy B15. Convert Offset T-Intersections to Four-Legged Intersections
NCHRP Report 500 / Volume 5: A Guide for Addressing Unsignalized Intersection Collisions
WHERE TO USE
Unsignalized offset T-intersections where through volumes on the cross street are very high.
For some unsignalized offset T-intersections with very high through volumes on the cross street, the best method of improving safety may be to convert the intersection to a single four-legged intersection. This conversion to a four-legged intersection can be accomplished by realigning the two cross-street approaches to meet at a single point along the major road, thus creating one four-legged intersection.
KEY TO SUCCESS
Depends upon the through volume of the cross street. If through volumes are low, the intersection may be safer if left as two offset T-intersections. Two offset T-intersections with low cross-street through volumes are generally safer than a four-legged intersection.
There should be no potential difficulties with this strategy, as long as the resulting four-legged intersection is properly designed and traffic control devices are properly used.
TIME FRAME: Medium
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 relocation and to communicate the plan to affected businesses 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.
Converting two offset T-intersections to a conventional four-legged 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.
TRIED: It is expected that this strategy would reduce crashes involving left-turning traffic from the major road onto the cross street at each of the two T-intersections. It can reduce or eliminate safety problems associated with insufficient spacing between existing offset T-intersections.
The conversion of two offset T-intersections to a conventional four-legged intersection may be used in conjunction with most other strategies for improving safety. In many cases, the purpose of relocating 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
FHWA Resource Center – Safety and Design Team
How to drive a roundabout (WSDOT)