Innovative Intersections and Interchanges
What Is A Roundabout?
- A circular, unsignalized intersection where all traffic moves counterclockwise around a central island
- Traffic entering the roundabout slows down and yields to traffic already inside the roundabout
- Roundabouts can be designed with one or more circulating lanes
- Design options allow for right turns to be channelized to bypass the circulating lanes
When Should It Be Considered?
- At intersections:
- With heavy left-turn traffic or with similar traffic volumes on each leg
- With crashes involving conflicting through and left-turn vehicles
- With limited room for storing vehicles
- Where there are limited nearby driveways
- Improved safety: Reduces the number of points where vehicles can cross paths and eliminates the potential for right-angle and head-on crashes
- Increased efficiency: Yield-controlled design means fewer stops, fewer delays and shorter queues
- Safer speeds: Promotes lower vehicle speeds, giving drivers more time to react
- Long-term cost effectiveness: No traffic signals means lower long-term costs for operations and maintenance
- Aesthetics: Allows for landscaping and beautification
How to Navigate
The number of conflict points (locations where vehicle travel paths intersect) is one metric that can be used to evaluate the safety of an innovative intersection or interchange.
There are three categories: crossing, merging or diverging.
In general, merging and diverging conflict points — where vehicles are moving in the same direction — are associated with less severe crash types than crossing conflict points where vehicles are moving in opposite directions.
The diagrams below compare possible vehicle travel movements and associated conflict points at conventional four-leg intersections to a roundabout.
These diagrams represent a general case, with one travel lane in each direction, and do not take into account pedestrian or bicycle movements at an intersection or interchange.
When compared to a conventional four-leg intersection, a roundabout has 16 fewer crossing, 4 fewer merging, and 4 fewer diverging conflict points.
Conventional Intersection: Conflict Points
Roundabout: Conflict Points
Design Considerations and Screening
For detailed roundabout design criteria, see the Virginia Department of Transportation (VDOT) Road Design Manual, Appendix F, Section 2 (Intersection design, spacing standards)
- Screening guidelines
- Intersection cost comparison spreadsheet user manual: Roundabout guidance (Version 2.5)
- Intersection cost comparison spreadsheet (Version 2.6)
- Roundabout design guidance (Version 1.1)
- National Cooperative Highway Research Program report 672: Roundabouts: An Informational Guide (Second edition)
- Policy and process scanning review technical memorandum