Roundabouts

Roundabouts have been shown to improve vehicular safety at intersections.

A roundabout is a circular intersection. Traffic maneuvers around the circle in a counterclockwise direction, and then turns right onto the desired street. All traffic yields to motorists in the roundabout and left-turn movements are eliminated. Unlike a signalized intersection, vehicles generally flow and merge through the roundabout from each approaching street without having to stop.

Roundabouts reduce the number of potential conflict points, compared with traditional intersections. Experience has demonstrated that vehicular crashes are significantly reduced when low-speed, single lane roundabouts replace four-way intersections.

Proper accommodation of pedestrians and bicyclists in roundabouts represents an area of continuing research and development. Properly designed roundabouts include sufficient deflection to ensure low speeds, and splitter islands at the approaches slow vehicles and allow pedestrians to cross one direction of travel at a time. Multilane approaches create multiple threats for pedestrians and are not recommended.

Wayfinding and gap selection cues need to be adequately addressed in the design of roundabouts so that roundabouts are not a barrier to pedestrians with vision impairments. Research is currently underway through the National Cooperative Highway Research Program (NCHRP) to further explore the problem and develop potential solutions. Refer to NCHRP Project 3-78, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities for the latest status report.

Bicyclists also may be disadvantaged by roundabout design. Unless the road is narrow (one lane in each direction), speeds are slow, and traffic very light, bicyclists may not be able to share the road comfortably. Marking bicycle lanes through the roundabout has not been shown to be safer. In larger roundabouts, an off-road bicycle path may be necessary to allow cyclists to use the pedestrian route. This is inconvenient and takes longer but it will improve safety. Refer to the FHWA report Roundabouts: An Informational Guide for more information related to the design of facilities for both pedestrians and bicyclists.

Geometric design slows vehicles as they enter the roundabout.

Purpose

  • Improve safety at intersections, particularly those experiencing a large number of angle collisions
  • Convert signalized intersection to improve traffic flow efficiency
  • Reduce speeds at an intersection
  • Create a gateway into an area

Considerations

  • Street widths and/or available right-of-way need to be sufficient to accommodate a properly designed roundabout.
  • Roundabouts have a mixed record regarding pedestrian and bicyclist safety—a low design speed is required.
  • Roundabouts are generally not appropriate for the intersection of two multilane roads.
  • Roundabouts often work best where the traffic flows are balanced on all approaches.
  • Deflection on each leg of the intersection must be set to control speeds to 24-29 km/h (15-18 mi/h).

Estimated cost

The cost for a landscaped roundabout varies widely and can range from $45,000 to $150,000 for neighborhood intersections and up to $250,000 for arterial street intersections, not including additional right-of-way acquisition. Yet, roundabouts have lower ongoing maintenance costs than traffic signals.