Roundabouts:

A roundabout is a circular intersection that eliminates some of the conflict traffic, such as left turns, that causes crashes at traditional intersections. 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 need to accommodate pedestrians and bicyclists. It is important that automobile traffic yields to pedestrians crossing the roundabout. Splitter islands at the approaches slow vehicles and allow pedestrians to cross one direction of travel at a time. Single-lane approaches can be designed to keep speeds down to safer levels and allow pedestrians to cross. Multilane approaches can 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. One possible solution is the use of accessible pedestrian signals placed on sidewalks and splitter islands to indicate both where to cross and when to cross. More 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 (at www4.trb.org/trb/crp.nsf/NCHRP+projects) 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 (online at http://www.tfhrc.gov/safety/00068.htm) for more information related to the design of facilities for both pedestrians and bicyclists.1



  Purpose
• Improve safety at intersections experiencing a large number of angle collisions.
• Convert signalized intersection to improve traffic flow efficiency.
• Reduce speeds at intersection.
• Create a gateway into an area.
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  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).
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  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.
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  Case Studies
Fort Pierce, FL 
Naples, FL 
Montpelier, VT 
Green Bay, WI, Metropolitan Area 
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Sketch by Michael Kimelberg


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Portland Office of Transportation
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U.S. Department of Transportation Federal Highway Administration