View Related Treatments -------------------------- Sight Distance Improvements Turning Restrictions Intersection Markings Curb Radius Reduction Roundabouts Merge and Weave Area Redesign

 

Example of a bicyclist sharing the lane through a single-lane roundabout http://www.pedbikeimages.org/ - Carl Sundstrom

 

 

 

Roundabouts

A modern roundabout is built with a large, usually circular, raised island located at the intersection of two or more streets and may take the place of a signalized intersection. Traffic maneuvers around the circle in a counterclockwise direction, and then turns right onto the desired street. Entering traffic yields to traffic 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. If properly designed, roundabouts force slow intersection speeds and reduce the number of conflict areas.

Roundabouts need to accommodate bicyclists and pedestrians. A properly designed roundabout (through widths and deflection) will have operating speeds that are slow enough for a bicyclist to navigate the roundabout comfortably in mixed traffic. Thus, at roundabout approaches with bike lanes, the bike lane will end and the bicyclist will merge into traffic to navigate through the roundabout. Alternatively, for bicyclists uncomfortable at roundabouts, a bicycle ramp may be provided to the sidewalk such that they may dismount and continue through the intersection using the crosswalks as a pedestrian. However, such ramps may be confused with pedestrian ramps and should be reserved for locations where the higher speeds or traffic volumes may result in uncomfortable conditions for bicyclists. Bike lanes are not recommended in the circulatory roadway of the roundabout.

The interest in mini-roundabouts (single-lane roundabouts with an inscribed diameter between 50 to 90 ft) is also on the rise. This type of roundabout provides the safety and operational benefits of a roundabout with a smaller footprint and lower cost and can be a good solution on lower volume roadways.

Multilane roundabouts tend to have higher motor vehicle speeds due to their location on multilane roads and create more conflicts between bicycles (and pedestrians) and motor vehicles. Given these higher speeds and volumes, they can present a challenge to bicyclists and may not make the corridor safer for their use.

Purpose

Roundabouts are circular intersections designed to eliminate left turns by requiring traffic to exit to the right of the circle. Roundabouts are installed to reduce vehicular speeds; improve safety at intersections through eliminating angle collisions; help traffic flow more efficiently and reduce operational costs when converting from signalized intersections; and help create gateway treatments to signify the entrance of a special district or area.

Considerations

• When determining whether to install a roundabout, general considerations include pedestrian and bicycle volumes, effects on pedestrian route directness, the design vehicle, the number of travel lanes, and available rights-of-way.
• Bike lanes should be discontinued when leading to roundabouts, so bicycles are expected to merge with the flow of traffic.
• Street widths and/or available right-of-way need to be sufficient to accommodate a properly designed roundabout.
• Yield lines should be provided at all entries of the roundabout.
• Roundabouts often work best where the traffic flows are balanced on all approaches.
• Roundabouts are not recommended if they would increase difficulty for bicyclists navigating the intersection or vehicle delay.
• Intersections with more than four legs may be good candidates for conversion to roundabouts. An engineering study should be conducted in order to determine where a roundabout would be most appropriate, or if a traditional intersection would be more suitable for the location.
• Roundabouts are not meant for high-speed roadways. Generally, entry speeds on each leg of the intersection should be designed for about 15-18 mi/h.
• Roundabouts are typically not appropriate for the intersection of two multilane roads.
• Intersections near active, at-grade railroad crossings are generally poor candidates for roundabouts because traffic will be blocked in all directions when trains are present.
• On low speed and volume non-arterial streets, consider installing mini traffic circles, or smaller-scale roundabouts.

Estimated Cost

For neighborhood intersections a roundabout can be installed for approximately $25,000 to $100,000, with landscaped roundabouts raising the cost from $45,000 to $150,000. For arterial streets the cost is approximately $250,000, but can be more than $500,000 depending on the size, site conditions, and whether right-of-way acquisitions are needed. Two-lane roundabouts cost approximately $330,000. Roundabouts usually have lower ongoing maintenance costs than traffic signals, depending on whether the roundabout is landscaped.

Safety Effects

References

Case Studies