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.1
Roundabouts need to accommodate bicyclists and pedestrians. It is important that motor vehicle traffic yields to pedestrians crossing at the roundabout. Splitter islands at the approaches slow vehicles and allow pedestrians to cross one traffic lane at a time. Single-lane approaches can be designed to keep speeds down to safer levels and allow pedestrians to cross. Multi-lane roundabouts tend to have higher motor vehicle speeds and create more conflicts between bicycles (and pedestrians) and motor vehicles.
Unless the road leading to a roundabout has two lanes, slow motor vehicle traffic speeds, and low traffic volumes, bicyclists may have difficulty navigating the roundabout. Marking bike lanes through the roundabout has not been shown to be safer and may actually be less safe. In high volume, mutli-lane roundabouts, an off-road shared path may be needed for bicyclists. Such a treatment delays and inconveniences bicyclists but may improve safety.
National Cooperative Highway Research Program Project 3–65, "Applying Roundabouts in the United States," is scheduled to be completed in 2006. The objectives of this project are to: (1) develop methods of estimating the safety and operational impacts of U.S. roundabouts, including a thorough examination of interactions between motor vehicles and pedestrians and bicyclists, and (2) refine the design criteria used for them.2
Purpose
- Provide good traffic management where the intersection is large and complex.
- Replace a traffic signal that is experiencing heavy traffic backup and congestion.
- Reduce speeds at intersection.
- Create a gateway into an area.
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Considerations
- Bike lanes should generally be discontinued when leading to low-speed roundabouts. Bicycles are expected to merge with the flow of traffic — a low design speed is required.
- Street widths and/or available right-of-way need to be sufficient to accommodate a properly designed roundabout.
- Roundabouts often work best where there is a high percentage of left-turning traffic.
- Deflection on each leg of the intersection must be set to control speeds to 24 to 29 km/h (15 to 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
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