Mini Traffic Circles
Mini traffic circles are raised circular islands constructed in the center of residential or local street intersections. Mini cricles are a traffic calming intersection treatment employing yield control. They may also be used at uncontrolled junctions. Signs should be installed directing motorists to proceed to the right around the circle before turning right, passing through or making a left turn. Entering traffic yields to traffic in the circle and both entering and exiting vehicles should yield to pedestrians crossing the legs of the approaches to the intersection. Mini circles are commonly landscaped (often with a center tree and low-growing shrubs, flowers, or grasses). In some communities, the city may require the neighborhood to maintain the plantings. In locations where landscaping is infeasible, traffic circles can be made more aesthetically pleasing by using special paving materials.
Generally, mini circles are not intended for use where one or both streets
are arterial streets (see section on Roundabouts). The primary
benefit to bicyclists is that, like roundabouts, mini circles slow traffic
approaching the junctions by forcing motorists to maneuver counterclockwise
around them. Mini circles also reduce the number of conflict points at
intersections. Mini circles have been found to reduce motor vehicle crashes
at the involved intersections by 90 percent or more in Seattle, WA. Mini
circles may provide one of the largest safety benefits of all the traffic
calming devices. Most impact studies suggest they have a nominal impact
on traffic volumes, so the reduction in crashes is apparently not due to
diverting traffic to other streets.2
Mini circles must be properly designed with enough deflection to slow vehicles to provide safety benefits to bicyclists, pedestrians and motorists. Pedestrians with vision impairments will, however, find fewer cues to identify a gap to cross when traffic does not stop. Additionally, right-turning vehicles are not (stop) controlled at intersections with mini circles, potentially putting pedestrians at risk. Therefore, narrow curve radii should complement this treatment to discourage fast right-turn maneuvers. Adding splitter islands with pedestrian cuts to the legs of the intersection makes crossing easier for pedestrians, especially wheelchair users. Splitter islands also direct vehicles entering the intersection but require additional space.
The occasional larger vehicle going through an intersection with a traffic circle (e.g., a fire truck or moving van) can be accommodated by allowing these vehicles to make left turns in front of the circle or by creating a mountable curb in the outer portion of the circle. Other possible solutions are discussed in Traffic Calming: State of the Practice, chapter 7.2
- Manage traffic at intersections where volumes do not warrant a stop sign or a signal.
- Reduce crash problems at the intersection of two local streets.
- Reduce vehicle speeds at the intersection.
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- Mini circles are typically not used on arterial streets.
- Consider whether bicyclists may be "squeezed" in traffic circles by overtaking motor vehicles.1 This type of problem is not likely on low-volume streets, but should be considered where vehicle and bicycle volumes are higher.
- Keep the turning radii low to reduce turning speeds and improve pedestrian and bicyclist safety.
- Larger vehicles that need access to streets (e.g., school buses and fire engines) may need to make left turns in front of the circle, or accommodation may be made with mountable curbs on the perimeter of the circle.
- Use yield, not stop, controls.
- Midblock speeds may not decline, or may even rise, if intersections and mini circles are widely spaced and no midblock traffic calming measures are introduced. Traffic circles are primarily used to manage traffic flow at intersections and reduce intersection speeds, but may be combined with other measures or frequent mini circles to achieve street-long traffic calming.
- Pedestrians with vision impairments will find fewer cues to identify a gap to cross when traffic does not stop.
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The cost is approximately $6,000 for a landscaped traffic mini circle on an asphalt street and about $8,000 to $12,000 for a landscaped mini circle on a concrete street (using existing curb radii).
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