Neighborhood Traffic Operations: Traffic Circle Program
Traffic circles are effective at reducing collisions and the severity at intersections, as well as speeds. Especially when installed in a series, traffic circles also provide an overall traffic calming effect along the entire street corridor. Over the last 30 years, the City of Seattle has installed over 1,000 traffic circles on city streets.
Due to high demand and limited funding, we have developed a process for prioritizing traffic circles. Because the primary purpose of a traffic circle is to reduce collisions at intersections, SDOT evaluates collisions City wide and prioritizes intersections based on the highest number of collisions.
Residents within one block of the intersection will be invited to an open house and SDOT will discuss the benefits and disadvantages of the traffic circles with the neighborhood, as well as other traffic calming devices.
Because traffic circles are popular traffic calming devices as well, communities are interested in installing them to calm traffic. Generally, if there are not at least two reported collisions in the intersection at the last three years, SDOT will not support their installation unless there is some other obvious safety need.
Design & Construction Overview
The traffic circle is made up of a 2’ wide concrete ring and reflectors are installed on the ring. The concrete ring is less than four inches high next to the road so fire trucks or other large vehicles can drive over a portion of the traffic circle without harming the landscaping. There are also reflective object marker signs in the center of the traffic circle so that the traffic circle is more visible at night. For more information on traffic circle design, see Traffic Circle Typical Design.
SDOT encourages landscaping in all traffic circles that are of sufficient size to support landscaping. Maintenance of the landscaping is the responsibility of the community, and several volunteers must be identified before the traffic circle is designed and constructed. If there are not enough identified volunteers, SDOT will eliminate the landscaping component from the traffic circle, and pave the interior of the circle instead.
It generally costs $20,000 to completely construct a traffic circle. SDOT staff designs the circle, and construction is normally completed by SDOT crews. Costs are kept down by not installing irrigation. Water must be brought from nearby residences to help establish the landscaping in the traffic circle.
Once a traffic circle is constructed, SDOT Urban Forestry provides a list of approved plants for the neighborhood to choose from. Plants are placed on this list because of their low mature height and ability to withstand the harsh environment of a traffic circle. Plants will be delivered to the home of a neighborhood volunteer either in the spring or fall. The neighborhood volunteer is encouraged to coordinate a planting party for all the volunteers on the block. Residents are encouraged to plant additional material from the approved plant list as space allows, as long as the mature height is not above 24" and stays within the borders of the circle. Future, ongoing maintenance is the responsibility of the community. Replacement plants are typically not available through SDOT, but access to free mulch and an invitation to the annual plant exchange is provided if you are registered with Urban Forestry.
If larger scale restoration is needed, community members can apply for a $250 sparks grant through Department of Neighborhoods at http://www.seattle.gov/neighborhoods/nmf/smallsparks.htm
If you have any further question regarding Traffic Circle Landscaping, please call SDOT Urban Forestry at 684-TREE (8733).