Traffic Circles

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.

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 for the last three years, SDOT will not support their installation unless there is some other obvious safety need.

Traffic Circle with flowers

Design & Construction Overview

Traffic circles are designed according to the existing geometry of the intersection. For example, if each street at the intersection is 25' wide, the diameter of the traffic circle will be 16'.

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.

Traffic Circle Landscaping

Traffic Circles Landscaping

Once a traffic circle is constructed, SDOT will landscape the traffic circle, generally in the Spring or Fall. Generally, the plants are placed low mature height and have an ability to withstand the harsh environment of a traffic circle. 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. If larger scale restoration is needed, community members can apply for a sparks grant through Department of Neighborhoods Neighborhood Matching Fund

Typical Traffic Circle Design