Bicycle Safety Guide and Countermeasure Selection System

 

Neighborhood Mini Traffic Circles

Seattle, Washington

Prepared by John Marek, Neighborhood Traffic Control Program Engineer, City of Seattle

Background

Seattle's Neighborhood Traffic Control Program (NTCP) started in 1968 when the city began to respond to resident requests to slow motor vehicle traffic and reduce the number of crashes at intersections of residential streets. Of all the treatments used in Seattle, the traffic circle has proven to be the most effective at solving this problem. Since 1973, over 1,200 circles have been constructed in Seattle and NTCP staff members receive about 100 resident requests for new circles each year.

Countermeasures

Traffic circle construction in Seattle.

Traffic circle construction in Seattle.
Photo by SDOT Photos. http://www.flickr.com/photos/sdot_photos/3808760113/.

Potential traffic circle locations are identified through community requests or investigation of high accident intersections. Each request is investigated and an initial assessment is performed to determine if a traffic circle is feasible and ensure that the location is not on an emergency response route. Residents' requests are responded to with a letter explaining the process for installing a circle and the likelihood of the location competing successfully for full city funding. To ensure that the city's traffic safety funding is allocated to intersections demonstrating the greatest need, a priority point system is used to rank the intersections where traffic circles are requested. Ranking criteria include the number of crashes that have occurred at the intersection in the last three years, traffic speed (85th percentile), and traffic volume. To compete for funding, residents are required to submit a petition with signatures representing 60 percent of the households within a one-block radius of the proposed traffic circle. Funding is allocated starting with the intersection with the worst combination of problems and continues as far down the list as funding allows. The cost to construct each circle ranges from $15,000 to $25,000.

Each traffic circle is individually designed to fit the intersection without having to modify the street width or corner radii. Many of Seattle's local streets are 25-feet-wide and traffic circles are usually 12 to 16 feet in diameter. A single unit truck having a 45-foot turning radius is used as a design vehicle to ensure that fire trucks can pass by the circle without running over the curbs. The fire department reviews all intersections where circles are to be constructed and field tests are conducted where they have a specific concern. While traffic circles are designed to allow fire trucks to pass by them, they are constructed with a two-feet-wide mountable curb that allows fire trucks or larger vehicles, such as moving vans, to run over the curb without damaging either the vehicle or the circle.

This traffic circle in a Seattle neighborhood also incorporates stamped concrete.

This traffic circle in a Seattle neighborhood also incorporates stamped concrete.
Photo courtesy of SDOT.

Traffic circles will be landscaped if their diameter is greater than 12 feet and if four landscape volunteers are identified who will maintain the circle. The pavement inside the traffic circle is removed during construction to allow for drainage and to accommodate tree roots. The landscaping plays two important roles—it makes the circle more attractive to the neighborhood residents, and changes the character of the street to make it less appealing for high-speed driving. The local residents are required to maintain the plantings, which consist of ground cover and one to three trees. Residents are allowed to add their own low-growing plants that will not block pedestrian or driver visibility.

Evaluation and Results

In 1997, a study of 119 traffic circles constructed between 1991 and 1994 showed a 94 percent reduction in all types of crashes. Since the study, subsequent spot checks of other locations have produced similar results. While most of the non-arterial intersections in Seattle have no right-of-way control, 32 of the 119 locations studied had existing two-way stop or yield signs, which were removed when the traffic circles were installed. These locations—which previously had right-of-way control—experienced crash and injury reduction rates similar to those found at uncontrolled intersections.

In addition to reducing crashes, traffic circles have been effective at reducing vehicle speeds and have not significantly reduced traffic volumes. The effect on speed generally carries over to the middle of the block, but to a lesser extent than near the intersection. Multiple circles at every intersection are more effective than an isolated circle. The minimal impact on traffic volumes allows circles to be used as a spot, or street-long, safety device without needing to address the impacts of traffic diverting to other residential streets.

Traffic circles have generally been well-accepted by bicyclists. The circles slow down motor vehicle speed, which reduces the speed differential between bicyclists and motor vehicles. Bicyclists have not complained of being "squeezed" by motor vehicles as they go around the circle since the speeds of the motor vehicles are comparable to the bicyclists.

The success of traffic circles is also measured by its acceptance among residents living near them. The vast majority of residents are enthusiastic about the traffic circles. For example, nearly 100 requests for new circles are received each year and about 400 signatures are received on petitions for new circles each year. Only two circles have been removed out of more than 1,200 constructed (the city will remove a traffic circle if, after construction, 60 percent of the households within a one block radius sign a petition to show support for the removal of the traffic circle), and surveys mailed to residents following construction of a traffic circle indicate that 80 to 90 percent of residents feel the circles have been effective and want to keep them permanently.

Conclusions and Recommendations

After more than 40 years of experience installing mini traffic circles, Seattle has found them an effective device for controlling neighborhood traffic and improving the safety of residential streets. Additionally, residents feel traffic circles have successfully addressed their safety concerns and make their neighborhoods better places to live. By slowing down motor vehicle speeds, they benefit neighborhood bicyclists. If a residential street has high volumes of bicyclists or is a bicycle boulevard, other treatments, such as diverters for motor vehicles, should be considered before installing a traffic circle.

Costs and Funding

$15,000 to $25,000, including staff time.

Contact

John Marek
Manager of Neighborhood Traffic Calming
Seattle Department of Transportation