John Marek, Neighborhood Traffic Control Program Engineer, City of Seattle
Peter Lagerwey, Pedestrian & Bicycle Program Coordinator, City of Seattle
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 800 circles have been constructed in Seattle and NTCP staff members receive about 700 resident requests for new circles each year.
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. 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. In order 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 proceeds as far down the list as funding allows. The cost to construct each circle ranges from $4,000 to $7,000.
Each traffic circle is individually designed to fit the intersection without having to modify the street width or corner radii. Most of Seattle’s local streets are 7.6 m (25 ft) wide and traffic circles are usually 3.7 to 4.9 m (12 to16 ft) in diameter. A single unit truck having a 13.7 m (45 ft) 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 0.6 m (2 ft)–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.
Landscaping is included in all the traffic circles as long as a neighborhood volunteer is identified who will maintain the circle (almost always). 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.
Traffic circles are evaluated by comparing the number of crashes occurring in the 12 months before and the 12 months after a traffic circle is installed. Additionally, surveys are mailed to residents following the construction of a traffic circle.
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 accident and injury reduction rates similar to those found at uncontrolled intersections.
In addition to reducing accidents, traffic circles have been effective at reducing vehicle speeds but 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. As might be expected, 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 generally have 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. A few bicyclists have complained that the circles cause them to slow down (in the same way they slow the motorists).
The success of traffic circles is also measured by its acceptance among residents living near them. By far, the majority of residents are enthusiastic about the traffic circles. For example, nearly 700 requests for new circles are received each year and about 3,000 signatures are received on petitions for new circles each year. Only two circles have been removed out of more than 800 constructed (residents are guaranteed that the city will remove a traffic circle if, after construction, 60 percent of the households within a one block radius have signed a removal petition), and surveys mailed to residents following construction of a traffic circle indicate that 80 percent to 90 percent of residents feel the circles have been effective and want to keep them permanently.
After nearly 30 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.
$5,000 to $8,000 including staff time.
Manager of Neighborhood Traffic Calming
Seattle Department of Transportation
700 5th Avenue, Suite 3900
P.O. Box 34996
Seattle, WA 98124-4996