"If we are to succeed in encouraging people to leave their cars at home, we have to offer them attractive, practical alternatives, including a safe environment for pedestrians. Traffic calming keeps our neighborhoods safe, encourages drivers to choose arterial streets, and, in a small but important way, lessens the dominance of the automobile in our daily lives."
- Mayor Schell

Of all the traffic calming devices used in Seattle, traffic circles have proven to be the most effective at solving neighborhood traffic concerns. Since 1973 over 700 traffic circles have been constructed in Seattle, and the Neighborhood Traffic Control Program (NTCP) staff receive about 700 requests for traffic circles each year. The program is currently funded to construct 30 traffic circles per year.

Site Selection Matched to Worst Problems

Potential traffic circle locations are identified through community request or investigation of high accident locations. A priority point system - based on the number of accidents that have occurred at the intersection, the speed of traffic, and the volume of traffic - is used to rank the locations where traffic circles are requested. Funding is allocated starting at the location with the worst combination of problems. Residents are required to submit a petition with signatures representing 60 percent of the households within one block of the proposed traffic circle. The cost to construct each circle ranges from $3,000 to $6,000.

Circle Design

Each traffic circle is individually designed to fit the intersection. Most of Seattle’s local streets are 25 feet wide or less, and traffic circles are usually 12 to 16 feet in diameter. All intersections where circles are to be constructed are reviewed by the Fire Department, and field tests are conducted where the department may have a specific concern. Designs may be adjusted or parking restricted to ensure that fire trucks can pass by the circle.

All the traffic circles currently being constructed are being landscaped. The landscaping plays two important roles, making the circle more attractive to the neighborhood residents and changing the character of the street to make it less appealing for driving at high speeds. The local residents are required to maintain the plantings.

Positive Results

Accident reduction is the greatest benefit of traffic circles. Between 1991 and 1994 a total of 119 traffic circles were constructed through the NTCP. An analysis was made of the accidents that occurred at these intersections - comparing the number of collisions in the calendar year before construction with the number in the calendar year after construction. The results were striking, revealing a 94 percent reduction in accidents. Furthermore, the number of injuries dropped from 153 in the year before construction to one injury in the year after construction. This accident reduction continued in subsequent years.

In addition to reducing accidents, traffic circles have been found also to be effective at diminishing vehicle speeds. They also lessen cut-through traffic once a street loses the wide open appearance that made it attractive as a short cut. However, traffic circles have not significantly reduced traffic volumes. As a result, the minimal impact on traffic volumes allows circles to be used as spot safety devices - without having to address the impact of diverted traffic being shifted to other residential streets.

After over twenty years of experience in installing traffic circles, Seattle has found them to be an effective device for controlling neighborhood traffic and improving the safety of residential streets.

Chicanes as an Alternative

Although traffic circles may be Seattle’s most common traffic calming device, there are some locations where they are not a viable option. At these locations, Seattle has looked at alternative approaches for mid-block speed control - with chicanes commonly used. Chicanes are two or three curb bulbs placed on alternating sides of the street and staggered to create a curved one-lane segment of roadway. To date Seattle has installed chicanes at over 20 locations.

Chicanes reduce vehicular speeds by requiring motorists to maneuver through the curb bulbs, one vehicle at a time. The size and spacing of the curb bulbs determines how easily motorists will be able to travel through the device. An important aspect of chicanes is that they visually change the appearance and character of a street, thus changing a driver’s perception. This is particularly true when they are landscaped.

Careful Assessment of Chicane Need and Support

Chicane locations are identified through community requests. Each request is investigated, and an initial assessment is performed to determine which traffic calming tool is best suited for the site. Less restrictive measures, such as our Neighborhood Speed Watch Program or traffic circle installations, are typically considered first. However, for those locations experiencing high speeds where these alternatives have been attempted or are not feasible, chicanes are considered. The community must submit a petition showing support from 60 percent of the residents on the street requesting the device and on those adjacent streets which may be affected. The cost to construct a chicane ranges from $6,000 to $14,000, depending on the design of the street and the chicane.

Effectiveness of Chicanes

The primary goal of chicanes is to reduce vehicular speeds on the street. Seattle recently completed a study investigating the effect that chicanes had on speeds and volumes at four locations. Based on this study, speeds were reduced by 5 to 13 miles per hour (mph) inside the device. At three of the four locations, speeds outside the chicanes were reduced an average of 5 mph. An important observation in the speed analysis was the reduction in high end speeders. At one location the number of motorists exceeding the speed limit of 25 mph dropped from 39 to 3 percent.

The study also showed that chicanes helped to reduce vehicular volumes. On three of the four streets studied, the average weekday traffic was reduced by 32 percent to 48 percent. While volumes on these streets was significantly diminished, the adjacent non-arterial streets experienced little or no change.

Potential Problems with Chicanes

Although chicanes have their advantages, there are some issues to consider before installing these devices. Chicanes can be problematic to design, especially with regard to curb bulb location and driveways. In addition, they may reduce available on street parking and increase emergency response time. However, where these issues can be properly addressed, Seattle has found that chicanes can be an effective tool for controlling neighborhood traffic.

Contact: Jim Mundell, P.E., Senior Traffic Engineer, Seattle Transportation, 206/684-0814.

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The United States Conference of Mayors

J. Thomas Cochran, Executive Director
1620 Eye Street, NW, Washington, DC 20006
Telephone (202) 293-7330, FAX (202) 293-2352

Copyright ©1996, U.S. Conference of Mayors, All rights reserved.