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close windowCase Study No. 53
Seattle, WA
Prepared by John C. Marek and Shauna Walgren, City of Seattle, WA.
As the City of Seattle’s arterial routes become more congested, motorists look for quicker routes and often choose to use non-arterial streets through residential neighborhoods. Communities have increasingly called for traffic calming device installations on their neighborhood streets to discourage cut-through traffic, reduce vehicle speeds and improve pedestrian safety and comfort.
Seattle’s Neighborhood Transportation Services (NTS) began as an outgrowth of programs to improve deteriorating neighborhoods. Residents of Seattle approved the Forward Thrust Bond Issue in 1968 with a major emphasis reducing traffic impacts and supporting for street improvements to re-vitalize deteriorating neighborhoods. Demonstration projects testing a variety of traffic control devices, such as traffic circles, diverters, chicanes and partial and full closures began in 1973 and continued throughout the 1970s and 1980s.
Since then, the NTS continues to emphasize citizen participation and has grown into a popular and highly visible program. Chicanes are one device used by the City’s Neighborhood Traffic Control Program to reduce vehicle speeds and improve pedestrian safety and comfort. To date, Seattle has installed chicanes at 13 locations.
Chicanes on NE 70th Street.
Chicanes on NE 98th Street.
Seattle’s chicanes are a series of two or three curb bulb-outs, placed on alternating sides of the street and staggered to create a curved one-lane segment of roadway. Chicanes help reduce vehicular speeds by requiring motorists to maneuver through the curb bulb-outs, one vehicle at a time. The spacing between the curb bulb-outs and the distance they extend into the roadway determine how easily motorists will be able to maneuver through the chicanes. These devices have a calming effect on streets (particularly if they are landscaped) by creating a visual narrowing of the street. They also enhance the local neighborhood appearance and improve comfort and safety for pedestrians using the roadway.
The City of Seattle studied three chicane installations for their effectiveness at reducing vehicle speeds and discouraging cut-through traffic.
In 1984, two sets of chicanes were installed on NE 70th Street between 12th Avenue NE and 15th Avenue NE. Each set consisted of three curb bulb-outs extended approximately 4 m (13 ft) into the street. The bulb-outs were spaced 15-24 m (50–80 ft) apart, with the two sets of chicanes located 128 m (420 ft) apart.
Two sets of chicanes were also installed on NW 55th Street and NW 56th Street between 3rd Avenue NW and 1st Avenue NW in 1992. These chicane curb bulb-outs are spaced approximately 18 m (60 ft) apart, narrowing the travel lane to 3.6 m (12 ft). The distance between the sets of chicanes is approximately 91 m (300 ft).
A single chicane was installed on NE 98th Street between 20th Avenue NE and 23rd Avenue NE in 1994. This device has 23 m (75 ft) between curb bulb-outs.
The chicanes have significantly reduced the speeds of vehicles traveling on the streets. At all the study locations, there was an initial reduction in 85th percentile speeds of 13–21 km/h (8-13 mi/h). Results of follow-up studies for NE 70th, NW 55th, and NW 56th Streets show that over time initial speed reductions eroded by only 1.6–5 km/h (1-3 mi/h) after the chicanes had been in place for a few years. Overall, speeds remained 18-35 percent lower than before installation. The slight increase may reflect motorists’ familiarity with the devices after driving through them repeatedly.
Chicanes also reduce speeds between sets of devices. While not as great as within the device itself, the speeds between sets of chicanes were reduced by up to 13 km/h (8 mi/h), or 28 percent. Northwest 55th and 56th Streets showed the greatest change with reductions of 10-13 km/h (6-8 mi/h) perhaps due to the relatively close spacing between the curb bulbs and the short distance between chicanes at these locations.
Vehicle volumes on NE 70th, NW 55th, and NW 56th Streets ranged between approximately 1400 Average Weekday Traffic (AWDT) and 2000 (AWDT) before the chicanes were installed. AWDT decreased by 48 percent on NE 70th, 32 percent on NW 55th, and 43 percent on NW 56th after installation. Interestingly, the volume on NE 98th Street remained relatively unchanged (increase from 1965 AWDT to 1993 AWDT) perhaps because no easy alternative routes exist.
Overall, Seattle has found that the chicanes have been very effective at reducing speeds and bringing mid-block speeds closer to the non-arterial limit of 40 km/h (25 mi/h). This benefits pedestrians because slower speeds reduce the probability of serious injury in the event of a collision and increase comfort for pedestrians walking along or crossing the street. Chicanes have also encouraged motorists to use nearby arterial routes, thereby lowering cut-through traffic.
John Marek
Senior Traffic Engineer
Phone: (206) 684-5069
E-mail: john.marek@ci.seattle.wa.us
Shauna Walgren
Senior Transportation Planner
Phone: (206) 684-8681
E-mail: shauna.walgren@ci.seattle.wa.us
Seattle Transportation Department
810 3rd Avenue
Central Bldg., Room 754
Seattle, WA 98104