The Traffic Calming Program (TCP) undertook a streamlined speed bump project in the Fall of 1995 to address traffic problems identified by residents along SE 108th Avenue from Burnside to Stark.
The goal of the project was to enhance street safety and livability by reducing the 85th percentile speed of vehicles using SE 108th Avenue closer to the legal maximum speed limit of 25 miles per hour (mph). Speed bumps are the only device considered for streets like SE 108th Avenue, which are classified as Local Service Streets, are not transit or primary fire response routes and have vehicle volumes between 400 and 1700 vehicles per day. Speed bumps have proven to be effective tools to reduce vehicle speed without impacting access. It is not an intended goal to reduce traffic volume on low-volume Local Service Streets.
SE 108th Avenue serves a single family residence commercial neighborhood. Burnside, to the north, is a Neighborhood Collector street. Stark, at SE 108th Avenue's south end, is a Major City Traffic street. SE 108th Avenue has neither sidewalks or curbs. A portion of the daily traffic on SE 108th Avenue is believed to be cut-through traffic due to a traffic signal at Burnside which allows crossing of the MAX rail line.
All residents along SE 108th Avenue were invited to an open house October 17, 1995 to review and comment on the proposed speed bump installation. Eight people attended the open house. Those who attended expressed approval for the proposed project. A petition was available at the open house for residents along SE 108th Avenue to sign and was circulated after the open house by a local resident. Of the 33 households along the street, 32, or 96.7%, signed the petition requesting speed bump installation.
Three 14-foot speed bumps, at 420 foot spacing, were constructed along the 0.23 mile stretch of SE 108th Avenue, Burnside to Stark, on September 28, 1996 by the Bureau of Maintenance.
As the graph shows, vehicle speeds, previously concentrated between 26 and 31 mph, have been shifted into the range of 20 to 25 mph. The average 85th percentile vehicle speed before the project was 33.3 mph. After bump construction the average 85th percentile speed is 25.6 mph and ranges from a low of 22 mph (near the bumps) to a high of 29 mph.
The traffic volume measured before bump construction averaged 730 vehicles per day (vpd) and varied from 720 to 740 vpd. After bump construction volumes averaged 640 vpd and ranged from 610 to 680 vpd. No parallel side streets were identified as potential diversion routes.
Traffic Calming on SE 108th Avenue, Burnside to Stark has successfully reduced the 85th percentile closer to the posted speed, enhancing street safety and livability.