Final Project Evaluation
The Traffic Calming Program (TCP) undertook a streamlined speed bump project, in the summer of 1997, to address traffic problems identified by residents along SE 115th Avenue, from Holgate to Harold.
The goal of the project was to enhance street safety and livability, by reducing the 85th percentile speed of vehicles using SE 115th Avenue closer to the legal speed limit of 25 miles per hour (mph). Speed bumps are the only devices considered for streets like SE 115th 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 are effective tools to reduce vehicle speed without affecting access. It is not an intended goal to reduce traffic volume on low-volume Local Service Streets.
SE 115th Avenue serves a single family residence neighborhood. Holgate Boulevard, to the north, and Harold Street, at SE 115th Avenue's south end, are both Neighborhood Collector streets. SE 115th Avenue has neither side walks nor curbs.
Residents along SE 115th Avenue were invited to an open house August 20, 1997 to review and comment on the proposed speed bump installation. Eleven 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 115th Avenue to sign and was circulated after the open house by a local resident. Of the 55 households along the street, 44, or 80%, signed the petition requesting speed bump installation.
Six 14-foot speed bumps, at 400 to 480 foot spacing, were constructed along the 0.45 mile length of SE 115th Avenue, Holgate to Harold, on September 20, 1997 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 34 mph. Since bump construction the average 85th percentile speed is 25.2 mph and ranges from a low of 18 mph (near the bumps) to a high of 28 mph in the 25 mph zone. Before installation of the speed bumps 65% of drivers exceeded the posted speed limit of 25 mph and 15% exceeded the limit by 10 mph or more. Since bump construction 11% of drivers exceed the posted speed limit and 0.6% exceed the limit by 10 mph or more.
Traffic volumes measured before bump construction measured 590 vehicles per day (vpd). After bump construction volumes averaged 640 vpd and varied from 440 to 860 vpd. Traffic volumes increase near the ends of the street, with the highest volumes measured at the south end.
SE 113th Avenue has had a measurable increase in traffic volume since speed bump construction on SE 115th Avenue and will continue to be monitored. SE 111th Avenue is a Neighborhood Collector street, and is intended to carry a high volume of traffic.
Traffic Calming on SE 115th Avenue, Holgate to Harold has successfully reduced the average 85th percentile speed closer to the posted speed, enhancing street safety and livability.