The Traffic Calming Program (TCP) undertook a streamlined speed bump project in the summer of 1997 to address traffic problems identified by residents along NE 128th Avenue from Halsey to Glisan.
The goal of the project was to enhance street safety and livability by reducing the 85th percentile speed of vehicles using NE 128th Avenue closer to the legal maximum speed limit of 25 miles per hour (mph). Speed bumps are the only devices considered for streets like NE 128th 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 affecting access. It is not an intended goal to reduce traffic volume on low-volume Local Service Streets.
NE 128th Avenue serves a single family residence neighborhood. Halsey Street, to the north, and Glisan Street, at NE 128th Avenue's south end, are both Major City Traffic streets. NE 128th Avenue has neither side walks nor curbs. Additionally, there is a hill at the south end.
Residents along NE 128th Avenue were invited to an open house August 26, 1997 to review and comment on the proposed speed bump installation. Sixteen 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 NE 128th Avenue to sign and was circulated after the open house by a local resident. Of the 46 households along the street, 33, or 71.7%, signed the petition requesting speed bump installation.
Six 14-foot speed bumps, at 350 to 470 foot spacing, were constructed along the 0.50 mile length of NE 128th Avenue, Halsey to Glisan, 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 33 mph. Since bump construction the average 85th percentile speed is 25 mph and ranges from a low of 18 mph (near the bumps) to a high of 27 mph in the 25 mph zone. Before installation of the speed bumps 63% of drivers exceeded the posted speed limit of 25 mph and 9% exceeded the limit by 10 mph or more. Since bump construction 10% of drivers exceed the posted speed limit and 0.2% exceed the limit by 10 mph or more.
Traffic volumes measured before bump construction averaged 890 vehicles per day (vpd) and varied from 800 to 940 vpd. After bump construction volumes averaged 600 vpd and varied from 490 to 740 vpd.
NE 127th Avenue has had a 100 vpd increase in traffic volume since speed bump construction on NE 128th Avenue and will continue to be monitored. NE 131st increased by 120 vehicles, compared to counts taken before this project. NE 131st is a Neighborhood Collector street, and is intended to carry a high volume of traffic.
Traffic Calming on NE 128th Avenue, Halsey to Glisan has successfully reduced the average 85th percentile speed closer to the posted speed, enhancing street safety and livability.