The Traffic Calming Program (TCP) undertook a streamlined speed bump project in the Fall of 1997 to address traffic problems identified by residents along NE Rocky Butte Road from the Monument to the Tunnel.
The goal of the project was to enhance street safety and livability by reducing the 85th percentile speed of vehicles using NE Rocky Butte Road closer to the legal maximum speed limit of 25 miles per hour (mph). Speed bumps are the only devices considered for streets like NE Rocky Butte Road, 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 Rocky Butte Road serves a single family residence neighborhood. NE Rocky Butte Road has neither side walks nor curbs and there are horizontal and vertical curves that limit sight distances. Rocky Butte Road is the only access to a hilltop monument and a popular recreational destination. Rocky Butte Road provides access to the monument from two directions. This project deals with the southern access. The northern access will be addressed separately.
Residents along NE Rocky Butte Road were invited to an open house December 4, 1997 to review and comment on the proposed speed bump installation. Twenty people attended the open house. Most of those who attended expressed approval for the proposed project. Concern was expressed about the number and location of the bumps. A petition was available at the open house for residents along NE Rocky Butte Road to sign and was circulated after the open house by a local resident. Petition results were as follows:
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Eight 14-foot speed bumps, at 380 to 510 foot spacing, were constructed along this 0.76 mile segment of NE Rocky Butte Road, Monument to Tunnel, on May 7, 1998 by the Bureau of Maintenance.
As the graph (over) shows, vehicle speeds, previously spread between 23 and 34 mph, have been shifted into the range of 20 to 28 mph. The average 85th percentile vehicle speed before the project was 34.7 mph. Since bump construction the average 85th percentile speed is 26.5 mph and ranges from a low of 20 mph (near the bumps) to a high of 30 mph in the 25 mph zone. Before installation of the speed bumps 66% of drivers exceeded the posted speed limit of 25 mph. Since bump construction 18% of drivers exceed the posted speed limit.
Traffic volumes measured before bump construction averaged 490 vehicles per day (vpd) and varied from 340 to 600 vpd. After bump construction, measured volumes averaged 550 vpd and varied from 450 to 630 vpd. Traffic volumes measured on NE Rocky Butte Road north of the monument showed a minor increase from volumes measured before speed bump construction.
Traffic Calming on NE Rocky Butte Road, Monument to Tunnel has successfully reduced the average 85th percentile speed closer to the posted speed, enhancing street safety and livability.