The Traffic Calming Program (TCP) undertook a streamlined speed bump project in the Summer of 1997 to address traffic problems identified by residents along SW 50th Avenue from Taylors Ferry Road to Orchid.
The goal of the project was to enhance street safety and livability by reducing the 85th percentile speed of vehicles using SW 50th Avenue closer to the legal maximum speed limit of 25 miles per hour (mph). Speed bumps are the only devices considered for streets like SW 50th 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.
SW 50th Avenue serves a single family residence neighborhood. Taylors Ferry Road, to the south, is a Neighborhood Collector street. Orchid Street, at SW 50th Avenue's north end, is a Local Service street. SW 50th Avenue has neither side walks nor curbs for its entire length and there is a steep hill near the south end.
Residents along SW 50th Avenue were invited to an open house August 21, 1997 to review and comment on the proposed speed bump installation. Eighteen people attended the open house. Most of those who attended expressed approval for the proposed project. Some considered the bumps to be excessive or had fears the bumps would cause unwanted noise. A petition was available at the open house for residents along SW 50th Avenue to sign and was circulated after the open house by a local resident. Petition results were as follows:
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Three 14-foot speed bumps, at 350 to 480 foot spacing, were constructed along the 0.28 mile length of SW 50th Avenue, Taylors Ferry Road to Orchid, on September 20, 1997 by the Bureau of Maintenance.
As the graph (over) shows, vehicle speeds, previously concentrated between 23 and 34 mph, have been shifted into the range of 20 to 25 mph. The average 85th percentile vehicle speed before the project was 31 mph. Since bump construction the average 85th percentile speed is 24 mph and ranges from a low of 21 mph (near the bumps) to a high of 26 mph in the 25 mph zone. Before installation of the speed bumps 54% of drivers exceeded the posted speed limit of 25 mph and 7% exceeded the limit by 10 mph or more. Since bump construction 7.9% of drivers exceed the posted speed limit and .02% exceed the limit by 10 mph or more.
Traffic volumes measured before bump construction averaged 1000 vehicles per day (vpd). After bump construction volumes averaged 890 vpd. Traffic volumes measured on SW 49th Avenue appear to have increased above permissible thresholds established before bump construction on SW 50th. SW 49th will be monitored for mitigation of shifted traffic volume.
Traffic Calming on SW 50th Avenue, Taylors Ferry Road to Orchid has successfully reduced the street's average 85th percentile speed closer to the posted speed, enhancing safety and livability.