The Traffic Calming Program undertook a Streamlined Speed Bump Project in the Winter of 1994-95 on NE 111th Avenue between Halsey Street and Glisan Street.
The goal of the project was to enhance safety and livability by reducing the 85th percentile speed of vehicles using NE 111th closer to the legal maximum speed limit of 25 miles per hour (mph). Speed bumps are the only device considered for streets like NE 111th, 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.
NE 111th serves a single family residence neighborhood and has some businesses at its North end. Halsey Street, to the North, and Glisan Street, at NE 111th's South end, are Major City Traffic Streets. NE 111th has neither sidewalks or curbs. A significant percent of the daily traffic on NE 111th is believed to be cut-through traffic due to a traffic signal at Halsey. Also 111th is designated as a Collector Street North of Halsey.
All residents along NE 111th were invited to an open house February 15, 1994 to review and comment on the proposed speed bump installation. Eight households were represented at the open house. The majority of households were in favor of the project. A petition was available at the open house for residents along NE 111th to sign and was circulated after the open house by a local resident. Of the 56 households along the street, 44, or 78.6%, signed the petition requesting speed bump installation.
Four 14-foot speed bumps, at 500 to 600 foot spacing, were constructed along the 1/2 mile stretch of NE 111th in March of 1995 by the Bureau of Maintenance.
As the graph shows, vehicle speeds, previously concentrated between 26 and 34 mph, have been shifted into the range of 20 to 25 mph. Eighty-five percent of the vehicles before the project were traveling 36 mph or less. After bump construction the average 85th percentile speed was 27.2 mph and ranged from a low of 21 mph (near the bumps) to a high of 30 mph.
Traffic Volumes measured after bump construction showed no significant change from the pre-bump volumes of 680 vehicles per day (vpd). NE 110th Avenue, which branches off of NE 111th, had an increase in traffic volume immediately after bump construction but has returned to acceptable levels. NE 110th will continue to be monitored to insure added traffic volume remains below the established ceiling. Other adjacent streets that were identified as unwanted diversion routes showed no significant increases in traffic volume.
Traffic Calming on NE 111th has successfully reduced the 85th percentile closer to the posted speed and enhanced street safety and livability.