Scott Batson, PE, Senior Engineering Associate, Traffic Investigations Section, Bureau of Transportation Management, Portland Office of Transportation
Portland’s Bike Program enlisted the help of the Traffic Calming section for a speed hump project in spring 1998. Speed humps were identified by local citizens as the most appropriate tool to address traffic problems on Southeast Clinton Street. Though three traffic circles were constructed toward the east end of Clinton in 1990, speeding vehicles continued to be a problem. Clinton had been designated a City Bikeway but did not have adequate curb-to-curb width to mark bike lanes without removal of parking. Reduction of traffic volume on the street was obtained in conjunction with the 1990 project that installed traffic circles, so speed reduction was the primary objective for this project.
The specific goal of the project was to enhance street safety for bicycle riders by reducing the 85th percentile speed of vehicles using Southeast Clinton closer to the legal maximum speed limit of 25 mph. Portland has determined speed humps to be an effective tool to reduce traffic speeding.
Southeast Clinton was divided into three segments for the undertaking of this project. A middle portion of the street, 21st Avenue to 26th Avenue, is part of a transit route that jogs through the neighborhood. This segment of Clinton necessitated a speed table design by City policy.
Southeast Clinton is a local service street and serves a mixed single-family residence and commercial neighborhood. Southeast Clinton is fairly level and straight. The entire length of Southeast Clinton has parking, sidewalks and curbs on both sides of the street.
Residents along Southeast Clinton were invited to an open house on June 3, 1998, to review and comment on the proposed speed hump installation. Forty-five people attended the open house. Most of those who attended expressed approval for the proposed project. Some considered the humps to be excessive or inadequate, while others expressed concern over noise and hump location. A petition was available at the open house for residents along Southeast Clinton to sign, and was circulated after the open house by local residents. Petition results in aggregate for the three segments were as follows:
|In favor of speed humps||Number||Percent of Total|
Five 4.3 m (14 ft) speed humps, at 121.9 to 161.5 m (400 to 530 ft) spacing, were constructed along the 0.7 km (0.44 mi) length of Southeast Clinton, from 12th to 21st avenues. Two 6.7 m (22 ft) speed tables were constructed along the 0.4 km (0.24 mi) length of Southeast Clinton, from 21st to 26th in the segment used by transit. Nine 4.3 m (14 ft) speed humps, at 97.5 to 182.9 m (320 to 600 ft) spacing, were constructed along the 1.3 km (0.83 mi) length of Southeast Clinton from 26th to 39th avenues. The projects were completed between September 26 and October 18, 1998, by Portland’s Bureau of Maintenance.
Standard velocity and volume counts, before and after speed hump construction, were used to measure the change in vehicle speed. Measurements taken after speed hump construction were averaged over the length of the street for comparison to speed before construction. The after velocities were weighted based on distance from the center of the nearest speed hump. Manual peak-hour turning movement counts were also conducted to assess the change in usage by cyclists. Counts were taken six months after construction was completed.
Table 1 describes the change in speed in the three sections of Southeast Clinton.
The changes in traffic speed associated with this project were typical of speed hump projects elsewhere in Portland.
|Segment||Speed Hump||Average Speed1, mph||Highest Speed1, mph||Speeders, Over Posted 25 mph||Speeders, Over 35 mph|
|12th to 21st||14 ft||33||25||27||80%||12%||10%||0%|
|21st to 26th||22 ft||26||24||26||15%||8%||0%||0%|
|26th to 39th||14 ft||31||25||28||58%||8%||15%||0%|
1. 85th Percentile Speed
Table 2 summarizes the change in traffic volume in the three sections of Southeast Clinton.
Typical daily fluctuations of traffic volume are expected to be 10 percent. The 25 to 30 percent reduction on Southeast Clinton is greater than normal (see table 2). The 1990 traffic circle project was constructed as part of an effort to deter use of Clinton as an alternative to parallel streets of higher classification.
|Segment||Speed Hump||Average Volume, vpd1||Change|
|12th to 21st||14 ft||3400||2300||-30%|
|21st to 26th||22 ft||3300||2450||-25%|
|26th to 39th||14 ft||2600||1800||-30%|
1. Vehicles Per Day
|Intersection||East-West 7-9 AM||East-West 4-6 PM|
|Clinton at 26th||40||66||40%||37||83||128%|
|Clinton at 39th||20||35||75%||24||36||50%|
Traffic volume measurements at over 40 locations adjacent to Southeast Clinton identified four that had volume increases that warranted additional monitoring. Subsequent reevaluation determined the volume increases to be anomalous.
The increase in usage by cyclists is another indication of the success of this project (see table 3). Feedback from local residents has been very positive.
Traffic calming on Southeast Clinton from 12th to 39th Avenues successfully reduced the average 85th percentile speed closer to the posted speed and produced an unexpected benefit of decreasing the number of cars using the street. The speed reduction associated with the use of speed humps will provide increased safety to cyclists using this bikeway.
Feedback from cyclists regarding a preference for speed humps versus speed tables has been mixed. It is unclear from this project if speed tables would have had as significant an effect on speeding if they were implemented along the entire project length. It is likely that speed tables will produce less discomfort to cyclists than do speed humps. A common theme with traffic calming projects is the tradeoffs such projects involve. The potential discomfort of the cyclist traversing a speed hump or table should be compared to the discomfort associated with the speed of adjacent vehicles.
Southeast Clinton is part of a dense grid of streets (typical 61-m (200-ft) block faces). Monitoring of adjacent streets for unintended diversion is critical. If diversion is identified as a significant issue and possibility, modification of the hump layout or use of the longer table design is recommended.
|4.3-m (14-ft) speed hump, including markings||$1,500||14||$21,000|
|6.7-m (22-ft) speed table, including markings||$1,800||2||$3,600|
|Warning Signs and posts||$160/group||7||$1,120|
|Total Construction Costs||$25,720|
Scott Batson, PE
City of Portland
City of Portland