Bicycle Safety Guide and Countermeasure Selection System

 

Bikeway Speed Humps

Portland, Oregon

Prepared by Scott Batson, P.E., Active Transportation Division, Portland Office of Transportation.

Background

Clinton Street in Portland, Oregon, had been designated a City Bikeway but did not have adequate curb-to-curb width to mark bike lanes without removal of parking. In 1990, three traffic circles were constructed toward the east end of Clinton, but speeding vehicles continued to be a problem. Input from local citizens helped to determine that speed humps were the most appropriate tool to address traffic problems on Southeast Clinton Street. In 1998, Portland's Bike Program enlisted the help of the Traffic Calming Section to carry out the project. The 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 mi/h.

A man rides a bicycle over one of the speed humps.

A bicyclist passes over one of the speed humps.

Southeast Clinton is a local service street and serves a mixed single-family residential and commercial neighborhood. The road is fairly level and straight. The entire length of Southeast Clinton has parking, sidewalks, and curbs on both sides of the street.

The street was divided into three segments for the purpose of this project. While speed humps were proposed as appropriate solutions for two of the segments, the middle segment, 21st Avenue to 26th Avenue, is part of a transit route that passes through the neighborhood, and city policy mandated that this street segment would need a speed table instead.

Countermeasures

Five 14-foot speed humps at 400 to 530 foot spacing were installed along the 0.44 mile length of Southeast Clinton from 12th Avenue to 21st Avenue. In the 0.24 mi segment used by transit, two 22-foot speed tables were installed. Nine 14-foot speed humps at 320 to 600 foot spacing were installed along the 0.83 mi length of Southeast Clinton from 26th to 39th Avenues.

Evaluation and Results

Portland has determined speed humps to be an effective tool for reducing speeding by motorists. 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 bicyclists. Counts were taken six months after construction was completed.

Traffic Speeds
The changes in traffic speed associated with this project were typical of speed hump projects elsewhere in Portland. Table 1 describes the change in speed in the three sections of Southeast Clinton.

Table 1: Before and after vehicle speeds

Table 1: Before and after vehicle speeds

Traffic Volume
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 was greater than normal. 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.

Table 2: Vehicle volume changes

Table 2: Vehicle volume changes

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 bicyclists is another indication of the success of this project (see Table 3). Feedback from local residents has been very positive.

Table 3: Cyclist usage of Clinton

Table 3: Cyclist usage of Clinton

From 2007 to 2010, throughout the project area (from 12th Street to 39th Street), average speeds remain in the 25-27 mi/h range, and the number of vehicles per day remains closer to the "after" range noted six months following speed hump and speed table installation.

Conclusions and Recommendations

A man rides his bicycle along Clinton Street. A speed table is visible in the background.

A man rides his bicycle along Clinton Street.

Traffic calming on Southeast Clinton from 12th to 39th Avenues successfully reduced the average 85th percentile speed and produced the unexpected benefit of decreasing the number of cars using the street. The speed reduction associated with the use of speed humps provides increased safety to bicyclists using this bikeway.

Feedback from bicyclists regarding a preference for speed humps versus speed tables has been mixed. Although speed tables are less uncomfortable for cyclists, 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. A common theme with traffic calming projects is the tradeoffs such projects involve. The potential discomfort of the bicyclist 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 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.

Costs and Funding

The following table gives the costs of installing the speed humps and speed tables in 1998. For more current cost estimates, please see countermeasure cost information in speed tables/ humps/ cushions.

Contact

Scott Batson, PE
Active Transportation Division
City of Portland