Final Project Evaluation
A traffic safety project was begun in the spring of 1997 to address traffic problems identified by residents on SW 57th at Dolph Drive. SW 57th at Dolph Drive serves a single family residence neighborhood, has no sidewalks but does have curb. The goal of the project was to enhance street safety and livability by eliminating an undulation on a steep section of the roadway. Approaching Dolph Drive from the north, SW 57th Avenue goes downhill with a street grade of up to 20%. It levels out at a T-intersection with Dolph Drive, then proceeds downhill again. Approximately 150 feet south of Dolph Drive, SW 57th has a slight bend to the east and begins climbing an equally steep hill and curving into Orchid Street. The excessive change in street grade, from level to steep in the downhill direction, was being exploited by thrill-seekers. At a high rate of speed, the grade break was reported to provide an automobile sufficient vertical momentum to become airborne.
Residents in the neighborhood surrounding SW 57th and Dolph Drive were invited to an open house August 5, 1997 to review and comment on possible solutions to the street surface, 26 people attended the open house. Alternative solutions to the problem were presented and discussed. A survey was distributed regarding the discussed alternatives. The majority of those surveyed (65%) were in favor of a traffic circle at the intersection. The Bureau of Traffic Management chose to change the vertical alignment of the intersection at SW 57th and Dolph Drive. This solution was perceived to solve the specific problem with the street without introducing the possibility of collisions with a circle or the impedance of emergency vehicle response. In light of resident’s concerns of speed increasing after the intersection was rebuilt, the Bureau of Traffic Management decided to include speed bumps on SW 57th Avenue north of the intersection with Dolph Drive.
The Intersection at Dolph Drive
It was expected that the vertical realignment of the intersection would contribute to an overall increase in speeding. While the excessive grade break attracted a small number of thrill seekers, the majority of traffic did not speed at this location. The intersection realignment was completed in November of 1998. As can be seen in graph one, the number of vehicles north of Dolph Drive traveling at a faster speed has increased.
The 85th percentile speed north of the intersection before the construction was 27 mph. Since construction the 85th percentile speed is 28 mph. Before installation of the speed bumps 20% of drivers exceeded the posted speed limit of 25 mph and 2.2% exceeded the limit by 10 mph or more. Since bump construction 27% of drivers exceed the posted speed limit but only 1.1% exceeded the limit by 10 mph or more.
South of Orchid Court the 85th percentile speed has decreased from 36 mph before construction to 34 mph after. The percent of drivers exceeding the speed limit (72% to 64%) and those exceeding the speed limit by 10 mph or more (16% to 11%) has decreased.
In combination the data suggests that speeding associated with the steep grade has only slightly increased since the excessive grade break was removed, contrary to expectations.
Speed Bumps on SW 57th Garden Home to SW 56th Avenue
Four 14 -foot speed bumps, at 420 to 460 foot spacing, were constructed along the 0.28 mile length of SW 57th, Garden Home to 56th, on April 4, 1998 by Portland’s Bureau of Maintenance.
As graph two shows, vehicle speeds, which previously peaked between 26 to 31 mph, now peak at 23 to 25 mph. The average 85th percentile vehicle speed before the project was 33 mph. Since bump construction the average 85th percentile speed is 27.7 mph and ranges from a low of 24 mph (near the bumps) to a high of 31 mph in the 25-mph zone. Before installation of the speed bumps 59% of drivers exceeded the posted speed limit of 25 mph and 6% exceeded the limit by 10 mph or more. Since bump construction 30.6% of drivers exceed the posted speed limit and 1.3% exceed the limit by 10 mph or more.
Traffic volumes measured before bump construction averaged 550 vehicles per day (vpd). After bump construction volumes averaged 500 vpd and varied from 440 to 550 vpd. Traffic volumes measured on side streets parallel to SW 57th showed no significant increases from measurements before construction.
The vertical realignment and speed bumps have been successful at enhancing street safety and livability.