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June 2002
Traffic "calming"- does it work?
It's a familiar scenario: homeowners want moderate traffic speeds past their properties; traffic engineers need a smooth flow of traffic to avoid localized congestion. Cities trying to strike a balance between safety and mobility often turn to "traffic calming" - roadway design and construction strategies to encourage speed reduction, without adding stop signs or other impediments to traffic flow.
But when is traffic calming appropriate - and how effective is it? Mike Marti of SRF Consulting Group began his presentation with an overview of the issues involved in helping property owners and traffic engineers work together. These issues include understanding the purpose of different types of roadway, agreeing on a way to measure the effectiveness of a particular traffic calming installation, and handling potential liability for accidents.
Marti found that little effective information is available to help planners decide on traffic calming strategies. After looking at such popular strategies as roadway narrowing ("chokers"), pavement marking, and raised crosswalks, he found that effectiveness seemed to vary widely in different settings and with different combinations of calming measures.
Faced with the difficulty of real-world experimentation on traffic calming, Kathleen Harder of the University's Center for Sustainable Building Research described her efforts to develop a methodology for testing the effects of traffic calming strategies in the controlled setting of a driving simulator. She and researcher John Bloomfield designed a series of tests based on a section of Franklin Avenue in Minneapolis that was slated to receive traffic calming treatments.
The simulator results proved fairly consistent with the observed behavior of drivers before and after the Franklin Avenue improvements. Based on this success, the two researchers then developed a generic "suburban collector street" simulation to examine the effects of curb treatments and plantings on driver behavior. While some results of this research may generalize to other traffic situations, Harder stressed that "more research is needed in order to develop a cost-benefit picture of traffc calming."
Finally, Tom Sohrweide of Short Elliot Hendrickson, Inc., walked the audience through two real-world traffic calming projects: Burnsville's busy Portland Avenue near County Road 42, and the economically vital Main Street in River Falls, Wisconsin.
By changing the four-lane roadway to three lanes with a two-way left-turn lane in the center, the city was able to reduce traffic blockages by left-turning vehicles while significantly lowering the accident rate. What's more, the project proved to be cheaper than the alternative approach of adding a median and widening the roadway—by almost $500,000—and popular with the driving public.
In River Falls, traffic volumes on Main Street were reduced, an effect attributed to more motorists getting on the freeway instead of driving through downtown. But a survey indicated that many drivers believed the traffic calming measures increased trip time and created more stress for roadway users. Sohrweide pointed out that the public may not see traffic calming the same way engineers do, and that such projects can have unexpected consequences such as increased volume on other roadways. He echoed Harder in stressing the need for a thorough cost-benefit analysis for any traffic calming project.