For cities trying to get a handle on traffic fatalities, dangerous motor vehicle speeds are an enormous problem. Once drivers exceed 20 mph, the chances that someone outside the vehicle will survive a collision plummet.
But even on city streets where many people walk and bike, streets with 35 or 40 mph traffic are common. Cities looking to reduce lethal vehicle speeds face a number of obstacles -- including restrictions on how they can set speed limits.
State statutes usually limit how cities set speed limits. In Boston, for example, the City Council has voted numerous times to reduce the speed limit to 20 miles per hour, but state law won't allow it.
Now Portland is taking on this problem. A pilot program expected to be approved by the Oregon Department of Transportation proposes a new way to evaluate what speeds are appropriate for urban areas.
The Portland Mercury reports that city officials are challenging the "85th percentile rule," the old traffic engineering practice of measuring travel speeds on a street, then setting the speed limit at the rate that 85 percent of drivers do not exceed.
One of the problems with this practice is that it doesn't even consider the presence of pedestrians or bicyclists -- only the speed at which drivers travel. The whole exercise can simply reinforce and legitimize dangerous driving speeds on poorly designed streets.
Portland has proposed to reevaluate speed limits according to a matrix of factors that account for motorists' proximity to walkers and bikers [PDF]. On streets with unprotected bike lanes, for instance, the maximum limit would be 30 mph, for instance, and the same would go for streets with sidewalks on each side. (Roads with sidewalks could still be signed for 50 mph, but only with an "impenetrable separation barrier" to protect pedestrians.)
In practice, the rules would mean that a street like Willamette Boulevard, where the 85th percentile speed is 41 mph and the speed limit is 35 mph, would be signed at 30 mph.
Street design is a more important safety factor than speed limit signs, of course, but lower speed limits can still send a signal to motorists to proceed more cautiously -- and they can set expectations for traffic calming redesigns in the future. If the speed limit is 30 mph but motorists consistently go faster, the design clearly needs to change.
On that score, other cities have taken more ambitious steps than what Portland is considering. New York City recently made 25 mph the default speed limit on its surface streets. And the "Total 20" campaign in the United Kingdom has enacted blanket 20 mph speed limits in some towns.
But Portland is turning the tables on the car-centric 85th percentile rule and devising a system that's more sensitive to the needs of people walking and biking.
And it looks like the city will be able to get its idea approved. State Traffic Engineer Bob Pappe, who needs to sign off on the proposal, told the Mercury, "I think in a city like Portland that’s probably very, very appropriate.”
