Portland, following the example of Seattle and other cities, is poised to lower speed limits to 20 mph on residential streets throughout the city.

The City Council will weigh the change Wednesday morning. But the city, under the direction of the council, had already lobbied to amend state law allowing the city to set lower speed limits. The state Legislature approved that authority last year.

The change would affect side streets in primarily residential areas, most of which currently have speed limits of 25 mph. Speed limits on busier collector and arterial streets wouldn't change.

Such streets add up to thousands of miles of roadway, and about 70 percent of the city's transportation network, the city's Transportation Bureau said.

The push is part of the city's Vision Zero campaign, an effort adopted in 2015 to eliminate traffic-related deaths. Fatalities have instead climbed -- from 37 in 2015 to 45 in 2017 -- as Portlanders drove more frequently.

Much of the effort to date has focused on routes identified as "high-crash corridors," typically busy through streets with few accommodations for pedestrians and cyclists. Those streets accounted for about 70 percent of last year's traffic deaths.

But city officials said they believe lower speed limits on residential streets could also help push the city closer to its zero-fatality goal.

"Even that 5 mph difference can make a big difference whether somebody survives a crash or not," said Dana Dickman, who manages the Transportation Bureau's traffic safety section. "A pedestrian (hit) at 25 mph is twice as likely to die compared with somebody at 20 mph."

That statistic comes from a 2011 AAA Foundation for Traffic Safety study that found the average person hit by a car traveling 25 mph has a 12 percent chance of death. That drops to a 7 percent risk at 20 mph.

Lower speeds also reduce crash rates, so the city could expect fewer people to be hit by cars in the first place.

But all of that assumes drivers observe the reduced speed limit.

"Broadly, the research would suggest that just changing out a sign isn't going to result in much change," said Chris Monsere, a Portland State University transportation researcher. "If it's combined with some restrictions in width or other traffic-calming devices, perhaps with some advertising and enforcement, it could have an effect."

Dickman said the city likely won't shift police deployment from busier areas, including the high-crash corridors, to routinely monitor speeds on quieter residential streets.

But police likely will conduct extra patrols on streets where residents frequently complain of speeders, and the Transportation Bureau plans a major marketing push once the new speed limits take effect.

That won't happen overnight. If the council approves the ordinance, the speed limit will drop immediately on residential streets that lack speed limit signs.

But where 25 mph signs are currently posted, that speed limit will remain in effect until the signs are replaced.

Crews are expected to start replacing signs in East Portland, which sees a relatively high number of crashes, and complete the transition citywide by April.

Seattle in 2016 lowered speeds on many of its roads, including bringing residential speed limits down to 20 mph. In New York City, 20 mph "slow zones" introduced in 2011 were credited with a 14 percent reduction in crashes resulting in injuries.

AFFECTED STREETS

The change is expected to affect most residential, non-arterial streets. This map from the Portland Bureau of Transportation shows all non-arterial streets.

-- Elliot Njus

enjus@oregonian.com

503-294-5034

@enjus