Seattle’s push for 25 mph speed limits will go far beyond downtown, as City Council members look to slow down the neighborhood arterials.

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As the Seattle City Council prepares to set a 25 mph speed limit downtown in November, members are also seeking to slow neighborhood arterials, stretching to all four corners of town.

Councilmember Mike O’Brien proposed last week that transportation staff identify, by March, more streets that can be reduced to 25 mph, without lengthy process.

“I would hate to keep those at 30 mph any longer than they need to be,” he told the Sustainability & Transportation Committee, which passed his amendment 4-0.

As announced in mid-September, the council is expected to vote Monday on changing the 30 mph arterial speed limit to 25 mph this fall for downtown, Uptown, South Lake Union, Capitol Hill, First Hill and the Chinatown International District.

Residential-area speed limits would drop citywide from 25 mph to 20 mph. Also, signs at the city limits would tell drivers that the default limit for all arterials would fall from 30 mph to 25 mph, unless otherwise posted.

The default speed of 25 mph is meant to cover most neighborhood arterials. But as a practical matter, drivers will be confused.

O’Brien’s proposal would, in many cases, lead city transportation crews to install 25 mph signs and remove 30 mph signs.

The March report would set the stage for the Seattle Department of Transportation (SDOT) to change road designs, such as striping, curbs or plantings, to help calm traffic to 25 mph, rather than rely solely on signs.

They haven’t named specific roads.

Possible candidates are easy to imagine, where two-lane roads mix passing traffic with bicycles and pedestrians: Fifth Avenue Northeast in Maple Leaf; 15th Avenue South atop Beacon Hill; Southwest Thistle Street at Chief Sealth International High School; or 34th Avenue West in Magnolia, are some examples.

Such reduced speeds, which also will take effect soon in Boston, are the latest milestones of a “Vision Zero” movement to eliminate traffic casualties by 2030. Seattle averages about 20 traffic deaths and 170 severe injuries per year.

Speed signs don’t necessarily achieve much — especially in Seattle, where the 60-officer traffic-enforcement unit is thinly spread. An exception is four-lane Admiral Way Southwest, where 45 mph driving was routine a decade ago. Nowadays, uphill drivers often slow to 30 mph when they see the full-time speed-display readerboard, combined with frequent police-speed traps.

But the city’s 30 mph signs in Interbay, on six-lane 15th Avenue West, have done little to reduce speed, city transportation Director Scott Kubly told O’Brien’s committee.

Overwhelming evidence confirms the lower vehicle speeds make it far less likely that a pedestrian will die or suffer severe injuries in a crash. Someone hit at 20 mph has a 95 percent chance of survival, which falls to 60 percent at 30 mph and 20 percent when hit at 40 mph, according to studies reviewed by the National Highway Traffic Safety Administration. Dr. Beth Ebel, director of the Harborview Injury Prevention and Research Center, has long cited similar figures in endorsing lower speeds in Seattle.

Lower speeds also give a driver more time to react, as well as a wider peripheral field of vision, to avoid hitting people or vehicles that suddenly enter the street.

But not much data exists yet to prove whether the latest wave of urban speed-limit reductions will save lives, though early findings in New York look positive. Pedestrian traffic deaths there in 2015, soon after the city imposed 25 mph limits, totaled 134, the lowest on record, reports Mayor Bill DeBlasio.

But severe crashes increased on some streets, leading an analysis by the New York Daily News to label the performance “mixed” a year ago. Activists urge DeBlasio to work faster on street redesigns to slow traffic, and reduce crossing distances for pedestrians.

In some cases, New York City slapped 25 mph limits on boulevards much wider than proposed in Seattle this fall, and council members in Queens complained traffic diverted to side streets. The SDOT safety team, like New York, is studying not just simple two-lane arterials for reduced speeds, but wider boulevards, such as Lake City Way Northeast.

London studies show a roughly 40 percent drop in deaths and severe injuries for all users, as a result of its 20 mph default speed limit.

It’s unclear to what extent a lower speed ordinance in Seattle might save lives.

“Frankly, the city is wasting money,” says Mark Jacobs, a Seattle-area traffic engineer and consultant, active in the West Seattle Transportation Coalition. Distracted driving, impaired driving and fast drivers more than 10 mph above normal are bigger threats, he said. “SPD struggles to enforce traffic now.”

As if to make his point, SDOT’s presentation to O’Brien’s committee included a photo of Leo Almanzor, killed in November 2014 in a hit-and-run near Westlake Center by someone speeding in a stolen car — not a scenario that new speed limits would prevent.

A study in Federal Way, cited by Jacobs, found that signs imposing lower speed-limits on four streets failed to slow down motorists overall, and at one spot, drivers zoomed at close to 42 mph in a 25 mph zone.

Lower speed limits alone cause only a 1 or 2 mph real slowdown, according to at least one federal study of rural and suburban roads.

A closer comparison is Seattle’s own Rainier Avenue South, where SDOT last year imposed a 25 mph limit through Columbia City, and trimmed other stretches from four lanes to three.

SDOT says collisions there are down 40 percent for pedestrians and bicyclists, while bus-trip times remain the same or faster. But the agency says it won’t release full data until next month.