An ordinance to lower central-city speeds to 25 mph unanimously passed Seattle City Council on Monday.

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Lower speed limits, including 25 mph through downtown Seattle, passed the City Council on Monday, paving the way for signs to change in November.

Arterial speeds will also be reduced from 30 mph to 25 mph nearby in Belltown, Uptown, South Lake Union, south Capitol Hill, First Hill and the Chinatown International District.

The ordinance also lowers residential roads to 20 mph. The initial cost is $375,000 for sign changes and education.

“I know some opposed this, but it is a really smart move to reduce deaths and injuries,” Councilmember Tim Burgess said before the unanimous 9-0 vote.

Council President Bruce Harrell repeated the “20 is Plenty” slogan popularized in cities such as London, which has reduced injuries 40 percent, and locally by the nonprofit Seattle Neighborhood Greenways.

Next year, the city will consider several “neighborhood arterials” beyond downtown, to redesign for 25 mph.

The council was eager to vote, despite inconclusive data.

In a recent test where 20-mph zones were marked, the Seattle Department of Transportation (SDOT) found 12 of its 22 checkpoints showed slight reductions and only two — both on Fairmount Avenue Southwest, a greenbelt road with speed bumps — slowed more than 1 mph.

“Lowering speed limits via reducing a number on a sign is a placebo,” said a letter to the council from Mark Jacobs, a local traffic engineer who obtained the draft SDOT findings. Traffic-calming designs like curbs or landscaped circles are needed, he said.

SDOT replied Monday “speeds were reduced in most of the pilot areas,” and several will see design changes soon.

Also, city staff members report 31 percent fewer injuries from speed and lane reductions on Rainier Avenue South, but won’t release full data yet.