Arterial speeds in Central Seattle are 25 mph while residential side streets citywide have been changed to a maximum 20 mph.

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Seattle transportation workers spent Sunday installing 145 signs announcing the new 25-mph arterial speed limit in the central city — an attempt to reduce traffic injuries and deaths.

Warnings are expected to be issued for the first two weeks to a month, followed by traffic fines, said Norm Mah, spokesman for the Seattle Department of Transportation (SDOT). A basic speeding ticket costs $136.

• The former 30-mph limit falls to 25 mph for downtown and connecting central-city arterials, including those in the South Lake Union, Capitol Hill, First Hill, Chinatown International District, Belltown and Uptown areas. Arterials are generally the streets with more than two lanes, or two-lane streets divided by yellow striping.

• The residential speed limit of 25 mph falls to 20 mph throughout the city, including hundreds of unmarked roadways — in many cases, too narrow for drivers to safely exceed 20 anyway.

• The default speed limit of 30 mph for arterials drops to 25 mph citywide, “unless otherwise posted,” according to new signs at entrances to the city.

This last point may confuse drivers because some outlying streets that were historically 30 mph have sporadic signs, due to abuse and neglect.

The City Council unanimously approved the changes Sept. 26 and is looking to extend the program next year.

Mike O’Brien, chairman of the council’s Sustainability & Transportation Committee, asked SDOT to find arterials where the signs should be explicitly changed next year from 30 to 25 mph in outlying neighborhoods, not just the city core.

In Seattle last year, 44 pedestrians, 23 bicyclists and 119 people in vehicles were killed or severely injured.

New York City lowered speeds nearly two years ago to 25 mph, including some boulevards as wide as six lanes. Boston is doing the same next year.

New signs alone don’t slow drivers, as Victoria, B.C., is finding this year on five corridors where neither speeds nor crash rates changed in 1½ years of a 25-mph limit, CHEK television reported.

Many Puget Sound communities such as Kent, Shoreline and Mill Creek use a 25-mph default speed for residential arterials. Neighborhoods ask for police stings and traffic-calming barriers, which are also expected in Seattle.