King County Metro Transit says it will increase its fleet by 625 battery-powered electric buses over the next 20 years and house them in Tukwila and a new base in the Green River Valley.

The plan represents an ambitious effort by the nation’s sixth-biggest public bus agency to reduce fossil fuels, following Metro’s renewal of its 174-coach Seattle wire-powered trolley-bus fleet in 2015, and its early adoption of diesel-hybrid buses in 2007.

Metro carries 122 million passengers annually with its 1,500-bus fleet. The agency says the expanded fleet will be carbon free by 2040 as older buses are replaced.

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Diane Carlson, capital-projects director, announced Wednesday that testing is underway to confirm battery buses can travel 140 miles — the distance a bus typically travels each day — between overnight charges. They’ll have to climb from the Green River Valley to International Boulevard South, east into Kent neighborhoods, and even up Seattle slopes.

Three finalist sites have been picked for a 250-bus base in Kent or Auburn, on industrial or commercial land. Metro chose the south county to spread its buses more evenly, which reduces time “deadheading” vehicles between south-county customers and the seven existing bases, farther north.

“Operating a zero emission bus base in South King County will provide cleaner air to a part of the region where residents have historically experienced a greater share of pollution from cars and trucks,” County Executive Dow Constantine said in a statement.

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Metro expects to spend $25 million for land and $480 million overall to open the valley base by 2030. Metro will build a related annex near the existing South Base in Tukwila by 2025, near the Duwamish River, preceded by an interim Tukwila base with electric-bus chargers around 2020.

Metro has been operating 11 Proterra quick-charge buses, costing about $975,000 each, that repower at Eastgate to circulate on Bellevue routes. Meanwhile, Washington state will gradually apply its $112 million share of the Volkswagen emissions-fraud settlement to help local agencies buy cleaner vehicles. That includes 50 electric transit buses in six counties to date, the Ecology Department said.

Cleaner school buses and port trucks also will be funded.

New battery-powered buses support the Metro Connects program, to increase countywide service 70 percent by 2040. San Francisco, Toronto, Denver and Los Angeles are also trying to electrify their fleets, a Metro study says.

Buses from New Flyer, Proterra and BYD will be tested this year, followed by initial orders, spokesman Jeff Switzer said.

Carlson said additional base capacity and bus purchases planned through 2030 should be affordable under existing sales taxes, but she can’t give a definitive answer, because Metro writes capital-spending plans only six years ahead. Expansions after 2030 would require new taxes.

Transit executives say new maintenance space will solve a shortage so acute that workers lose time maneuvering buses in crowded lots, sometimes leaving late for their routes. Metro last year was unable to add enough buses at its seven bases to satisfy all of Seattle’s peak-time service requests, or match Sound Transit’s future Eastside growth.

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Metro is the nation’s only major public bus provider to increase customers this decade, about 10 percent. But ridership grew less than 1 percent last year.

Carlson said more electric buses and service will create more ridership. Transit executives and operators say worsening congestion forces Metro to deploy more hours and vehicles to sustain frequent trips. The Puget Sound region is projected to add 1 million people by 2040, Switzer said.

Finalist sites for the new bus base are:

  • Kent, 25 to 38 acres at South 196th Street and 68th Avenue South, zoned industrial.
  • Auburn, 18 to 26 acres at South 277th Street and D Street, a former Valley 6 drive-in theater.
  • Auburn, 38 acres at 37th Street Northwest and B Street Northwest.

Metro has posted a Bus Base Expansion website, and soon will begin public outreach. So far, Auburn city officials don’t have a site preference and haven’t heard community opposition, communications director Kalyn Brady said. The former theater site is near wetlands, she said.