SEATAC, Wash. » On Sunday, the county transit system for the Seattle metropolitan area began hurtling down a road that few cities have traveled before: pricing tickets based on passengers’ income.

The project, which is being closely watched around the nation, gives discounts on public transportation to people whose household income is no more than 200 percent of the federal poverty level – for instance, $47,700 or less for a family of four under the 2014 guidelines. The problem it addresses is that many commuters from places like SeaTac, an outlying suburb, are too poor to live in Seattle, where prices and rents are soaring in a technology-driven boom. If these commuters are pushed out so far that they cannot afford to get to work or give up on doing so, backers of the project said, Seattle’s economy could choke.

"I would characterize this as a safety valve," said Dow Constantine, the King County executive and chairman of Sound Transit, a transportation agency serving multiple counties in the region. From 1999 to 2012, Constantine said, 95 percent of the new households in King County have been either rich or poor, earning more than $125,000 a year or less than $33,000, with hardly anything in between.

"It’s people doing really well, and people making espresso for people who are doing really well," he said.

At 12:01 a.m. on Sunday, buses, trains and passenger ferries in the county began charging only $1.50 per ride – more than 50 percent off peak fares – to riders like Basro Jama, who lives in Tukwila, just south of Seattle.

Jama, 27, an immigrant from Somalia who is raising two young children by herself, earns less than $25,000 a year after taxes from her full-time job cleaning office buildings in downtown Seattle at night. She enrolled last week in the first wave of sign-ups for the ORCA Lift reduced-fare program, which transit administrators said could reach perhaps 100,000 people. (ORCA, or "one regional card for all," is King County Metro Transit’s name for its fare card.)

ORCA Lift is run by King County Metro Transit, but the discount will apply to all passenger public transit in the county, including that of other agencies, like Sound Transit.

The program hinges on smart-card technology, an aggressive outreach effort by King County officials to people like Jama, and a liberal political establishment that believes the region’s economy is unbalanced and vulnerable in its growing divisions of poverty and wealth. Politicians and voters have raised the minimum wage based on that argument, with an increase to $15 an hour, more than twice the federal level, approved by voters in SeaTac in 2013 and by the Seattle City Council in 2014.

The reality of public transportation in America is that almost all of it is heavily subsidized by government, no matter how rich or poor the riders are. And those budgets, not least in Seattle, have been under severe stress. More than 70 percent of the nation’s transit systems cut service, raised fares or both during the recession and its aftermath, according to the American Public Transportation Association, a trade group. King County Metro Transit has raised fares six times since 2008, including an increase of 25 cents that kicks in on Sunday.

But income-based pricing is logistically complicated, which is partly why it has rarely been tried on any large scale, transportation experts said. San Francisco, which many in Seattle see as a kind of big brother to the south – sometimes to be emulated, other times to be scorned – got there first with a fare program called Muni Lifeline, which started in 2005. But after 10 years, Muni Lifeline remains tiny, with fewer than 20,000 card holders in a system that serves about 350,000 people a day.

Smaller, tentative experiments are underway elsewhere. Greene County, Ohio, near Dayton, recently started a program for low-income riders, with social service agencies buying travel vouchers and distributing them to their clients. In other places, like western Pennsylvania, nonprofit groups have jumped in to provide bus service to the poor.

But at least for the moment, all eyes are on Seattle, transportation experts said.

"What Seattle has done is what others might consider," said Art Guzzetti, vice president for policy at the American Public Transportation Association. "Everyone is watching."

The distinctions start with scale and ambition: King County has about two and a half times San Francisco’s population, and, in aiming for enrollment numbers San Franciscans could only dream of, it is relying on what transit experts say is the most innovative idea of all: tools honed by the Affordable Care Act. A countywide system of more than 40 health clinics, food banks, community colleges and other sites run by nonprofit groups was put together to enroll residents in health insurance, and those partners were re-enlisted in the last few weeks to start registering people for ORCA Lift.

That is how Jama got her little blue card. She went to a health clinic here in SeaTac on Tuesday morning to have a doctor look at one of her children. While she was there, a worker for a nonprofit called Global to Local – a member of the ORCA Lift team tasked with connecting residents to health and social services from a kiosk in the lobby – asked Jama if, by chance, she was a transit rider.

Jama said yes. She also happened to have a recent pay stub in her pocket, verifying her income. Fifteen minutes later, she had the card in her hand.

Though it works like a regular transit pass, the card will remove only $1.50 for a fare. It is good for two years without the need for users to reconfirm their income. County officials said they did not anticipate big wage increases.

"What’s the trick in it?" Jama asked softly, glancing down at the card and up at Amy Samudre, the program manager at Global to Local who had processed her application.

"No trick," Samudre replied with a little shrug.

The discount ORCA Lift provides might sound trivial to some, but for Jama, it is clearly not. The $10 or more a week that she will save on commuting represents a raise of almost 2.5 percent in her take-home pay. She said she might use the money for a special treat.

"That’s like taking my kids to McDonald’s," she said. "Two Happy Meals is $9."

Kirk Johnson, New York Times

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