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 they 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, Mr. 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.