Seattle, like Denver, is experiencing an economic boom that's bringing jobs to the area but also pushing the cost of living up and low-income residents out to distant suburbs. 

To help those workers afford increasingly pricey commutes, the area's transit authority began on Sunday to price bus, train and ferry fares based on travelers' income. 

It's a move designed to ward off future economic problems, The New York Times reports: 

“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.

In Denver, low-income and homeless Coloradans are upset about proposed fare increases on the area's mass transit system.