After the Utah Transit Authority offered free fare two days last week on its buses and trains, the Utah House advanced a bill Monday that could provide 17 more free-fare days over three years as an experiment to help reduce air pollution.

It passed HB353 on a 47-26 vote, and sent it to the Senate for consideration. It would create a $1.2 million pilot program that would cover the $70,000 a day in cash fares that UTA figures it would lose when fares are free.

“We’re trying to change behavior,” said Rep. Joel Briscoe, D-Salt Lake City, sponsor of the bill. “Can we incentivize people to do something different than they normally have done?”

Unlike the free-fare days last week — which occurred on relatively clean air days — the future ones would be triggered on days that are expected to be especially dirty just before or during inversions.

Briscoe said the Utah Division of Air Quality and UTA will measure how many vehicles were likely removed from the road and the effect on the air — and help see if more free-fare days should be considered in the future. It is part of a $100 million package for clean-air legislation requested by Gov. Gary Herbert.

Briscoe noted that a survey by Envision Utah found that residents say the biggest incentive that could get them to use mass transit on bad air days is free fares.

UTA offered one free-fare day in concert with Salt Lake County two years ago, and said it increased ridership by 23 percent that day and removed an estimated 17,500 vehicles from the road, saving 3 tons of particulate pollution and preventing 200 tons of greenhouse gases.

UTA said results of its free-fare days last week will not be available for about a week.

The free-fare days last week were funded by $80,000 contributed by Salt Lake City, Salt Lake County, Davis County and Intermountain Healthcare. UTA also contributed by giving up its estimated $70,000 a day in cash fares.

Some bus systems in Utah — in Park City and Cache County — already offer free fares at all times. UTA has been offering it, thanks to a federal grant, on its new Utah Valley Express bus rapid transit system in Provo and Orem. It has said that has quintupled ridership from the old bus routes it replaced.