Legislators signed off Wednesday on an experimental program to help reduce air pollution by offering free fares during bad air days on Utah Transit Authority buses and trains.

HB353 originally proposed to spend $1.2 million to fund 17 free-fare days over three years. But the final version cut funding to $500,000, which would fund about seven free fare days. UTA estimates that each one costs about $70,000 in lost cash fares.

“It's basically to get people to try mass transit,” Sen. Curt Bramble, R-Provo, told the Senate, as it approved but amended the bill on a 21-0 vote. The House later concurred with the change on a 44-30 vote, and sent the bill to Gov. Gary Herbert for his consideration.

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

Unlike two free-fare days UTA offered earlier this month — which occurred on relatively clean air days — the future ones would be triggered by forecasts of an inversion.

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 Herbert.

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.

The free-fare days earlier this month 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.