Come wintertime, the same features that make the Salt Lake region an idyllic ski destination—tall mountains, lots of sunny, blue-bird days—can turn the lowlands into a smoggy nightmare. The metro area is surrounded on nearly all sides by mountain ranges that hold weather systems in place over the valley below. The area is also prone to temperature inversions, where a layer of warm air at higher elevations traps colder air down low. If the inversion gets locked into place, the air stagnates and the pollution and particulates from automobile emissions, chimneys and factories collect in a low-laying haze over Salt Lake County.

“Pollution gets trapped and builds and builds over a two- or three-week period and all of a sudden it looks like Beijing,” says Carl Arky, a Utah Transit Authority (UTA) spokesman. “It’s not very healthy. We see a lot of respiratory problems.”

In December, the Salt Lake region experienced one of its first bad inversions of the season. A soupy haze settled in. The Utah Department of Environmental Quality rated the air between orange (air unhealthy for sensitive groups) and red (unhealthy for everyone).

Automobile emissions account for more than half the pollution in Utah. As such, getting people out of their cars during “red flag” days can make an important impact during an inversion. On December 22, in an effort to do just that, UTA, Salt Lake City Council and the Salt Lake County Mayor’s Office partnered to fund a free fare day on all UTA buses, commuter rail and light rail.

The free fares had the desired effect. UTA saw a marked system-wide bump in ridership and estimates a big reduction in greenhouse gas emissions. But, free rides cost the agency money and, short of a dedicated funding stream or new partnerships, it doesn’t think it can afford to make free fare days a recurring event. Local transit advocates were happy to see the bump in ridership, but rather than focusing on free fares, think the agency should spend money to improve transit service in a way that will naturally draw in new riders.

On Free Fare Friday, ridership increased 23 percent across the entire system compared to the previous five weekdays. On FrontRunner commuter rail, ridership jumped 66 percent. On TRAX light rail, ridership increased 32 percent. Bus ridership remained mostly flat. In total, UTA had an extra 22,800 people riding transit. They estimate the event got about 17,560 cars off the road that day, which equates to 200 fewer tons of greenhouse gas emissions.

“We had a lot of people who either hadn’t used the system or hadn’t used the system in in a while. It shows that people do like public transit. It doesn’t mean there aren’t hurdles to getting more and more people using transit, but the response we heard was ‘we like this,’” says Arky. In a poll of riders on rail transit, UTA found that, “approximately 90 percent of riders who knew it was Free Fare Friday stated that they would ride UTA on another free fare day.”

Though the free fare day accomplished its goals, hosting another will require another inter-agency partnership, funding from the state or sponsorship from a company or foundation. UTA estimates the free fare day cost the agency about $70,000 in lost fare revenue and the increased operating costs from accommodating the bump in riders. In December, UTA, the city council and county mayor’s office split the bill.

“In a perfect world, we could do free fares—and hopefully we’re striving towards a more perfect world,” says Arky. “But it comes down to dollars and cents. If we can find the money, I’d imagine we’ll do this more often.”

Salt Lake is not the first city to experiment with free or reduced fares as a tool to address air pollution. Recently, Seoul made transit free during morning and evening rush hour for three consecutive days when air pollution got so bad that a layer of yellow dust settled over the city. Paris also temporarily made transit free and restricted driving to combat air pollution in 2016. In both examples, critics worried about the high cost of making transit free.

Still, transit advocates in Salt Lake City were happy to see the bump in ridership.

“We were pretty encouraged by it,” says Christopher Stout, Utah Transit Riders Union president. “Free fares worked and did what we thought they would do.”

But, Stout thinks that if UTA does somehow find a new revenue stream, free fares are not the best way to spend it. “Our focus has been primarily on trying to get them to expand the late night and weekend service and create a real high-frequency network. Even if they could find more money to offer free fares, the service would still have big holes in it.”

As is often the case, UTA’s bus service primarily serves 9 to 5 downtown workers. If you fall into that demographic, Stout says, you’re likely to be able to ride a bus line with 15-minute frequency. If you need to travel in the evenings or mid-day or early morning, you might need to wait 30 minutes or even an hour for your bus to arrive. “That leaves a significant hole for the trips mothers, seniors, students that are outside of the normal 18-24 age group had to take.”

Unless they have dedicated travel lanes, buses will always struggle to compete with cars on travel times. But the Transit Union thinks a revitalized bus network could help close the gap a bit. According to Stout, a bus trip across the Salt Lake Valley takes as long as an hour and a half, compared to about a half hour in a car.

“If the high-frequency service was there, we think the people would be there. Until they revitalize the bus system, ridership will be flatlined,” says Stout.

UTA may, in fact be getting a funding boost in coming years. The state legislature is currently considering a bill that would increase sales tax and impose new fees on electric vehicles to help fund mass transit.