Utah Transit Authority buses and trains usually prevent more pollution than they create by reducing single-rider car trips. But a new study says that is not always the case.

At times and places with low ridership — such as weekends, evenings and outside the crowded urban core — the emission-cutting benefits of UTA dwindle and even disappear, according to the study by University of Utah researchers Daniel Mendoza, Martin Buchert and John Lin.

Still, it says, UTA eliminates about 1.5% of all pollution created by vehicles along the Wasatch Front. While it may sound small, roughly “half of the total pollution here comes from vehicles” so cutting that by 1.5% is significant, Mendoza said.

Also, if the agency would replace some of its older dirtier vehicles with newer models, including those powered by clean diesel, natural gas and electricity, “that would significantly improve numbers,” Mendoza said. The study, published in Environmental Research Communications, said vehicle upgrades could cut some types of UTA emissions by 50% to 75%.

“With this information, UTA can go to stakeholders and funding agencies and say, ‘Look, we’ve done this analysis,' " Mendoza said. “This is how much less we can pollute” with new vehicles.

(Steve Griffin | Tribune file photo) The Salt Lake City skyline is obscured by dense smog as an inversion settles over the valley Tuesday Dec. 26, 2017.

The study provides more detail about exactly where and when UTA likely cuts pollution by incorporating data from electronic tap-on, tap-off fare cards — used by about half of all UTA riders — to show times and places that riders are avoiding single-rider car trips.

It also used data from machines that count other passengers and technology that tracks the location of transit vehicles in real time. Additionally, researchers used data about how much pollution is caused by different types of vehicles, plus state measurements of pollution by area.

Mendoza said the study found the emission-cutting benefits of transit vary by area and time of day and even time of the year. For example, benefits are less in the summer, when universities have fewer students. Such students are among UTA’s best customers. Lower ridership without them reduces pollution benefits.

“In the central parts of the service area,” essentially the most crowded urban areas, “and primarily during the peak hours and sometimes during the middle of weekdays as well, it [transit] is an emissions reducer,” Mendoza said.

But “in the outer parts of the service area and during very early mornings, late in the evening and on the weekends, that may not necessarily be the case.” Sometimes it creates more pollution there than it prevents.

He said pollution benefits also tend to be greater with TRAX — which uses electricity produced outside the Wasatch Front — and FrontRunner trains that carry more passengers.

(Lee Davidson | Tribune file photo) FrontRunner train arrives at Orem station as crowds wait to board on June 27, 2019.

Mendoza said a key for greater pollution benefits is to boost ridership and to use cleaner vehicles.

For example, he praised a new partnership in which Salt Lake City this month funded UTA increasing the frequency of several downtown bus routes plus extending their hours.

“It’s definitely an encouragement for those who may not have tried transit, or those who may have tried it and were disappointed,” he said. “It’s definitely one way to reduce emissions and to actually have a little bit more time on your hands because you don’t have to contend with the driving.

Mendoza himself is an avid transit user.

“Maybe 90% of my trips are through transit or walking," he said. “I try not to drive at all.”

His reasons include trying to be environmentally friendly and using the extra time on transit to read or do other work. He also said sometimes it actually saves time — if he does some planning.

For example, he recently had to travel at peak time, about 5 p.m., from downtown Salt Lake City to Sandy. “It took me 50 minutes — even using TRAX, FrontRunner and a bus.” Because of congestion, “it would have taken me 90 minutes to drive.”

He added, “So there are times when it is actually faster, especially if you’re taking TRAX or FrontRunner because they have their specific guideways and don’t have to really worry about traffic.”