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Back when fuel prices topped $3 a gallon, the Utah Transit Authority imposed temporary fare surcharges to help cover its extra costs. It also said high gasoline prices made many people abandon cars for buses and trains.

So with recent low fuel prices, how much is UTA saving on diesel — and should that lead to lower fares, at least temporarily? And are lower prices at the pump prompting some transit riders to return to their cars?

Despite repeated requests by The Salt Lake Tribune for an interview on the topic, UTA officials did not grant one.

The agency provided some information by email, but did not directly answer some key questions — including whether low fuel prices should or could reduce fares.

UTA does insist that lower gasoline is not hurting ridership so far. It also acknowledges some fuel savings, but sees that as only temporary.

The agency uses about 6.2 million gallons of diesel per year, according to an email from Jerry Benson, UTA vice president of operations.

In December, as fuel prices plunged, Benson said UTA's diesel cost was $2.33 a gallon — a savings of $1.07, or 31 percent, below the $3.40 it had budgeted.

Savings were even bigger in January, as UTA paid $1.78 a gallon for diesel, Benson said. That was 46 percent, or $1.52 a gallon, less than the $3.30 it had budgeted for that month.

A savings of $1 to $1.50 per gallon could add up quickly for the agency that buys around a half-million gallons of diesel a month.

Of course, fuel prices have been rising again. Benson wrote that UTA's costs were back to $2.20 a gallon as of March 10, still well below prices UTA had been using recently in budgets.

The Tribune asked if UTA would consider lowering fares if fuel prices stayed low over an extended period.

Spokesman Remi Barron did not provide a direct answer, responding in an email, "We don't base operational decisions on fuel-price fluctuations. We may save money from time to time on fuel, but we can lose money when fuel prices go up."

In 2008, 2010 and 2011, UTA imposed fare surcharges of 25 to 50 cents when diesel prices had averaged more than $3 a gallon in the previous three months — and removed them after prices fell.

The agency changed that policy in 2011 so fuel surcharges may be imposed when diesel reaches $4 a gallon. An extra 25 cents may be charged when diesel is between $4 and $4.99 a gallon, 50 cents if it is between $5 and $5.99, 75 cents if it is between $6 and $6.99 and so on.

The UTA Board has created a fund to set aside some savings when fuel costs are low to help it better afford periods when prices rise rapidly. Asked for details and how much may be set aside now, Barron wrote, "Check your notes from last year," and said the fund was not new.

Meanwhile, UTA says ridership seems not to have been hurt by riders returning to cars while fuel is cheap.

"We are not seeing a clear impact of lower fuel prices on ridership," Benson wrote. "December 2014 ridership was slightly higher than December 2013."

But in January, public data show that ridership dipped from the previous January, by 0.2 percent. February and March data are not yet available publicly.

Barron said that January decrease is a bit deceptive.

"Average weekday, average Saturday and average Sunday ridership were all up in January." But he said "January's total ridership was flat over the previous year because there were fewer weekdays of operation" and more weekend days, when ridership is lower. January 2014 had eight weekend days compared with nine this January.

Any assertions "that lower fuel prices have hurt UTA ridership are not right," Barron said in an email.

A study by the American Public Transportation Association in 2012 says gas prices strongly affect transit ridership. "Analysis shows that 44 percent of the variation in ridership can be explained by changes in the price at the pump. This is considered a strong correlation and is statistically significant."

Nationally, APTA consistently has seen more transit riders when gas prices rise, and fewer when prices drop, though the study notes there often is some lag time.