That shift is good if you're among the remaining rush-hour drivers or someone with easy train access. It's not so good if you're a state road builder relying on gas taxes to fill potholes and fix bridges.

Traffic on the nation's roads fell 3.7 percent in May from the same month a year ago, the third-largest monthly drop in 66 years of record-keeping.

An even bigger share of Utahns parked their cars, the Federal Highway Administration reported Monday. Utahns drove a collective 2.13 billion miles in May, down 4.4 percent, or 97 million miles, from May 2007.

The Beehive State's decline was the second highest among 13 Western states. Only Nevada, at 4.8 percent, saw a steeper plunge in driving.

Salt Lake City attorney Ben Bates is one face of the evolving rush hour. Stepping off of a FrontRunner train Monday with a briefcase strapped over one shoulder and a pair of RollerBlades slung over the other, the Farmington resident said the commuter train saves on gas and vehicle wear, but not time. He switches to the skates to reach his downtown office so he isn't at the mercy of the TRAX light-rail schedule to finish his journey.

He figures FrontRunner takes 10 minutes longer than his car commute, though it's time he uses to review documents and e-mails.

"The only thing that will cause me to drive a car," Bates said, "is if I have a meeting in a place that's hard to get to."

And when Bates does drive, he notices that traffic is thinner than before FrontÂRunner cranked up in April. Last year he would have been mired in traffic whenever he headed north before 6:30 p.m., he said, but now rush hour dissipates before that.

The thinning traffic is perhaps most pronounced on Interstate 15 in Davis County, a corridor that's about to clear out even more when the four-lane Legacy Parkway opens in September.

Utah Department of Transportation traffic sensors on that stretch of I-15 are tracking 6,000 fewer vehicles every day since FrontRunner started. Traffic monitors say rush hour may have shortened by 15 minutes so far.

The upshot, from UDOT's perspective, is that the road fund for fiscal 2009 is heading for a $10 million to $20 million shortfall because of a decline in gas-tax receipts, spokesman Nile Easton said. In coming weeks the department's five regions will prioritize maintenance projects and decide what can be pushed back to next year.

The funding gap isn't attributed just to those who quit driving, Easton said. Those remaining on the roads are turning to more efficient cars.

"People are putting away their SUVs," he said.

Federal transportation officials say the trends will require a new funding mechanism for decaying highways and bridges. Without a change in revenue sources, the federal road fund that taxes 18.4 cents on every gallon of gas and 24.4 cents on diesel will run dry next year.

"Less driving means less money for the Highway Trust Fund," acting Federal Highway Administrator Jim Ray said in a written statement. "The status quo cannot and will not work in the 21st century."

It's not so bad in Utah, Easton said, but only because the state puts so much of its own money into roads. With help from a 24-cent state gas tax, Utah pays for most of its big improvements such as bridge replacements and freeway reconstruction. It's the smaller maintenance projects that suffer.

Still, without a funding change, he said, "you'd see a lot more bumpy rides."

So far this year, Utah Transit Authority trains and buses have provided about 1.2 million more rides than they did for the same fraction of 2007, a spurt of nearly 7 percent.

bloomis@sltrib.com