WASHINGTON, July 5 (UPI) -- U.S. drivers put in 30 billion fewer miles behind the wheel in the past six months than they did in the same period last year, the federal government estimates.

Meanwhile, the number of people using mass transit rose by 3.3 percent in the first three months of the year, McClatchy Newspapers reports.

The reason for both trends is obvious -- gas prices that average more than $4 per gallon. The American Automobile Association estimates that a family with two cars will spend $6,200 a year just on gas.

"That's unsustainable. At some point, the math begins to not work," said Stephen Reich of the Center for Urban Transportation Research at the University of South Florida.

Reich and Alan Pisarski, a transportation consultant, are reluctant to declare an end to the U.S. love affair with the automobile. Pisarski points out that only 1 percent of the trips in the country are now done on public transportation, so even a big jump in transit use has little effect on the big picture.

In fact, the number of commuters using public transportation now is about the same as it was 50 years ago, when the population was 60 percent of its current size.