SAN FRANCISCO (MarketWatch) -- Gas prices may be falling fast from record highs, but travelers are avoiding the highways and jumping on buses and trains at a record pace, according to a report from the U.S. Department of Transportation on Friday.

“ "Relying on the gas tax is like relying on cardboard to keep the rain out - the longer you use it the less it works." ”

In fact, Americans drove 15 billion fewer miles in August than they did in the same month a year ago, according to federal data, which puts the streak of declining driving habits at 10 months.

That represents a 5.6% decline from August 2007, marking the biggest year-over-year drop ever recorded, U.S. Secretary of Transportation Mary Peters said in a statement.

At the same time, public transit ridership jumped 6.2% across the country this summer compared with a year ago.

Peters pointed out that Texas, where she spoke during a visit to a light-rail station under construction in Dallas, saw an increase of 15% in its DART rail system.

Gas began its swift descent from record highs in mid-July and currently costs a national average $2.78 for a gallon of regular unleaded, according to recent numbers from AAA. That's down almost a dollar from a month ago and is more or less the same as it cost in October 2007.

The trend, likely exacerbated by the dismal state of the economy, is making it difficult for the government to pick up the tab. Peters warned that the lower income from gas taxes will make it difficult for the federal agency to continue to fund future projects.

"We pay for transit the same way we pay for road and bridge projects -- with federal gas taxes," she said. "Relying on the gas tax is like relying on cardboard to keep the rain out -- the longer you use it the less it works."