updated 7/28/2008 11:32:27 AM ET 2008-07-28T15:32:27

The soaring price of fuel and other strains on the U.S. economy have caused Americans to cut back sharply on driving, which jeopardizes the federal fund for building and repairing the country's highways.

The federal highway trust fund — which relies on per-gallon taxes that don't rise with price — faces a multibillion dollar shortfall next year, down from a surplus of more than $10 billion just three years ago.

According to Federal Highway Administration data released Monday, Americans drove 9.6 billion fewer miles in May 2008 than in May 2007, the third-largest monthly drop in the 66 years the data has been collected.

The May decline continues a seven-month decrease in driving that has amounted to 40.5 billion fewer miles traveled since November 2007 compared with the same period a year earlier.

May's drop comes during a month that traffic usually rises due to the Memorial Day holiday and the start of the summer vacation season.

What can fund our bridges, highways?
Not only are Americans cutting back on their own driving. They are increasingly using fuel-efficient vehicles, carpooling and taking mass transportation.

Transportation Secretary Mary E. Peters said in a statement Monday that the drop in driving miles demonstrates that the federal gas tax is no longer sufficient to finance the nation's transportation infrastructure.

The highway trust fund gets 18.4 cents per gallon from gasoline sales and 24.4 cents per gallon for diesel sales.

"We must embrace more sustainable funding sources for highways and bridges through more sustainable and effective ways such as congestion pricing and private activity bonds," Peters said.

Last week, the House approved by a veto-proof margin an $8 billion infusion into the highway trust fund for the fiscal year beginning in October.

Reps. Jerry Lewis of California and Paul Ryan of Wisconsin, top Republicans on the appropriations and budget committees, have criticized the bill for redirecting funds from the general Treasury that should be used for other priorities.

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