In the latest reflection of $4-a-gallon gas, Americans drove 9.6 billion fewer miles in May than in May 2007, a 3.7 percent decline that is the largest drop in miles traveled for any May, the federal government reported yesterday.

Traffic normally increases in May, which ends with the Memorial Day holiday weekend that marks the traditional start of the summer driving season.

The decline was the third-largest monthly drop in the 66 years that the Federal Highway Administration has tracked miles driven.

The data released yesterday show that Americans drove 29.8 billion fewer miles in the first five months of this year compared with the same period last year, a 2.4 percent drop. The dip continues a seven-month trend beginning in November. Americans have driven 40.5 billion fewer miles from November through May compared with the same period a year earlier.

Some analysts, including Alan Pisarski, author of "Commuting in America," link the cutbacks to high gas prices and a slow economy.

Transportation Secretary Mary Peters said the continued decline in driving signals long-term changes in transportation habits and highlights the need to find new ways to pay for roads and bridges. "Fewer trips are being made to conventional service stations, and we expect that to continue," Peters said. "Our approach to transportation in America needs to change."

The federal Highway Trust Fund, which funds road construction with gas taxes, faces a $3.1 billion shortfall in fiscal year 2009 because drivers are buying less gas. Peters plans to unveil a plan in Atlanta today that will "refocus, reform and renew" the government's approach to transportation funding.

If the trend continues, with Americans driving less but transit ridership surging, Peters said she expects priorities to shift. Now, most of the 18.4-cent-a-gallon federal gas tax goes for highways and a much smaller amount for transit. "I would see more money going to transit and less to highways in the future," Peters said.

Pisarski described the "real impact" of the decline in miles driven: "Those trips that are not being taken are directly affecting the economy."