Americans continued to drive less at the start of the summer, new Department of Transportation data shows.

Since last November, Americans have driven 53.2 billion miles less than they did over the same period a year earlier, topping 1970s total decline of 49.3 billion miles.

"We can’t afford to continue pinning our transportation network’s future to the gas tax," said U.S. Transportation Secretary Mary E. Peters. "Advances in higher fuel-efficiency vehicles and alternative fuels are making the gas tax an even less sustainable support for funding roads, bridges and transit systems."

Americans drove 4.7 percent less, or 12.2 billion miles fewer, in June 2008 than June 2007. The decline is most evident in rural travel, which has fallen by 4 percent -- compared to the 1.2 percent decline in urban miles traveled -- since the trend began last November.

The national average price of regular unleaded gas in June went from $3.97 to almost $4.10 per gallon.

Travel Industry Association spokeswoman Cathy Keefe said the June driving decline "is not surprising, given the environment that we were in." But she was optimistic that the recent drop in gas prices to below $4 a gallon in many parts of the country will have travelers on the road again.

Last month, Peters unveiled a transportation reform plan which offered lawmakers several options to consider when Congress takes up highway and transit legislation next year.

"It really makes little sense to try to upgrade our infrastructure using a revenue source as ineffective, unsustainable and unpopular as the fuel tax," she added.

As Americans drive fewer miles, less revenue is generated for the Highway Trust Fund from gasoline and diesel sales -- 18.4 cents per gallon and 24.4 cents per gallon, respectively.

During the first quarter of 2008, motorists consumed nearly 400 million fewer gallons of gasoline, or about 1.3 percent less than during the same period in 2007, and 7 percent less -- or 318 million gallons -- of diesel.