According to Mass Transit Mag:
"Secretary Peters said that Americans drove 9.6 billion fewer vehicle-miles traveled (VMT) in May 2008 than in May 2007, according to the Federal Highway Administration data. This is the largest drop in VMT for any May, which typically reflects increased traffic due to Memorial Day vacations and the beginning of summer, and is the third-largest monthly drop in the 66 years such data have been recorded. Three of the largest single-month declines – each topping 9 billion miles – have occurred since December.
This continues a seven-month trend that amounts to 40.5 billion fewer miles traveled between November 2007 and May 2008 than the same period a year before, she said."
To learn more, review the FHWA’s "Traffic Volume Trends" reports for May 2008.