I really like the charts from Doug Short showing the total number of vehicle miles driven per person. The data starts in 1971 and shows a very steady increase in the number of miles driven per person for 36 years, until it peaks in 2005 and begins a decline. In terms of absolute miles driven in the country, a plateau is reached in 2007.
Both of these are interesting trends because it shows that we are undergoing a major shift in transportation in this country. That there had to be an upper bound on both trends is pretty clear, but a decline in miles driven per person? And why so sharp of a decline? Of note is that all of these peaks predate the recession.