Update: Traffic Volume
Thanks to John Thacker for a pointer to a longer historical data source for traffic volume back to 1971. There were two previous periods of significant decreases in traffic volume, comparable to the most recent 9-month, 2.08% decrease in miles driven (moving 12-month total) from November 2007 to July 2008.
There was a 11-month, 2.12% decrease in traffic volume from December 1973 to October 1974 (see shaded area in graph above), when gas increased from 40 cents to 55 cents per gallon. I think price controls were in effect then and prevented an even higher increase in gas prices, since oil went from $4.30 to $11.16 per barrel during that period.
A 3.25% reduction in traffic volume occurred over a 12-month period from May 1979 to May 1980 when oil almost tripled from $15 to $40 per barrel, and retail gas prices almost doubled from 62 cents to $1.11 (see shaded area in graph).
Therefore, I stand corrected - there were several previous periods in U.S. history with greater reductions in traffic volume than the recent 9-month period of a cumulative 2.08% decrease in miles driven. With gas prices falling now ($2.68 in Missouri) in October, the trend may reverse, although we'll probably see monthly declines in August and September when those data become available.