According to the most recent report on traffic volume trends from the Federal Highway Administration (Dept. of Transportation), "Travel on all roads and streets changed by -1.7% for January 2008 compared with January 2007. Travel for the month is estimated to be 226.7 billion vehicle miles."

The January 2007 to January 2008 decline of -1.7% in miles driven was the largest percentage drop in January traffic volume in at least 16 years (see chart above, data available here and here, back to 1992). One explanation could be that consumers are responding to higher gas prices by driving less. The biggest declines in January traffic volume were in the West (-2.9%) and the South Atlantic (-3.1%), see map below.