A couple of charts from the government are very illuminating on the subject of how Americans have responded to increasing gasoline prices. The first chart shows the moving 12 month average of miles driven on all US roads; the detail data shows that between Jan 1, 2005 and May 31, 2008 the change in this number has been essentially zero. This number has essentially been in a flat range since December 2004. So US drivers were modifying their driving well before gas prices got to $4.00 per gallon; as is demonstrated by the second chart showing gas prices.
The second chart shows the trend in gasoline prices from mid-2007:
So when you combine no growth in miles driven and consumers shifting to more fuel-efficient vehicles you get negative change in demand for gasoline.