Not everyone changes their habits based on gas prices though apparently. Brookings reports that most people just continue with their daily lives- including their driving habits- and cut back in other areas to save money when gas prices increase. That's because many pick up kids, grocery shop, and do everything else by car and don't believe they have another option just because gas is more expensive.
The graph below (click toenlarge) shows the close correlation between the price of gas and how much the average American family spends on it, in comparison to other expenses. As you can see, the two pretty much move together.
The report also says that the amount spent on gas is a fairly small portion of all household expenses, representing only about five percent of total household spending.