How expensive does gas have to get before you have to start making lifestyle-reduction choices?

Let's say you want to take a roadtrip, like to goneMOAB and let's say Moab is 1200 miles from your home. Assuming 16mpg, that's 150 gallons roundtrip. At $2.50/gallon that's $375 for gas just to get to Moab. That doesn't include gas consumed while wheeling. If gas goes to $3.00/gallon, that's $75 less you have for stuff like food, etc.

I wonder how high gas will have to get to significantly reduce driving-based tourism in the US...