At what gas price would Americans make changes?

From a comment on The Big Picture: More on Gas Prices:

At what price would gas affect consumer behavior?

I think gas in my birthplace, Quebec (Canada - still), goes for about $6/gallon US. You don't see a lot of SUVs there.

I suspect the price in BC is similar. It would be interesting to compare the per-foot costs of suburban vs. urban homes in Vancouver vs. Seattle. If $6/gallon gas changes behavior then the suburban/urban per foot costs ratio in Vancouver area should be smaller than in the Seattle area. (One might have to adjust for tax structure differences.)

I've long guessed that US middle-class behavior will change significantly (mass transit choices, preference for urban living) when gas hits $7/gallon. Can't say why, but I figure it's about the adjusted Canadian price. Mostly it would be a trend to smaller homes, smaller cars, urban condominiums, etc with shorter commutes or access to transit. Terrorism or social unrest could counteract a trend to urbanism though, so probably the most reliable marker of changed behavior would be the obvious and dull one -- smaller cars.