Dave Roberts makes a good point
-- gas prices have to get punishingly high to make a serious dent in people's driving habits because most people don't have the option of not driving so much. There's this pervasive "Ten things YOU can do to save the Earth!" mentality that makes us over-focus on pushing individuals to make different decisions. But our decisions are highly constrained by collective, infrastructural conditions like the presence or absence of public transit and bad zoning laws and the sprawling development that they cause. I suppose if you incentivize people enough they'll vote for government action to change those collective conditions, but if government policy is your incentivizing lever, why not just cut to the chase?
In related news, here's a case of incentives gone awry: I'm intending sometime in the next week or two to drive an hour to Mesa in order to ride the new Phoenix light rail just for the sake of riding it. I doubt I'll ever actually use it for getting around because it doesn't help me get anyplace that I need to go.