UPDATE 6/13/07:John Norquist gets the issue exactly right in a recent-ish op-ed. Here’s the crux:

In the next 30 years, our country will build 70 million new dwellings somewhere. With urban life emerging as a market favorite, it’s looking more as if building a good portion of them in livable, walkable traditional neighborhoods is one of the most convenient – and effective – remedies for the inconvenient truth.

In some ways, this new policy in California just seems incredibly obvious: of course low-density car-dependent sprawl makes it difficult to reduce greenhouse gas emissions. The energy impacts of sprawl—both in transportation and in residential consumption—are amplydocumented. So places like California that have serious pledges to reduce climate changing emissions more or less must take steps to corral sprawl and provide alternatives.

But I guess I’m wrong: it must not be very obvious. Climate change is seldom mentioned in discussions about transportation or land-use. Case in point: Puget Sound is considering a massive increase in road capacity. And so far scarcely anyone’s bothered to mention just how difficult that’s going to make meeting the governor’s vaunted climate targets.

Building new road capacity in lightly developed areas is like begging for sprawl—and that directly undermines our attempts to put the brakes on greenhouse gases. It’s a bit like promising to go on a diet and then taking a job at Krispy Kreme.