Just to follow up on the point about public transportation in the previous post, it's no secret that Congress has always spent far more to promote driving than it's spent on public transit—note that the White House requested $40 billion for the federal highway budget in 2008, versus $1.08 billion for railroad funding. But that's only the beginning. While doing some searching around, I came across an old Brookings report from 2003, which usefully compared the funding process for highway and mass transit projects, and laid out some glaring differences.

Under current law, the federal government usually covers about 80-90 percent of the costs for a new highway project, compared with only 50 percent of the costs for a transit system. Local communities have to pick up most of the rest of the tab for public transportation, with state governments chipping in what's left. Since doing that usually requires raising property taxes, most local governments just prefer to build highways. (Indeed, some 30 states restrict their gas-tax revenues to highway purposes only.)

Moreover, transit projects have to undergo intensive scrutiny: a cost-benefit analysis, a land-use analysis, an environmental-impact analysis, and, usually, a detailed comparison among various alternatives. That all sounds pretty reasonable, except that highway projects don't have to undergo any of this—save for a (considerably less strict) environmental analysis—federal oversight is rather minimal. Highway money is basically a gift to states and local governments.

Not surprisingly, most communities find it far easier to build new highways than to set up, say, a light-rail system, no matter how popular the latter might be. (The Brookings report gives an example of a popular light-rail proposal in Milwaukee going down in flames for exactly this reason.) So, sure, any decent plan for reducing emissions and curbing gasoline use should include more money for public transit. But it also seems like a lot of funding rules need to be changed, so that transit and highway projects can compete on a more level playing field.

--Bradford Plumer