Krugman made a couple of interesting points:

First:

Public transit, in particular, faces a chicken-and-egg problem: it's hard to justify transit systems unless there's sufficient population density, yet it's hard to persuade people to live in denser neighborhoods unless they come with the advantage of transit access.

Which is a good argument to fund transit before it would be completely usable, as a government-led way to increase density and walkability.

Second:

And there are, as always in America, the issues of race and class. Despite the gentrification that has taken place in some inner cities, and the plunge in national crime rates to levels not seen in decades, it will be hard to shake the longstanding American association of higher-density living with poverty and personal danger.

To be put under the heading of our "national conversation about race", are white suburbanites hesitant to move into cities because...why?  Or is it because of bad public education, which means that improving public education is an environmental issue, because the transportation system is more efficient if people are living in cities, and people will be more willing to live in cities if the education system is decent.