When I was in Japan (for that whole week!) although the statistics told me that it was very densely populated (especially cities like Tokyo) the actual city itself didn't feel that crowded.

My guess, is that they had less spare space, narrower streets, smaller gardens. Stuff like that.

The point about density and public transport is absolutely vital. Railways, above all, simply cannot exist below a certain density (unless they are heavily subsidised).

I haven't really made up my mind on these communities. The Kentlands one in particular I am fairly familiar with. The times I've been there, during the day, there's NOBODY around. I do agree with Patrick that population density is the absolute key factor in the support of a public transit system, but there's no chance of any kind of rail system going anywhere near there, and the Washington Metrobus system does go on the main road alongside, through all the same traffic that the people do (I'm extremely familiar with the roadway construction going on there), and I really don't see the people there riding the bus.

I think the main reason for Kentlands was to try to get the biggest profit out of this small portion of land in the DC suburbs. The land values go well over $500,000 per acre, and in that development, they were charging up to $1,000,000 per acre (for a 1/5 acre lot, and that's expensive even for that area). If the idea were implemented in less exclusive neighborhoods, where the point was really to get a higher density, and more neighborly feeling (I didn't get a neighborhood feeling there), I think it's a good idea, as an alternative to traditional suburban development. But if it's just going to be another way to make exclusive communities, I feel it will fail badly.

Kentlands (and other similar 'new' urbanist communities in the U.S.) are so expensive because there are so few of them. The market clearly shows that people want to live in this kind of environment (they're even willing to spend very large sums of money and to put up with the D.C. government in order to live in this kind of environment in places like Georgetown), but for a variety of well-meaning but entirely wrongheaded reasons it's generally illegal for developers to supply the housing product that's in demand in the United States. (A product, incidentally, that should also be more profitable for the developers.)

Places like Kentlands are not expensive in order to be 'exclusive' or because they're built on particularly expensive land; they're expensive because new construction in the United States means, almost exclusively, tract houses on a treeless plain or nasty apartment complexes built to the lowest possible standard.

The inherent cost of building at higher density is lower (to a point), since each housing unit has to pay for less land. If we stop restraining developers from satisfying the current pent-up demand for dense housing, eventually you'll wind up with more low-cost housing -- and it'll be low-cost housing that doesn't require that the residents all either own cars or spend large amounts of time wrestling with a mediocre transit system.

One more transport angle: aside from facilitating mass transit systems, denser neighborhoods make it possible to get places without using any transport at all other than one's feet. In most American suburbs built in the last thirty years (or more), it's nearly impossible to walk anywhere, so every trip out of the house (or office) is necessarily a trip in a car. Just creating a situation where it was possible to walk to a restaurant for lunch or dinner -- much less to a train that will take you into the city -- would be a big improvement.

I remember an American telling me a few years back that "Americans don't consider walking as a means of transportation". I was too polite to point out the connection between that and the fact she was virtually spherical in shape....