To get people out of their cars, you have to do two things. First, you have to create enough density to make transit, walking, and bicycling conceivable, and, second, you have to make driving sufficiently expensive, inconvenient, and unpleasant to force people to consider alternatives. As Portland and Seattle have discovered, you don’t get people out of their cars just by building attractive transit systems. Washington D.C. has a beautiful subway system, but no one with a car feels compelled to take the train because there’s always a place to park.

Anyone who has spent any time in Manhattan has had the experience of being stuck in traffic in a taxicab and watching a little old lady on the sidewalk overtake them and disappear into the distance. That’s a very green experience. Traffic jams are underappreciated by mainstream environmentalists.

Environmentalists and urban planners sometimes say that, in order to get people out of their cars and onto their feet, developed areas need become more like the country, by incorporating extended “greenways” and other attractive, vegetated pedestrian corridors. It’s true that such features, along with parks and natural areas, can encourage some people to take walks. But, if the goal is to get people to embrace walking as a form of practical transportation, oversized greenways can actually be counterproductive. Walking-as-transportation requires closely paced, accessible destinations, not broad expanses of leafy scenery. If you want to see people moving around under their own power under the sky, don’t go to the country or the suburbs; go downtown.

Because urban density has such high environmental value, we must find ways to shift new residential and commercial development away from places where population growth and economic growth exacerbate critical environmental problems and toward places where population growth and economic growth help to relieve them. For American cities, that will mean first understanding and then extending the benefits of population density and the thoughtful mixing of uses, as well as acknowledging that in a dense city the truly important environmental issues are less likely to be things like solar panels on building roofs than they are to be old-fashioned quality-of-life concerns like education, culture, crime, street noise, bad smells, resources for the elderly, and the availability of recreational facilities—all of which affect the willingness of people to live in efficient urban cores rather than packing up their children and fleeing to the suburbs.