Recently I visited the largest green home development in the country, Civano, in Tucson, Arizona. Started in the late 1990s, it consists now of some 600 homes; developers expect to eventually construct 1,500 homes. The Civano development operates by a strict energy and water conservation code that cuts annual bills for electricity (think hot, dry, desert-climate air conditioning) by 50% and annual water use by 60%, compared with similar suburban developments in the area.40 Civano is also denser than most suburban developments. It has a community feel to it, resulting from two public swimming pools, a community center, several small businesses (including a wonderful nursery) and narrow streets that keep down the heat in summer and serve as traffic-calming devices. Density as a sustainable design virtue reduces energy use from automobiles and allows for a more walkable community.

Certain environmental benefits of density are unquestionable. Some have even argued that New York, especially Manhattan, is the greenest city in the country (certainly for per capita use of energy and gasoline) because so many people take public transit, not only to work, but for everyday errands, shopping and leisure activities. My late father-in-law lived his entire adult life in New York City. Well into his 90s, after a car accident forced him to give up driving, he got along just fine by walking, taking the bus or subway, occasionally hailing a cab for city travel or hiring a limousine for day trips. Lots of New Yorkers don't own a car at all because of all the costs and trouble of ownership.

Having lots of high-rise residential and commercial buildings is also energy efficient, because the ratio of exterior walls to total occupied space is lower, reducing the potential for gaining or losing energy. On the basis of annual energy-use per square foot of floor space, such buildings can be much more energy efficient than a single-family home or a low-rise office or apartment. They can also support a qualified operations and maintenance staff, helping to keep equipment in good shape.

The drawback to cities is, of course, that they are largely devoid of nature. Imagine a New York City without Central Park, or any large city without parks and street trees. Additionally, cities tend to have lots of sun-absorbing asphalt, brick and concrete surfaces, which heat up during the day and don't cool off rapidly at night. This means air conditioning is required on summer afternoons and evenings, even when air temperatures should be dropping and breezes increasing. Many cities are now 8°F to 10°F hotter on summer evenings than they were 30 or 40 years ago, causing urban energy use to rise.

But cities may also be healthier than suburbs because people tend to walk more. One of the classic comedic movie scenes I recall is in Steve Martin's L.A. Story. It opens with him pulling his car out of the driveway one morning and driving to the house next door, getting out and going in for a cup of coffee. Having grown up in Los Angeles, with its dominant "auto-erotic" car culture, I can tell you this imagined scene isn't too far from the truth! We are all much less healthy these days because of excessive driving and infrequent walking.

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