Since the end of World War II, most of the growth in U.S. metropolitan areas has taken place in the suburbs.1 In 1940, 48 percent of the U.S. population lived in a metropolitan area, and 68 percent of metropolitan area residents lived in center cities. By 1990 the first figure had grown to 78 percent, and the second had fallen to 40 percent.2 In 1970 the average urbanite lived in a community with 10,452 people per square mile. By 2000 urban population density had fallen over 25 percent, with the average metropolitan area resident living at a density of 7,358 people per square mile.3
This chapter examines the environmental costs of this trend, which has come to be labeled "urban sprawl." Sprawl, which I define as the migration of homes and jobs to low-density areas, poses several sustain-ability challenges. It typically increases land consumption and vehicle use, which in turn increases carbon dioxide production and requires the building of new roads. In addition, sprawl increases the proportion of middle-class households that are likely to oppose policies, such as expanding mass transit, that improve urban sustainability. Instead, these voters have a strong incentive to support policies that subsidize private transportation.
1. Margo (1992); Mieszkowski and Mills (1993); Glaeser and Kahn (2004).
2. Altshuler and others (1999).
3. This calculation is based on all census tracts within twenty-five miles of a major central business district.
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