The mass production of the automobile gave new impetus to suburbanization in the early decades of the 20th cent., allowing commuters to live yet further from their place of employment. The automobile virtually eliminated restrictions on travel, and the subsequent demise of much public transportation left millions dependent on the automobile. State and local governments responded with massive road-building projects, and the federal government with a major expansion of the interstate highway system in the 1950s.
Congestion in the central city and a consequent deterioration of living conditions there provided additional incentive for people to move to the suburbs. In some cases, such migration diminished the city's tax base to the point that it could not afford to provide adequate services, inciting further suburban flight of business as well as population; some older U.S. cities, such as Philadelphia, Cleveland, and Detroit, have been especially affected by this trend, which became apparent soon after World War II.
The rise of modern suburbia has also been encouraged by the appeal of the suburban lifestyle, often characterized by an image of urbane society living graciously in an idyllic setting, where neighborhoods of single-family houses on large, private lots are combined with convenient proximity to the city's business and employment opportunities and cultural attractions. However, the traditional attractions of the central city are increasingly themselves located in the suburban ring. Most suburban communities in the United States grew spontaneously, although a few were carefully preplanned by architects and real-estate developers. These include, at the beginning of the 20th cent., Shaker Heights, a wealthy suburb of Cleveland, and, in the 1940s and 50s, Levittown, N.Y., a middle-class suburb of New York City, which was the model for two more Levittowns near Philadelphia.
Most conspicuously shaped by suburban sprawl are the metropolitan areas of the Sun Belt that have boomed since World War II. The Los Angeles area, often held up as an exemplar of the suburban metropolis, spans c.34,000 sq mi (c.88,000 sq km). With a population of more than 14.5 million, although ranking second in the nation, it has a relatively low population density of about 430 people per square mile, less than one fifth that of metropolitan New York. Many newer U.S. metropolises are distinctly suburban in character even within the corporate limits of the central city.
The largest suburbs in the United States have the population of a middle-sized city; Mesa, Ariz. (1990 pop. 288,091), a suburb of Phoenix, is more populous than Newark, N.J. (1990 pop. 275,221). Many suburbs remain racially as well as economically exclusive; efforts at integration often result in racially segregated neighborhoods within the larger suburban municipality. The shift of population out of the central city has had the effect of attracting industry and commerce to the suburbs. The rise of suburban industrial parks (areas zoned primarily for office space) and shopping centers has led to the further decline of the central city. Suburbs have been particularly successful in attracting newer, often high-technology, industries.
Since 1980, huge new concentrations of economic activities have developed in the most accessible suburban locations. These suburban downtowns in the 1990s reached a scale and diversity reminiscent of the central city downtown. Some notable examples are Tysons Corner, Va., near Washington, D.C., Stamford, Conn., and Costa Mesa, Calif. In densely populated regions, the suburban expanses of large urban areas may coalesce, creating a megalopolis. The Atlantic seaboard from Washington, D.C., north to Boston includes the metropolitan areas of Baltimore, Philadelphia, and New York and is sometimes considered a single economic and social unit; in recent decades, the Pacific coast from San Diego to the San Francisco Bay area has witnessed a similar development, as have heavily populated regions in England, Germany, and Japan. By 1990 the majority of U.S. citizens lived in suburbs, and during the next decade that majority increased.
Suburban growth slowed and urban growth increased somewhat in the 1990s, as density in the first tier of suburbs neared and sometimes reached urban levels. During this period a significant difference became apparent in suburban development in the West and the South. In the West, where land and resources precluded growth, suburbs became increasingly dense. Population increases were particularly high around Las Vegas, Los Angeles, San Diego, San Francisco, Phoenix, Sacramento, Seattle, and Portland. In the South, with fewer natural barriers to growth, the density of first-tier suburbs was lower, suburban growth spread further afield, the creation of suburbs far from cities was more prevalent, and converging bands of suburbs began to rival the great metropolitan corridors of the Northeast and industrial Midwest. Suburban development in the Northeast and Midwest during this period fell somewhere between these extremes.
For years many critics, themselves largely urban, have criticized suburbs as cultural deserts where neighbors are strangers, women are virtually imprisoned, and environmental concerns are scorned. While many concerns relating to suburbia continue, by the beginning of the 21st cent. many stereotypical views were fading with the proliferation of suburban colleges and museums, the increase of local employment opportunities, the enrichment of surburban women's lives, and the realization by academics that in today's suburbia there is often a positive sense of community as well as a complex social structure.