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Density

The term density indicates the degree to which people within a settled area are living close together or spread out. Since the residents of cities generally live close together, that is, at higher densities than in other types of communities, density figures can help distinguish cities from suburbs and suburbs, in turn, from rural areas where the population is most widely dispersed.

Measuring Density

Density is typically measured as the number of people residing per square mile or square kilometer. While there is no minimum density requirement for U.S. cities, the Bureau of the Census does specify that a “block group,” a defined portion of census tract, have at least 1,000 residents per square mile to be included in an “urbanized area” (population 50,000 or more) or in an “urban cluster” (population 2,500 to 49,999).

Table 1 shows substantial variations in densities among the 10 largest U.S. cities. Phoenix and Houston, which have incorporated large areas relative to their populations, have much lower densities than Philadelphia, which has the relatively confined boundaries typical of older cities. Since these figures represent citywide averages, a lowdensity city might still encompass crowded public housing projects, condominium complexes or mobile home parks; conversely, a highdensity city might have neighborhoods of singlefamily homes like those in nearby suburbs.

Table 1 Population Densities of the Ten Largest U.S. Cities
CityPopulation (per square mile)Area (square miles)Density
New York City8,008,278303.326,404
Los Angeles3,694,820469.17,876
Chicago2,896,016227.112,752
Houston1,953,631579.43,372
Philadelphia1,517,550135.111,233
Phoenix1,321,045474.92,782
San Diego1,223,400324.33,772
Dallas1,188,589342.53,470
San Antonio1,144,646407.62,808
Detroit951,270138.86,853
Source: U.S. Census Bureau; Census 2000.

These figures show only where people live, not where they work. It is, however, workplace densities (derived from journeyto-work, employment, and commercial real estate data) that are critical for determining the kind of transit service appropriate for a given city. Since Manhattan's office population during working hours soars to more than 200,000 per square mile, subway and rail commuter services are a necessity. In cities where jobs are more likely to be dispersed, bus or trolley (light rail) service may be used by commuters headed downtown, but for the majority, traveling in various directions, there may be no practical alternative to the car.

The Urban Revolution

The first census—in 1790—recorded that 3.9 million Americans were so dispersed across the country, mostly in farms and villages, that only 1 in 20 could be considered a city resident. While the population of the biggest city, New York, was only 33,000, all such ports, including Philadelphia, Boston, Baltimore and Charleston, had densities considerably higher than do most American cities today; in the absence of public transit or cars, city residents lived close together and within walking distance of where they worked. The busy streets, shops, and marketplaces of these compact towns gave them the crowded feeling we often associate today with big cities.

Beginning in the 1820s and continuing almost every year for a century and a half thereafter, thousands of predominantly young men and women from outlying farms and villages and from overseas streamed into American cities seeking work in downtown shops, offices, factories, and homes. In 1820, only New York could boast of more than 100,000 residents, but by 1900, there were 38 cities of that size or greater. And by then, New York, Chicago, and Philadelphia all had populations exceeding a million. Never before in history had so many people lived at such high densities—then referred to as “congestion of population.” The prospect of living in crowded cities was exhilarating because they offered excitement, jobs, and entertainment; but urban slums were associated with crime, vice, and epidemics.

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