centers.11 Centrality is a measure of the extent to which the development within a metropolitan area spreads out from a point of highest density. Closely related to centrality is the density gradient, a measure representing average density at increasing distances from the center.

Residential Location

As population density within some metropolitan areas has declined, so have density gradients. Kim (2007) analyzed population data for a consistent group of 87 cities with populations of at least 25,000 and their metropolitan areas from 1940 to 2000 to examine changes in population density and density gradients.12 He assumed a monocentric metropolitan area to estimate density gradients. He found that average population density levels have declined since 1950, and the estimated density gradient has declined consistently over the entire period studied (see Table 2-1). Kim suggests that the accelerated flattening of the density gradient since 1950 is likely due not only to the suburbanization of the population but also to the expansion of suburban land area, as found by Fulton et al. (2001). The rate of change in both average density levels and the density gradient appears to have begun slowing in the 1990s, but this trend cannot be definitively established because Kim’s city sample excludes cities that failed to meet the metropolitan area definitions of 1950.

Monocentric models and average measured density gradients, while reasonable for capturing broad trends in urban form, mask internal dynamics that may be more useful in ascertaining the evolution of


Various terms have been used to denote employment centers outside the CBD—activity centers, subcenters, subcity employment centers, edge cities, job concentrations, employment poles, and employment centers (see Guiliano et al. 2008 and Lee 2007 for discussion of each). The term employment center is used in this report.


Kim (2007) notes that population density is typically measured as persons per square mile. The density gradient is usually estimated by using a negative exponential function: D(x) = Doeyx, where D(x) is population density at distance x from the center; Do is the density at the center; and y, the density gradient, is the proportional rate at which population density falls with distance from the center.

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