Academic journal article Economic Quarterly - Federal Reserve Bank of Richmond

The Evolution of City Population Density in the United States

Academic journal article Economic Quarterly - Federal Reserve Bank of Richmond

The Evolution of City Population Density in the United States

Article excerpt

(ProQuest: ... denotes formulae omitted.)

The answers to important questions in urban economics depend on the density of population, not the size of population. In particular, positive production or residential externalities, as well as negative externalities such as congestion, are typically modeled as a function of density (Chatterjee and Carlino 2001, Lucas and Rossi-Hansberg 2002). The speed with which new knowledge and production techniques propagate, the gain in property values from the construction of urban public works, and the level of labor productivity are all affected by density (Carlino, Chatterjee, and Hunt 2006, Ciccone and Hall 1996). Nonetheless, properties of the distribution of urban population size have been studied far more than properties of the urban density distribution.

Chatterjee and Carlino (2001) offer an insightful example as to why density can be more important than population size. They note that though Nebraska and San Francisco have the same population, urban interactions occur far less frequently in Nebraska because of its much larger area. Though the differences in the area of various cities are not quite so stark, there are meaningful heterogeneities in city densities. Given the importance of urban density, the stylized facts presented in the article ultimately require explanations such as those given for the evolution of city population.

This article makes two major contributions concerning urban density. First, we construct an electronic database containing land area, population, and urban density for every city with population greater than 25,000 in the United States. Second, we document a number of stylized facts about the urban density distribution by constructing nonparametric estimates of the distribution of city densities over time and across regions.

We compile data for each decade from 1940 to 2000; by 2000, 1,507 cities meet the 25,000 threshold. In addition, we include those statistics for every "urbanized area" in the United States, decennially from 1950 to 2000. Though we also present data on Metropolitan Statistical Area (MSA) density evolution from 1950 to 1980, this definition of a city can be problematic for work with densities. A discussion of the inherent problems with using MSA data is found in Section 1. To the best of our knowledge, these data have not been previously collected in an electronic format.

Our findings document that the distribution of city densities in the United States has shifted leftward since 1940; that is, cities are becoming less dense. This shift is not confined to any particular decade. It is evident across regions, and it is driven both by new cities incorporating with lower densities, and by old cities adding land faster than they add population. The shift is seen among several different definitions of cities. A particularly surprising result is that "legal cities," defined in this article as regions controlled by a local government, have greatly decreased in density during the period studied. That is, since 1940, local governments have been annexing territory fast enough to counteract the increase in urban population. Annexation is the only way that cities can simultaneously have increasing population, which is true of the vast majority of cities in our sample, and yet still have decreasing density.

This article is organized as follows. Section 1 describes how our database was constructed, and also discusses which definition of city is most appropriate in different contexts. Section 2 discusses our use of nonparametric techniques to estimate the distribution of urban density. Section 3 presents our results and discusses why cities might be decreasing in density. Section 4 concludes.

1. DATA

What is a city? There are at least three well-defined concepts of a city boundary in the United States that a researcher might use: the legal boundary of the city, the boundary of the built-up, urban region around a central city (an "urbanized area"), and the boundary of a census-defined Metropolitan Statistical Area (MSA). …

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