The Evolution of City Population Density in the United States

T he 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