Estimation and recent behavior of urban population and employment density gradients

The monocentric model of urban structure predicts that urban population density declines with distance from the central business district. Using the negative exponential function to approximate the decline, Mills (E. S. Mills, “Studies in the Structure of the Urban Economy,” Johns Hopkins Press, Baltimore, Md. (1972)) estimated population and employment density gradients from 1948 to 1963 for a sample of 18 SMSAs. This paper updates Mills' estimates and examines recent patterns in population and employment suburbanization. The updated series estimated here is obtained using a “corrected” version of Mills' method. The original procedure incorporated a bias which Mills noted and later corrected (E. Mills and K. Ohta, in “Asia's New Giant” (Patrick and Rosovsky, Eds.), The Brookings Institution, Washington, D.C. (1976)). The comparability of the series begun by Mills and extended here is not interrupted, however, because, as Mills suspected, the bias is indeed small provided SMSA data are used. On the other hand, Urbanized Area definitions of metropolitan areas cause the original and corrected versions to yield significantly different results. This finding has implications for the appropriate choice of data for urban studies.