A previous version of this working paper was originally published in 1998.

To account for this deconcentration of metropolitan employment, the authors develop a system-of-cities model in which an increase in aggregate metropolitan employment causes congestion costs to increase faster for the more dense metro areas. A calibrated version of the model reveals that the (roughly) two-and-a-half-fold increase in postwar aggregate metropolitan employment implies, by itself, more deconcentration than actually observed. Thus, rising aggregate metropolitan employment appears to be a powerful force favoring deconcentration, although some benefit of greater employment density appears to have partially offset the effects of rising congestion costs for the more dense metro areas.