Changes in Transportation Infrastructure and Commuting Patterns in U.S. Metropolitan Areas, 1960-2000.

Population decentralization has been a salient feature of the landscape of most U.S. urban areas since 1950. Nathaniel Baum-Snow (2007) documents that the aggregate population of central cities of the 139 largest metropolitan areas (henceforth, MSAs) declined by 17 percent between 1950 and 1990 while aggregate MSA population growth was 72 percent during this period. Expansion of the highway network in urban areas thus accounts for about one-third of the gap in central city and MSA population growth rates. While transport network expansions clearly generated urban population decentralization, there is little evidence to date on how this decentralization manifested itself as changes in employment locations and commuting patterns. In this paper, I present evidence indicating that employment decentralization occurred apace with residential decentralization between 1960 and 2000 such that their relative spatial concentrations remained remarkably unchanged. While in 1960 50 percent of working residents of MSAs over 250 thousand people lived in their central regions, this had fallen to just 23 percent by 2000. During the same period, the fraction of metropolitan area jobs in central cities also fell by 28 percentage points, from 0.60 to 0.32. As is evident from these numbers, most commutes of MSA residents in 2000 did not involve central cities at all. Central cities as defined by their geographies in 1960 were the origin and/or destination of only 38 percent of commutes made by MSA residents in 2000, down from 66 percent in 1960. Results presented in Section III confirm indications from these trends and the estimates in Baum-Snow (2007) that radial highways caused the decentralization of both residences and