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 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. A byproduct has been that most commutes of MSA residents no longer 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. Using planned portions of the interstate highway system as a source of exogenous variation, estimates reported in Section III indicate that urban highway construction played a pivotal role in generating this shift. New highways primarily increased the number and fraction of commuting flows within suburban areas at the expense of commutes within central cities. Because within suburb commutes are longer than other types of commutes on average, results are consistent with Gilles Duranton & Matthew Turner’s (2009) evidence that the elasticity of kilometers driven with respect to lane-kilometers of highways in urban areas is one.1