The Political Consequences of Spatial Policies: How Interstate Highways Facilitated Geographic Polarization

In the postwar era, Democratic voters have become increasingly more likely than Republican voters to live in urban counties. Public policies that shape geographic space have been a major contributor to this geographic polarization. This article examines the effect of the Interstate Highway System, the largest public works project in American history, on this phenomenon. Drawing on a database of US highway construction since the passage of 1956 highway legislation, it shows that suburban Interstate highways made suburban counties less Democratic, especially in the South and where highways were built earlier. Metropolitan areas with denser Interstate networks also became more polarized. Analysis of the Youth-Parent Socialization Panel Study (1965–97) reveals individual-level mechanisms underlying these changes: Interstates drew more white and affluent residents, who tended to be Republican, to the suburbs.