RESEARCH IN PROGRESS

Interstate Access & Employment Growth: Evidence from a Design-Based Instrument [scrolly explainer webpage]

• The Federal-Aid Highway Act of 1956 authorized the construction of the 41,000-mile Interstate system. Since then, employment has become increasingly concentrated in counties linked to the network.

• These highways, of course, were not placed at random - they were planned to connect large cities. Therefore, this employment growth could be driven by concurrent forces of urbanization, rather than by Interstate access itself.

• I develop a new design-based instrumental variable using an overlooked planning quirk that shaped counties’ Interstate access. Leveraging this source of quasi-experimental variation, I estimate the causal effect of Interstate access on county employment growth.

• I find that, among counties that as-good-as-randomly gained Interstate access, employment only increased where agriculture was initially important.

Highways and Segregation: Evidence from the US South

• Highways can have disparate impacts across racial groups, impacting patterns of racial segregation. 

• However, the empirical relationship between dissimilarity indices and the presence of Interstate highways is sensitive to the size of the spatial unit at which race counts are aggregated. Dissimilarity indices constructed from Census tracts yield systematically different results to those constructed from Census block groups. 

• I examine this further to determine how much of this difference is explained by (1) the scope and scale of racial sorting and (2) bias, arising from endogenous Modifiable Areal Unit Problems.