Staggered Health Policy Adoption: Spillover Effects and Their Implications.

This paper investigates the direct and spillover effects on mobility caused by the staggered adoption of Stay-at-Home orders (SHOs) implemented by U.S. counties to contain the spread of COVID-19. We find that mobility in neighboring counties declines by a third to a half as much as in the counties that first implement the SHOs. Further, these spillovers are concentrated in counties that share media markets with treated counties. Using directional mobility data, we find that declines in internal mobility in the neighbor counties account for a much larger proportion of the overall decline in mobility than decreases in traffic originating in the treated county and headed to the neighbor. Together, these results provide strong evidence that SHOs operate through information sharing and voluntary social distancing. Based on our estimates and a simple model of staggered SHO adoption, we construct counterfactual scenarios that separate the impact of policy coordination from that of adoption timing. We demonstrate that staggered implementation of SHO policies can yield mobility reductions that are larger than coordinated but delayed SHO policy adoption.