Staggered Health Policy Adoption: Spillover Effects and Their Implications.

This article examines the direct and spillover effects on mobility resulting from the staggered adoption of stay-at-home orders (SHOs) by U.S. counties during the early COVID-19 pandemic. Using smartphone-based mobility data and a difference-in-differences design comparing contiguous county triplets, the study finds that neighboring counties reduce mobility by about one-third to one-half as much as counties implementing SHOs, with spillovers concentrated among counties sharing the same designated market areas (DMAs), indicating an information-sharing mechanism. Decomposing mobility changes into internal (within-county) and external (between-county) traffic reveals that voluntary reductions in internal mobility by neighbors contribute twice as much to spillovers as changes in external traffic, supporting the role of voluntary social distancing driven by awareness. Counterfactual analyses suggest that staggered SHO adoption, through positive spillovers, could achieve greater overall mobility reductions than delayed but coordinated adoption, highlighting the importance of both timing and coordination in public health policy implementation.