“…2 Source: 5.3 millions across US State borders, Census Bureau, 2011; 3.6 million across Länder borders, Regionaldatenbank, 2018;4.6 million across Département borders, INSEE, 2010. 3 See, among others, Baum-Snow (2007); Duranton and Turner (2012); Faber (2014); Tombe and Zhu (2015); Fretz, Parchet, and Robert-Nicoud (2017); Caliendo, Parro, Rossi-Hansberg, and Sarte (2018).…”