The whole point of this paper is that it's not just showing a correlation between proximity to toll booths and infant mortality, it's showing that adding E-ZPas

The whole point of this paper is that it's not just showing a correlation between proximity to toll booths and infant mortality, it's showing that adding E-ZPass reduced infant mortality, which makes the method essentially the same as an interventional study even though E-ZPass system wasn't installed for the study.