You could have a mixture of exact scores like for whole cities < https://aqicn.org/map/usa/ https://aqicn.org/map/usa/ > and then combine that with localized sc

You could have a mixture of exact scores like for whole cities < https://aqicn.org/map/usa/ https://aqicn.org/map/usa/ > and then combine that with localized scores based on things like being near a freeway. Walk Score isn't an exact science, but is still a useful measure.