A very common way of measuring traffic volumes is via bluetooth MAC address broadcasts. The Wisconsin DOT even makes the traffic counts derived from this data interactive and public:
https://wisdot.maps.arcgis.com/apps/webappviewer/index.html?...
350 karma · joined November 13, 2018
https://serc.berkeley.edu/compostable-plastics-are-they-play...
PurpleAir sells an indoor sensor, but it appears to be limited release and somewhat 3D printed. https://www.purpleair.com/