Egregoros

@mWare it's 30 seconds to 2 minutes IIRC but even without specialized receivers you can detect it over 150ft away and through buildings. now imagine what happens when you have specialized hardware -- I bet the range is scary far. 314.9-433.92 MHz can probably go waaaaaay farther than people think.

The original traffic congestion detection was by a company called TrafficCast (we hosted their websites and stuff) and that's what they do -- boxes on the side of the road detecting TPMS sensors.

These days it has expanded to bluetooth as well, but originally it was all TPMS. A few datapoints on a long highway can tell you their avg speed and you can guess their route if they turn off the main highway and you pick up their serial number somewhere else etc etc

edit: maybe TrafficCast didn't make TPMS stuff public, I can only find BlueToad mentioned which does Bluetooth