So in every area, they have a good estimate of how many cars have Bluetooth and have it turned on? For example, our household (HS senior, college sophomore, my wife and I) have 4 cars total and only one of them has Bluetooth. It is turned on. But, the rest of us have BT headsets - which are not in pairing / discoverable mode. So I guess they only "see" one of us on the road?
They don't need many samples to determine road speed conditions. On a busy highway, even if only 1% of the cars have bluetooth discovery enabled, there will be valid data. More valid as the sample size increases. Similar technology is used in airports to understand the speed of the people moving thorugh the security line.