I'm wondering how accurate this might prove measuring automobile traffic on a relatively busy street?

I've been pondering some way to track the number of vehicles that pass my house each day, and would like (but not ideal) to get their average speed too. Was thinking about computer vision, and even audio doppler approaches, but this seems simpler - if only the bluetooth sensing might work on a device moving at 50mph+ at a distance of maybe 20 to 100 yards.