I'll bet that you're thinking that this will have to wait a decade or two, until every car on the road has a new generation of computers installed. Look up a Mercedes product called Distronics Plus and a company called Autoliv. These guys are already mounting stereo cameras and long-wavelength IR (heat) cameras on production luxury cars to detect turn signals, pedestrians, and license plates of other cars. When 50% of cars have radars linked to their cruise control, it's a matter of iteration to let them share data, then to characterize the remaining vehicles on the road based on the aggregate observational data.

Your '95 Civic may not be announcing its velocity, but when the car behind you and the car in front of you have both read your license plate and agree on your speed, you may as well be.