How about something much more simple- put a trivial directional RF transmitter on brakes lights, then you could have a receiver to give you a bit more advance warning of large numbers of cars braking ahead. This would be particularly useful on high-traffic freeways where a slow down up ahead can quickly turn into chain reaction rear-enders as people aren't leaving nearly enough following distance but still driving rather fast. It would be an obvious advantage in fog or around curves.

In fact, there is already part of this system in place. Many emergency vehicles are now carrying transmitters that send out a simple encoded message to radar detectors. But for the "brake light" frequency, we could even forgo the encoding to keep costs down, and a retrofit unit could simply plug into a taillight socket.