Love the brake light idea, simple solution. A radio signal would be one of many, the one directly in front of you needing to be differentiated from the one behind or to the side or on a side street requiring a much more complicated system and protocol that might never be agreed upon and be expensive to implement for retrofitting legacy cars. It makes up for in simplicity what it lacks in features and that could really drive adoption.
Yeah, combined with short range radio you could get a useful system. The lights would announce the sender's vehicle ID (new random number each trip for privacy) and speed / acceleration / whatever from three or four vehicles up front. The radio could announce more detailed vehicle-id-tagged information. A car's receiver can easily filter out unknown vehicle ids.
Random vehicle IDs are probably vital to acceptance.
