This would be huge, and Cadillac has already deployed this in some of their models. The basic idea is simple: your car broadcasts its position and velocity vec

This would be huge, and Cadillac has already deployed this in some of their models. The basic idea is simple: your car broadcasts its position and velocity vector multiple times per second (and your "car id" is periodically randomized for security), and the signal can be detected by any receiver within 500-1000 meters. The simplest receiver could then trigger a flashing red "collision imminent" light if things look dicey. A complex receiver could employ active controls to "platoon" a set of vehicles, co-ordinate lane changes, or negotiate intersections. The missing links, I think, are the need for a) 1-2 meter position accuracy for platooning, and b) 100-200 cm accuracy for lane-keeping/lane changing. You'd still need to be an active, engaged driver: more "auto-pilot" than autonomous.