I was wondering if (instead of level 5 self driving) there could be a simple inter car protocol to communicate presence/distance/range so that car could warn or even slow down (smoothly) when signal is caught.
That sounds workable, and could be implemented in parallel - with two caveats: what of unconnected vehicles (not using this transponder), and what of malicious data (e.g. faking road jams for fun and profit)
