Re: V2V vs. on-board sensors
What everybody seems to have missed is that an active V2V system is pretty much useless until a significant fraction of vehicles are equipped with it.
In the gliding world we have FLARM, a short-range active GPS-based system in which every set broadcasts its 3D position and velocity vector while using the data sent by other sets to determine whether a collision is likely. Virtually every glider, helicopter and light plane operating in the Alps are now equipped and elsewhere in Europe coverage is, I believe pushing up to 50%. Experience has shown that FLARM was pretty much useless when less than 25% of local gliders carried it. Now it is starting to become worthwhile as usage exceeds 50% and so the remaining parts of the fleet are seeing that carrying it is a positive benefit and installing it too.
I'd say that V2V is less use on the road because a driver's traffic scan only has to cover the horizontal plane and in any case road vehicles already carry conspicuity features (lights, horns) and intentional signalling equipment (turn indicators and brake lights). Now add in the fact that FLARM is a small, self-contained box the size of a mobile phone that's easy to install in almost any cockpit while a road V2V system will need both something on the dash and (probably) external front and rear sensors. As a result V2V retrofit would probably not be easy or cheap. Consequently, V2V installation is likely to only be a feature of new vehicles and will be opposed on cost grounds by many owners.
The fleet coverage statistics for gliders are likely to apply to road users as well so, if we assume that a hypothetical V2V system is only available as an optional extra on new vehicles, how long is it likely to take for over half of all vehicles to be fitted with V2V sets?