Vehicle-to-Vehicle
From BBC: Connected cars 'promise safer roads'.
To be valuable, this must work in a mixed environment. Not all cars will have communication devices reporting to them, and even cars with such devices might see them disabled from time-to-time. Communication between cars is fine, but the key to the future of smart cars is the ability to sense the environment independently, and operate in that perceived (rather than reported) environment. Only that can be deployed.
Consider the first year when only a small percent of cars will have the technology. If the system requires all the cars to have the technology, who will pay extra for it? If it requires no other cars, but adds value, people may buy it.