I don't know of any current networking technology in active use that solves that problem.
Specifically, most of the technologies require too much handshaking/setup to work well in traffic when cars move relative to each other all the time. (Doesn't matter if it's ZigBee mesh or Bluetooth or 802.whatever -- setup/discovery is slow)
Also, there's the question of knowing *which* car you're talking to -- there are cars in front, back, to the sides, and in the oncoming lanes, and matching up which ones are which electronic ID is another challenge that isn't really solved for real yet.
If you have a better description of what particular data you'd like to communicate between cars, and what your performance requirements are (speed of discovery, distinguishing facilities, throughput, power, etc) then perhaps a slightly less negative answer can be found. Parked cars telling navigating cars which parking lots are already taken is a different problem domain than two cars meeting on a narrow road trying to negotiate who brakes and pulls to the side to let the other one pass!
