Hackers will easily figure out a way to spoof the communication, and could play with traffic.
There are mitigations for most issues, but it's a complex topic.
Just imagine some scenarios:
-) Spoof an emergency break advisory that causes tailing cars to also do an emergency break. (could be mitigated by first observing that cars in front are actually slowing down before breaking)
-) Spoof a command from a smart traffic light at an intersection to stop immediately for police / other emergency traffic. (need to check if traffic light is actually red)
-) Spoof speed restrictions issued by a smart highway traffic jam prevention system.
-) A system for police to force a car to stop immediately and pull over, eliminating car chases. Just spoof this signal and stop anyone you want. (mitigate by checking if there is a police car trailing you, and ignore otherwise).
And so on...
A way around would be to maintain a national database with public keys for each registered vehicle, and make cars only accept those keys. But that would be hard to maintain and still hackers could just get a hold of some PK.
In the end, the driving system will always have to correlate such car 2 car communication with observations it makes itself.
And an autonomous system can react almost immediately anyway. So coordination doesen't give you all that much.
-- There are some useful ideas though, like:
-) Traffic lights can announce an ideal speed for a route, taking into account traffic and traffic light timings, so you can optimize throughput and minimize fuel consumption