More scare tactics. Automobiles already use multiple network protocols for communication in vehicles. And the powertrain BUS (engine, transmission, bakes) is typically separate from other buses and only way to talk to them is through passing messages. You can't "take over" or "reprogram/flash" your engine computer (for example) by hacking into the car cellular connection.
As to communicating between cars, there's a simple solution. DON'T store the data. And why would you? If a car in front of you hits the brakes which then signals your car to also apply brakes to avoid a collision, what point is there to store this data? Likewise, if an accident ahead is slowing traffic, why would you want to store the message your car received warning you about it?