My guess is one best thing for autonomous cars would be a combination of the interactions among surrounding cars, and the safety sensors (using cameras and IR/whatever sensors) system to detect their surrounding environment for any movements and moving objects. That would be never available by the existing manual driven cars to manage so many aspects and to take safe responses.
It will be either that or they'd be networked to a fully automated system driving them all remotely. Frankly, apart from perfecting the actual driving system the other major concern will likely be protecting these cars from hacking. We saw what happened with the recent hacking attacks against hospitals and businesses, can you imagine the devastation if someone malicious decided to do that to automated cars for whatever reason?
Latency in the communication to/from the cars prevents a remote-controlled concept. That is why the cars will be autonomous, but will communicate with other cars in their vicinity to take their inputs into consideration. And to anticipate what is happening ahead of them. Hacking is indeed a concern, as is the injection of "fake messages" into the system. Research is ongoing how to avoid getting a fake car sending out messages that "everyone have to brake immediately!!!".
I guess this kind of hacking can be fixed easily by after soonest eliminating the hacking source, simply resetting the whole ecosystem. Meanwhile there should be a standby/backup sub-system, catering for scenarios like this kind, (probably controlled by the local police) to cover/control the local surrounding areas within a certain distance.
Don't worry. It took the experts in the industry a few years to figure this one out as well. Especially the telco's were pushing hard for a mobile-phone based system.
A system where the cars communicate with each other individually seems a lot more complicated though. I wonder how long will it take to be properly developed to ensure safety.
The cars will not communicate with other cars in the neighbourhood individually. Each car will broadcast data such as location, speed and other diagnostic information. All receiving cars need to decide whether they use the incoming messages from this car, and what they want to do with those messages. The data filtering and processing is therefore done by the receiving cars. This includes security related steps, such as detecting and removing "fake messages" and/or "fake transmitters". The Wikipedia page itself does not contain a lot of information, but it could be a start for further study, if you're interested in more technical details: https://en.wikipedia.org/wiki/Vehicular_communication_systems
That is certainly a very fascinating subject. I have read some articles about it, mostly in passing while doing other things. I should read in more detail, apparently.