But modern cars are already wide open to hacking, allowing attackers to take over the engine control, brakes, and in some cases the steering. Some think the NSA cyber warfare branch is already doing this. A clever foreign attack could kill or injure millions of drivers simultaneously when they all slam into each other.
We should be building robustness instead of adding even bigger vulnerabilities.
Modern cars aren't "wide open to hacking", but people have demonstrated the ability to take over a car and control the engine, brakes, shifting, etc. The difficulty in doing so, however, is that direct physical access to the car was required. In order to do it, the hacker had to be plugged into the OBDII port; it wasn't done wirelessly.
I like the idea of driverless cars because of the potential to increase travel efficiency, decrease time, decrease accidents, etc. However, in order to achieve this, cars will need to be able to either (1) communicate with others in the near vicinity, or (2) be controlled by some central system coordinating the actions off all the cars in a certain area. Either one requires some sort of wireless communication system that will open the cars to hacking. Of necessity, the same system that controls the brakes, engine, steering, etc. will have to be connected to that communication system.
Even if the systems were made to be very robust and could handle any system malfunction/road hazard/physical breakdown/etc., it would never be hack-proof.