Chris Valasek, director of security intelligence for IOActive, opined that the defense against cars being remotely hacked would be "a device you could plug into the car to stop any of the attacks we've done and that others have done." He went on to say that the solution would be as simple as "an algorithm that detects attacks and prevents them."
You mean like anti-virus which does not work against zero-days?
One reason vehicles connected to the Internet are vulnerable is that they have one big system. Once a hacker is in, he's got access to the entire vehicle. Car makers could have designed them with firewalls in between the various systems, but they often didn't.
Vehicles also could have designed with an Internet kill switch, but automaker market research has told them that most buyers want a traveling router. You'd think they'd understand that a significant minority would purchase a unhackable vehicle.
The requirements of brakes and steering for passengers in a self-driving vehicle need to be well crafted, not least because shysters will sue when an unusual accident occurs.
I forgot to mention a few things in my last post.
Each vehicle needs a specialized hammer which would be used in case the power windows and locks are not functional, with the passengers needing to break the glass and depart.
Each vehicle needs a manually operated fire extinguisher for obvious reasons.
And each vehicle needs a means of communications with the vehicle operator and Enhanced 911, but that's probably already in the plans.