I recall learning, oh, something like 30 years ago, that the Space Shuttle had multiple computers, running at least two different operating systems, managing all vital systems on a space shuttle.
With all the concern about self-driving cars being cracked, or otherwise running into problems, why is no one demanding something similar? The computers themselves are pretty cheap these days - and will be cheaper by the time we start putting this in every car. Just have a minimum of three computers running a minimum of three different operating systems, determining what the car does. One of those computers can have priority for decisions about where to drive and such (with a human override, which could be as simple as changing which computer got priority for these decisions), but the other two computers would monitor every move for safety. If a single computer returned results outside of safety parameters, the car would shut down until the problem could be resolved - and control of safely slowing down and stopping would be according to the majority of the computers.
Cracking the automobile's control system would still be possible, but it would require that at least two different kinds of systems be cracked, almost simultaneously.