As much as I love Python, you need realtime processing for this kind of things. You can't have a slow language with gc pauses and no way to guaranty execution time for a given operation.
CPython spent actually need the GC. If you make sure you have no cycles, you can disable it, since the rest uses reference counting (which AFAIK is predictable).
It's not necessarily about speed of reaction, although that certainly plays a part.
Such software needs to react in real time though, if the task that's turning the steering wheel gets preempted in the middle of taking a curve on a cliff your self-driving car will become a self-flying car.
Such a system would be the equivalent of a driver that suddenly starts texting at all sorts of poorly chosen times.
