I was in a small bus on a switchback mountain road in Peru. The bus stopped at a low lying turn, and we could see that a muddy stream was racing across the road

I was in a small bus on a switchback mountain road in Peru. The bus stopped at a low lying turn, and we could see that a muddy stream was racing across the road at its low point, pouring away into the valley off the the downhill edge it was eroding. The pavement under the stream was gone. The driver got out and found a couple of what looked like 2-by-8 pieces of lumber, and placed them across the stream, adding some rocks and rubble underneath like track ballast. He then slowly tiptoed the bus across these creaking muddy boards, leaning out the window to stay on track. We were not swept over the edge, as far as I recall. Needless to say, not a situation for "auto-steer". I'm sure this has been thought through, and I imagine the solution involves zones requiring different levels of autonomy and capability, some means of zone discovery or classification for unmarked areas, and self-driving cars refusing to continue automatically when overmatched.