[jonathanberger]

I'm not sure I'm understanding you when you say "definitively". Do you mean if I can't be 100% sure about my answer?

The answer to your question is: the same way you determine when it's safe to cross the road on foot. When there are no cars in sight I'm 100% sure it's safe. When there are cars in sight, I'm not 100% sure but I still cross because I've developed judgement about the situation and the risks are acceptable.

[h0l0cube]

I'm not sure, as a road user, how you've not encountered traffic situations that changed from perfectly normal to dangerous very rapidly. Sometimes an obscured vehicle could pull out, or a child, or some other obstacle, can pop out of nowhere even on an otherwise quite suburban street. The examples are too numerous to mention, and certainly many would be capable of confounding an AI.