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This approach is all backwards anyway. You don't need to detect obstacles per se, you need to detect safe conditions to proceed.

Clear roadway is safe. Following behind a detected vehicle is conditionally safe.

Cow in the road? Not clear road way, not a detected vehicle. Does not match any safe conditions, so do not proceed.

This approach is why you see so many reports of Waymos stopped somewhere that's outside traffic rules. But it's so much better to stop safely in situations where it's not really needed than to not stop in situations where it is needed.