Traffic law is definitely not my forte, so other lawyer-types can feel free to correct me, but my sense is that this is a matter of "last chance" doctrine. If you could have avoided the accident by stopping, but didn't, then you had the last chance at avoiding the accident and you have at least partial liability for the ensuing accident. (At the very least, the hypothetical other guy could make things quite lengthy, unpleasant and expensive for hypothetical you. My trial advocacy professor always said that there's no case more complicated to try than what looks like a simple traffic case.)