Sure, but the goal isn't "perfect", it's merely to do better than human drivers. Human drivers make that mistake constantly, so the case you're describing is on

Sure, but the goal isn't "perfect", it's merely to do better than human drivers. Human drivers make that mistake constantly, so the case you're describing is one where the cars will do MUCH BETTER than people because cars are better at sharing information over time. So we go from a world in which every driver independently has to update their own personal worldview and is thus individually surprised by the addition of a new sign to one in which a few cars are surprised and have to slow down and update but within a few days every subsequent car through the intersection automatically knows the new data.