The obvious counterpoint to "operating from stored data" is an accident or construction causing a lane to be closed, or a child chasing a ball across the road.
In the hypothetical world in which self driving cars are the norm and not the exception detours would be in the system so that SDCs would not need to read signs. Even now a lot of traffic maps get updated to show detours, delays, and temporary closures due to accidents.
FWIW I am a near-term self-driving car skeptic and have been for a long time. I just think that these are not the kind of issues that really pose a major obstacle whereas drunk people wandering across the street are.
