What makes you think computers can't see the signs marking detours? It's perfectly within the possibilities of current OCR technology; in fact, there have been usable implementation of road sign processing for years.
Same for changed traffic pattern, especially if there's more than one autonomous car in the zone (since they can communicate in ways that humans can't).
They even understand the policeman who waves at them.
Yeah, and so can an Xbox with a $100 addon. It's really not that hard nowadays to track and recognize those kinds of movements.