In huge parts of the world these are standardized and readable as well - where Vienna Convention signs are being used. (Those with have more pictograms).

Detours are also labelled by standard signs in a pre-determined font with a specific reflectivity. Easy for a car to recognize.

And even without text recognition is one of the really easy parts of self driving. I've done development on document recognition for random bank statements, builinng plans and legal documents - all the paperwork for financing a house - before. Those documents come in in various fonts, layouts differently formatted etc.