UK motorways have varying speed limits communicated via road-side signage. When repairs are being made there are sandwich board style signs that say "dead slow loose chippings" [chippings being small stones] or sometimes they have a 15mph label on a similar sign.

You'd have to guarantee perfect information from road repair crews, at least, if you were going to ignore situational signage.

Probably right, because when I use google maps' navigation mode, it sometimes has a little speed limit sign in the top right corner.

Self-driving cars need to support unplanned changes to the road, such as construction zones.