It hopefully uses it's GPS in combination with the Google mapping data for speed limits along with signage on the road.

Hilarious to think about nonetheless.

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.

All GPS have speed limits for all road, and they can beep when you go over the speed limits.

The speed is part of the map.

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

They currently have a max speed of 25 mph or something, even on roads with faster speed limits. It's a bit annoying when you're stuck behind them.

Isn't that only for the custom-designed Prototype cars? I thought the modded Lexus cars were allowed to drive at highway speeds.