[zinok]

However, as far as I can tell, the GPS data, in at least some cases, comes from Streetview-type camera cars which drive along the roads and capture the roadsigns.

On French highways there are context-dependent speed limit signs which look exactly like normal signs but with a small additional panel below which tells you when they apply. Eg, big 70 in a round red circle, with a small picture of a car towing a trailer below. Eg https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2F... which obviously only applies to cars with trailers.

Google maps (on my phone, not built-in to the car in any way) would constantly tell me that the speed limit of the road was that of the last such sign regardless of the specificity.