Different manufacturers probably use different systems but no. BMW attempts to read the speed limit signs using the frontal camera with a mix of some sort of stored info - it knows that the speed limit is about to change (Mobileye?), but it is very often that it won't catch a sign in the bend or when the weather is bad. Also, it does not recognize time restricted speed limits, for example 30kph from 7:00 to 17:00 Monday to Friday so it would keep driving 30kph outside of those hours while 50kph is allowed. In some places in Germany, it does not recognize the city limits and carries on showing 70kph for a kilometer longer than it should.
I'm not sure how the cars know the speed limit. Maybe someone else knows? My guess is combo of GPS/camera to position correctly on road and the lookup of known speed limit data. Perhaps it reads signs though?
My car shows the speed limit of roads it knows. It uses GPS and stored limits. It also doesn't know the limits of non-major roads and doesn't attempt to show a limit then. My car is a 2013, and I've not paid the $$ to update the maps in that time (seriously, they want $200-$400 to update the maps).
Since I bought my car, Illinois (where I live) has raised the maximum limit on interstates by 10 MPH. My car doesn't know about it. If my car limited me to what it thought the limit was, I'd probably be driving 20 MPH slower than prevailing traffic, a decidedly unsafe situation.
The rental car I am using now certainly a) reads road signs for speed limit information, b) is definitely fooled by signs on off ramps etc.
It’s hard to imagine how speed limit systems would work without some sort of vision capabilities — a database of speed limits would never be up to date with roadworks and so on.
Does it always get this correct, or does it sometimes read a 30mph sign on a side road and then slow the car on the motorway down to that speed?