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.