It sounds like you're describing a system based on GPS and map data, not one of the systems that read the actual signs posted as you drive by them. The only problems I've had with the speed limit sign reader in my car are conditional speed limits (e.g. school zones during certain hours, truck-specific limits). Otherwise, they're just limited by the prevalence of the physical signs.
Even reading the signs is non-trivial, I drive several stretches of road where an access road runs parallel to a highway, and the 20mph lower speed limit signs for the access road are clearly visible from the highway and only slightly farther off the shoulder than the correct signs.
