I think OCR may have reliability issues, but most road signage has consistent properties (size, height, font, etc.) that could be easily optimized for.
It also covers the (admittedly limited) situations where no GPS is available (tunnels, skyscrapers). If there are temporary changes in speed limits due to construction or other events, OCR could immediately identify those with no need to download new information.
It also covers the (admittedly limited) situations where no GPS is available (tunnels, skyscrapers). If there are temporary changes in speed limits due to construction or other events, OCR could immediately identify those with no need to download new information.