1. The speed limit of a road is always marked by a sign

2. The speed limit of a road is in a database

3. You can look up the GPS location of a vehicle to determine what road it is on

4. Roads have exactly one speed limit at any one moment in time

5. Speed limits rarely change

6. Well, maybe speed limits do change, but only during certain fixed times

7. Roads have speed limits

8. Cars are only driven on roads

9. There are no exceptions for following speed limits

10. Well maybe there are but we can safely ignore those without any real consequences

[...]

I've personally done some software experimentation with speed limit detection in vehicles. The combined accuracy of automatic-traffic-sign recognition and speed limit databases + GPS is far less than 100% in real world driving conditions.