Your adaptive cruise doesn't "know" what the speed limit is. Self-driving cars have this information, whether from a database or by reading a sign. It remains to be seen how much enforcement the software will perform.
I believe the tesla adaptive cruise control knows it in some cases. It will go max 5 mph over the limit on 2-lane farm roads (adaptive cruise control + self steer).
I set my adaptive cruise control over the limit so I catch up to the car in front of me, for example.