THOMAS KNAPP: Rule of what?

Imagine, for a moment, that you are driving at 30 miles per hour through a town with a clearly posted speed limit of 35, and are pulled over for speeding. When you object that you were going well below the posted speed limit, the cop points to one of the signs, which now declares that the speed limit is 15 mph. A few seconds later, the sign changes to 20. Then 10.