YOU'RE
A good example are school zones. You ever see a 20mph school zone sign where there's a perpendicular line drawn on the street in alignment with the sign? Hence sign = the physical point where the speed limit is enforced. Or...read your damn driver's ed book since I assume you never did in the first place.
Please tell me where you live, so that I'll never head to a town where the teachers are just as dumb as their students.