The potential consequences of mass surveillance come to mind.
While it is true that many people do speed, that doesn't make their speeding "the real speed limit".
Anyway. I come from the UK where we've had camera based enforcement for aeons. This of course actually results in people speeding and braking down to the limit as they approach the camera (which is of course announced loudly by their sat nav). The driving quality is frankly worse because of this, not better, and it certainly doesn't reduce incidence of speeding.
Of course the inevitable car tracker (or average speed cameras) resolve this pretty well.