Average Speed Cameras

My trip to work takes me down the A127 in Essex, over the last year average speed cameras have been installed from Southend to the outskirts of Basildon reducing the speed limit from 70mph to 50mph.
Without going into the rights and wrongs of that decision the one thing I'm always amused with is the amount of people who don't understand the concept of "average speed", every day I see drivers slowing down for the cameras then speeding up between them and slowing when they approach the next one.
I guess they're all bemused when the fine drops through their letterbox informing them they'd averaged 65mph.