Two issues with average speed cameras are:
1. Lane discipline goes out the window as all the BMW and Merc drivers who used to sit at 90 in the third lane are forced to sit with everyone else, and there is a lot of undertaking and switching lanes to try and get past the guy doing 1mph slower than them.
2. 50mph average speed cameras prove what complete bastards HGV drivers are; they finally achieve their life's ambition to keep up with the cars, so they sit 3 feet from your rear bumper to shit you up, or overtake you if they think they can get away with doing 52mph. This is rather unnerving if you're in a low-slung car, because you never know if they can actually see you once they're alongside. A relative learned the hard way not to sit alongside a lorry for any longer than is absolutely necessary - she was sideswiped by a foreign HGV and the police weren't interested because they said you can't expect the driver of an LHD lorry to see you