Not just motorways they are all over the place, I use Waze on my motorcycle with a blue tooth headset and it announces them as you enter leave and shows a moving average (which I'm always way under, I don't speed but I like to know where they are because they can make cars erratic, same with speed cameras, if I get the alert there is one ahead you can predict all the cars will start braking earlier).

Honestly I'm completely in favour of both speed cameras and moving average zones, the stuff I see on a week to week basis is shocking, same with red light cameras, I wish every major to minor junction had them.

Yes of course. I never said they don't - I was doubting that people pointlessly speed and then stop and wait for their average speed to drop. It sounds implausible.