The yellow ones on the side of the overhead gantries on the M25 are very real & live all the time.
As are the ones on the 4 lane section of the M3.
They seem to be set at 78mph, ie 70 + 10% + 1 mph, based on the speed of the traffic. Unless a lower limit is shown I set the cruise control at 77mph (GPS 75mph) and so far have not had a ticket....
