I was also told that if you approach a revised lower speed limit on a smart motorway, and are still slowing down as you pass the gantry, the sensor will think that you are braking for an incident, and post a lower speed limit. That's why they seem to change randomly. On the course, they said they'd rather have people on the course than fine them, so that more people understood, and the limits would be maintained higher.
they also said that very soon the limits would be enforced RIGIDLY.
