M1 was down to 60 some weeks ago when practically empty. When i enquired as to why, I was told it triggers automatically when the pollution hits a certain level. Even when working at its maximum, it only improves air quality by 4%! The system for speed is also automated i'm told.
I was flashed just after Meadowhall heading north. Limit on the gantries was 50. It flashed me at 62 so it does work. Some are also set to trigger when there is no speed limit on the gantry, just set to national speed limit.