The speed gantries on the M1 which now stretch from the M18 junction south, beyond the A38 to J25 are "active", even when no limit is displayed.
Having travelled that way more than normal recently I've seen them flash when the traffic flow is normal high density. Not all gantries have the cameras, and they can be spotted by looking to the extreme left as you approach - there are two small grey boxes cantilevered out from the main structure. On one occasion there were six flashes on the opposite carriageway, and the speed of lane 3 didn't appear to be excessive. So I'm guessing that these are set at 70 + 10% +3mph so 80mph.
I always work on the basis that my speedo will be up to 10% fast, so to be doing 80 I would be showing nearer 88, which I no longer do. I've steadied down a lot as I've got older!
