Two things are apparent on the M1 gantries.
The first is that not all gantries have the cameras. Those that do have them cantilevered out from the left hand support. There isn't a camera on each lane, it's a twin box installation, and they are hard to see flash on your carriageway.
But it also seems, when I'm showing 75 on the speedo, so probably 69 in reality, those who hurry past at 80 don't seem to get flashed. So I'm guessing its the +10% +3mph rule.
But I've seen the opposite carriageway flash quite a lot. One day I saw six flashes as it nailed three nose to tail cars in lane 3. They didn't appear to be going that fast.
Regulars on the Nottingham-Sheffield stretch have soon got wise to this and it's noticeable that traffic flow is slower, which is the intention. Some drivers lift off to go under the gantries but if you were on there every day you would soon remember which gantries carried the cameras.
I've also seen flashes on the M42 around the NEC.
