A few groups of drivers who seem to be routinely above speed limits :
LGVs - speed limit on single carriageway roads is 40, get they routinely travel at 50 or even 60 mph. Certainly round us where there are no duel carriageways, all the police would need to do is look at their tachos to see how long they have been doing 50 or 60 & issue tickets, but no. I wonder what the difference in braking distances are for LGVs at 40, 50 & 60 mph.Overtaking a LGV at 50 is much more difficult & dangerous than on doing 40.
Motor bikes - generally the high powered "road legal" racing bikes. I have been passed frequently by bikes doing well in excess of 60, might be 80 or 90 given how quickly they disappeared into the distance. Can't say have seen a Harley or other large cruising bike exceeding the limit.
Private hire care - either seem to do 10/20 mph under or over the limit, depending on whether on hire or not.
Cyclists - another couple in dark clothes which do not show up well even with headlights, do they have a death wish ? {edit}
Just some thoughts
On the 20 mph limits at schools, the flashing lights are frequently on during the night, at weekends & in the holidays at our local school, when it is obvious the gates are locked & no lights are on - naturally everyone keeps to a needless 20 mph.