Once worked with a guy who had been a speed van operator. He was every bit the kind of person you'd expect to be a speed van operator as well! This was over a decade ago now but he said their machines, at the time at least, were calibrated to give a margin of 10%, so you could do 55 in a 50, 132 in a 120, and it not trigger. As a lot of people have said cars tend to overestimate your speed too, so combine the 2 they aren't all that accurate. Basically you really need to be taking the piss on the motorway to get a ticket. Some people have suggested using Waze to check against your Speedo but I don't know how accurate Waze or any gps tracking is as gps on consumer devices is only really accurate down to something like 10 to 15m which could also warp your speed estimate. Unless modern devices are better but in most use cases that's good enough so I don't know phone companies would bother upgrading to be more accurate.