CHP in California calls the guy changing lanes and passing every one a volunteer, they volunteer to get a ticket.
Most mechanical speedometers have 10% error, the electronic units are a bit more accurate, and that is dependent on what method of pic up they use, so most Police Departments will give the driver at least that margin.
We used to calibrate the police vehicles speedometers with a dynamometer every 6 months so that the officer had an accurate speed curve to work with, to many ticketed clients wanted to challenge the tickets by wanting to know when the vehicles where calibrated.
Same for the radar guns and motorcycles, and then there is the bright guy that wants to know if the officer is qualified to operate the radar gun.
navegator