Source of said standard? Older cars (pre-1990) had pretty inaccurate speedometers, 5-10% off. Modern cars are very accurate. There is no such need for cop to "

Source of said standard? Older cars (pre-1990) had pretty inaccurate speedometers, 5-10% off. Modern cars are very accurate. There is no such need for cop to "prove" you "knew" you were speeding. Ignorance of the law (or the condition of your vehicle, or whatever) is no excuse, as the saying goes. When you sign up for a driver's license (in the US) you agree that you will abide by all traffic laws.