Electrodynamics and Relativity — David J. Griffiths
Physics · proper-velocity
Problem
A cop pulls you over and asks what speed you were going. ``Well, officer, I cannot tell a lie: the speedometer read 4×108 m/s.'' He gives you a ticket because the speed limit on this highway is 2.5×108 m/s. In court, your lawyer (who, luckily, has studied physics) points out that a car's speedometer measures proper velocity, whereas the speed limit is ordinary velocity. Guilty, or innocent?