The comments above have been interesting to read. We, however, need to remember that modern vehicles no longer have speedometers that work by attaching a cable to a vehicle wheel and the amount of revolutions that a wheel turns is transfer to the speedometer that works with internal weights that measure how far the separate the higher the revs.
Modern speedometers are electronic that receive a signal from a sensor on the vehicles gearbox and the analog needle moves to the speed that the vehicle is moving. There are small electromagnets in a servo that move the needle around. If the power to the vehicle is shut off due to a violent crash there is no residual power left to return the needle to its resting place. That is why the speedometer can give a good indication of what the vehicles speed on impact was. This method is accepted by Police in USA, Canada, UK and Australia. I hope this explains why this method is used. Black boxes are not fitted in all vehicles in other countries.