It seems to me that, in the U.K., most people's knees will tremble when stopped by the plods and if they're pulled over then they must be guilty.
The lesson I learnt was to stop the quivering and analyse what these tax collectors were saying and writing. Inevitably, since they are not used to being challenged, traffic police ooze confidence and as a consequence, they often make the simplest of mistakes to the public's advantage.
Above all, even if you are guilty, never admit it. Think first, act second!