I also posted this in response to a question by R1Geezer:
I took the view that as the first duty of a police officer is to prevent crime, Mr Thompson’s action was assisting the police in that duty and therefore the prosecution was wrong. However, having read the details of an appeal in the High Court which barmaid kindly provided:
http://www.bailii.org.../Admin/2005/2333.html
I’ve had a change of heart.
Their Lordships in their wisdom determined that there was a finer point of law involved. To save you trawling through the whole case notes, in summary they decided that the issue is what else had occurred which the defendant’s action may have influenced. They decided that if a crime (i.e. speeding) had already been committed or was about to be committed a police officer would have a duty to take action to detect or prevent that crime. Anybody interfering with this duty (i.e. obstructing) is guilty of an offence. However, if no offence was committed or about to be committed there could be no obstruction.
The Magistrates obviously took the view that Mr. Thompson was warning drivers who either were speeding or were possibly going to speed that they would be apprehended by the speed trap and so they decided he had obstructed the police.
The appeal case addresses a fine point and I can understand their Lordships findings with regard to drivers who were already speeding, but cannot quite agree with the decision with regard to those who were about to speed. They may have slowed down if they saw the speed trap themselves or if they saw Mr Thompson’s warning. Either way a crime has been prevented and Mr. Thompson assisted the police in that duty.