Image courtesy www.speedcam.co.uk
A man who was warning motorists of a mobile speed camera was acquitted recently of obstructing a police constable in the execution of his duty.
A mobile camera van had been set up on a dual carriageway, and the man, who in the report has been named only as 'G', was making slow-down signals to drivers. He was arrested and charged with obstruction, but was acquitted on appeal on the grounds that the cars were not exceeding the limit anyway, and so weren't slowing down. If his actions had no effect, then he was doing nothing wrong, was the gist of the judgement.
This of course ignores the issue at the heart of the action: is a speed camera there to slow cars down -- the official justification for the arsenal of devices -- or is it there to raise money? If the former, then G was performing a public service. If not, then the law could (and did) argue that, if his actions had obtained a reaction, then he was indeed obstructing the law.
Funny old world, ain't it?