"Oh Shit" moments...

It's a dangerous game. 'Behaviour' as judged by a flock of slightly up tight Daily Mail types is clearly different than that judged by someone more socially relaxed. People have a tendency to group their responses and to respond to the prevailing mood too. I'm not saying that witnesses can't be useful in determining motive or aggression for example "he was running at the victim shouting and raising his fists, he made no attempt to move away or calm the situation" etc. But being drunk is not so straight forward.

Sure, but witness accounts are used as part of the process of convicting someone for murder, for example, which involves proving intent. Surely intent is a state of mind even more, er, don't know what the word is, but y'know, even more something than being drunk. Basically, witness statements are used in court, and there are mechanisms in place, such as one witness not being as strong a case as two unrelated witnesses, reliability of witnesses (not being drunk/shortsighted themselves) to prevent or reduce abuse/misuse/mistakes. So why not here?

I imagine it is just custom or accepted practice that they only use breath or blood tests. Or is it the case that this is prescribed in law?