MikeF wrote: ... I didn't phrase my comment very well. Perhaps I should have written "Curious how many have assumed the driver had been drinking." There could be other reasons for the bad driving.
FWIW, I think I understood your broad point which is a good one, but it's that now well-known phrase or saying "keeping an open mind." Erratic driving is often caused by drink or drugs and breath testing confirms or eliminates that suspicion. "Over the limit" does replace the vague term "drunk" with a measurable standard.
I may well have previously recounted a case of my own: back to my patrol sergeant days - 1974 ish, I drove off the station car park at the start of a night shift and was almost immediately behind a VW Beetle - a current model back then - weaving about all over the road. Like something from a cartoon animation. I stopped the suspect driver who was an older man who got very annoyed. He explained that his erratic driving was not erratic at all but he had been avoiding the bumps in the road. I called for a breathalyser kit to be brought out to me - glass tube and bag in those days - and while we were waiting he got stroppier still: not falling about drunk in the road. Positive test and down to the Town Hall for the samples procedure. He gave urine and it was a matter then of waiting for the lab analysis to come back. On the way back to the nick I called round to his house - he was almost home when I stopped him - to check his address and to notify his wife where he was. Her reaction was that he was obsessed about not damaging his car on the bumpy road and always weaved round the bumps. A week or so later and back on days, I walked into the front office and one of the PC's typing up process had the lab analysis: something like 407 - a big reading. It turned out that he had a metal plate in his head - the result of a war wound - and drinking was his pain relief. By a sad irony, his surname was a spelling variation of a well-known pain-killer.