Having to factor things in just shows how unworkable and relatively pointless having a level like zero would be. It'd always have to contain a bunch of factors to make it actually make any sense. Zero means zero, except for alcohol from food consumption, natural variation, medicine and tinctures and metabolic conditions below a certain threshold ... got it boss, zero means zero.
The problem is always "the limit is xyz" - great so how much can I safely drink? "you can't drink anything safely" - well I don't want to hear that, so how much can I drink? They'll always be people like that who want to push it because of their attitude towards driving.
I drink at least half a pint of beer throughout the day at work. Later I drive home. Saying anybody who drinks and drives should be banned for life where the criteria for such a ban is a road side blood test result of greater than zero is unworkable madness. Maybe one day they'll change the limit, probably by winding everybody up with tragic stories of those who've lost loved ones due to alcohol related accidents. Maybe one day I'll get banned for life for having alcohol in my blood stream which was unlikely to have been a factor in the accident, like I'm hit from behind waiting at the lights. The story will be man loses license after accident resulting in injury due to being over the legal limit. Sounds awful. I can see how I'd have totally deserved that.