Setting a zero limit for alcohol for drivers under 20 is a sensible move.
There is no guess work about a zero limit – either someone has had an alcoholic drink or they haven’t and if they have they’re breaking the law if they drive.
I don’t have any objection to reducing the limit for adult drivers to .05 either, it’s what most people believe is the case already anyway.
But it could cause problems for people who don’t realise that a big night can put them over the limit next morning.
That could be particularly problematic for people under 20 because even a trace of alcohol from the night before would put them over the zero limit if they’re tested on the morning after.
Would it be too optimistic to hope that an unintended consequence of lower drink-driving limits could be a reduction in binge drinking if people who need to drive next day will know they can’t afford to drink too much the night before?