12 penalty points ≠ ban

Good question - and the one I am noe asking you, in a way.

Let me put it like this, as this is how I suspect the majority would view it:

Why is it fair to stop people having an amount to drink that does not (legally, currently) effect their ability to drive?

I.e if it has no effect, why are you banning it?
Well to infringe on people's liberties to impair their bodily functions and risk lives I agree that I need some evidence. That is solidly medical territory, and I can only offer a quick google-survey...there are studies demonstrating impairment at levels below the current limit and studies demonstrating no impairment.

Assume that it is impossible for alcohol to impair driving performance at concentrations lower than the legal limit (which I don't think is the case).

The only argument in favour of allowing people to drink is that it allows people to drink, which raises revenue and avoids infringing on personal liberties.

There is still an argument in favour of banning it to be made -- the people who can't count, miscount, are lighter than they think. They have a glass or three, and without deliberately flouting the law, they drive with an illegal level of alcohol in their blood.

If a total ban was made, all of those cases would be eliminated if people had a basic respect for the law (which most do, I think). The idea is that people would no longer be able to try to drink up to the limit.

Now, if you assume that there is a non-zero effect on driving skill below the current limit, the argument gets stronger.