My point is that if the law had a zero-tolerance policy on alcohol and driving, then at the very least, the matter of people trying to subjectively decide if they have had too much to drink before driving would never come up. It wouldn't stop people from breaking the law intent upon doing so and hoping they can get away with it without incident, but it *WOULD* stop at least some people who may sincerely believe they should be okay to drive after having some number of drinks, only to discover too late that they weren't. And since most people who might have had too much to drink do not generally deliberately get into a car intent upon driving in full awareness that they may not be capable of driving safely, a zero-tolerance policy would, at the very least, leave such people without any excuse for driving in the first place, because regardless of how safe *THEY* might think they are okay to drive, they would still know that the law had a zero tolerance policy, and that should hopefully be enough to dissuade them from engaging in the practice. If it isn't enough for any one person, then it isn't... but at that point, they would be actively intent upon breaking the law, and hoping they will get away with it anyways. Some people might do that, but probably not all.
And while it might seem that people who do not drink irresponsibly before driving (and I don't refute that they exist and even that most people who do drink probably fit into that category) would be unfairly discriminated against by such a law, consider first of all that driving itself is not a right, but a privilege, and that different amounts of alcohol can affect people differently, and a person who has had *ANY* alcohol before driving, even if they were below the legal limit, can still be charged with impaired driving if they were seen to be driving unsafely, or if they were involved in an accident. In accidents where the issue of fault is slightly ambiguous, the issue of alcohol, even if below the legal limit, can tip the scales of determining fault in an accident enough that a person can still be considered legally impaired.
In that context, therefore, I sincerely think that having a zero tolerance policy on alcohol for drivers would still be a good thing.