IMO... I don't see why it can't be set to zero.
We have to be practical and pragmatic.
I can see the moral case you're making here. We shouldn't drink and drive at all, people shouldn't be stupid. You're operating a 1 tonne hunk of metal thats going fast. I get that. Problem is not everyone shares your moral standpoint, people are stupid sometimes.
It's great to be moralistic but there is probably little practical difference between 0.00 and 0.01.
People will drink. You can't avoid that. If you set the limit at 0.00, people will constantly be breaking it. They're not going to stop because its illegal. In-fact, it might be worse, if we have it at 0.05, people know they have to stay under that and only have a couple drinks or get busted. If we set it at 0.00, it hardly matters anymore to those that would of had a drink anyway.
Say you go to a restaurant or something and have a single glass of wine (which isn't all that unusual), you're already over dead 0. Soy sauce (if its properly made) is even mildly alcoholic.
The way your body processes alcohol is different to a lot of drugs. For most drugs your body removes a fraction/% of that drug, rather than an absolute number. So, maybe, 50% every 4 hours. If you take 100mg, its 50mg eliminated every 4 hours, take 200mg, its 100mg. Alcohol is different. It's a constant number. It's not proportional. So, your body might only be able to break down one standard drink every hour. Plenty of people can have a heavy night out and still be over 0.00 the next day.
It's fairly impractical, it'll probably tie up police and court resources as well. I'm not usually with the libertarian crowd but something like this would be a fairly big intrusion on our civil liberties, to not be able to have a glass or two of wine when you drink. You can drink responsibly, plenty of people do it.
Even 0.02 applies to what i've said.
If you violate the law though, there are things they can do. I had a relative who was 0.098 or something (almost twice the legal limit). Lost their license for 6 months. Once you're 0.10 or over, they take your license for 2 years as far as i know, its pretty much automatic unless you choose to contest it in court (you'll have a hard time wining).
I understand we're relying on the fact that these people are just randomly picked up by booze buses to get caught but even if we lowed the limit, it wouldn't stop those who already break the law from breaking it. People know you can go down backstreets and enforcement is patchy. All it'll really do is create a massive hassle for the rest of us.
http://www.vicroads.vic.gov.au/Home/SafetyAndRules/RoadRules/Penalties/DrinkDrivingPenalties.htm