This is why we should outlaw impaired driving, of any sort. The effects of an x% blood-alcohol level (for whatever x), on real people's actual impairment, seems to vary quite a bit. If we had a good way of testing impairment, it wouldn't matter what the cause of that impairment was. A driver impaired by fatigue or stress is just as dangerous as one impaired by any chemical. The problem is that it's a difficult thing to test, I guess. Something like the old field-sobriety tests could probably work, though.
The fact that it is so difficult to design reliable FITs (field impairment tests), to me, says a lot about the questionability of sobriety testing in general.
If we can't measure impairment, how can we justify x% as an "impaired" alcohol level ?
The problem with associating impairment with levels of some chemical is that there's always going to be a new chemical on the scene - and people can have wildly differing reactions to a lot of chemicals. Bring on the self-driving cars!
I have always thought a "breathalyzer" should be some mechanical device that measures the same skills one needs to capably drive a car. If you can not adequately handle that then you can not drive no matter what the "impairment"; whether it be booze, pot, old age or simply some other cognitive or motor skill issue. And although I would like to see it happen faster, even google, benz, etc. admit that fully autonomous cars are a long way away.