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.