I'd be curious to know if blood alcohol reliably predicts impairment. From what I know, modern drunk / intoxicated driving laws are mostly about ideology and have little to do with public safety. To be clear, drunk driving is dangerous and a concern, but "impaired" has lost all real meaning as with all ideological terms so a 200 pound regular drinker who's had two drinks is treated the same as an alcoholic who's blackout drunk and a serious danger.

And urine tests only test for a by product of the intoxicant with a half life of days to weeks, so are pretty useless.