Comment by arcfour

It's also terribly misunderstood by pedants since you can be charged with a DUI with a 0.00 BAC by doing drugs. The point isn't that it's a definitive line in the sand between impairment and not, but if people are trusted to drive a car (generally or broadly speaking, not pedantically speaking), being above or below said limit is a reasonable litmus test for "visibly/obviously impaired" or not.