Impairment is relative. I know a guy that can drink a 12 pack to himself, but is he drunk and/or impaired? No, he is not. He can drink 12 beers, (sad really), and be behind the wheel with virtually NO impairment. Can't say much about the blood-alcohol level though, but he's never been pulled and has no accidents, go figure. Then the other spectrum - a light-weight drinker where one alcoholic drink is enough to impair.
I'll agree about cell phones. Worst are texters/facebook checkers that drive, worse than impaired when constantly glued to the phone. Know one that's hit and been hit while on phone, female. She must have her social life while driving. Little does she realize that life might be over in a crash.