Breathalyzing is binary but "being too drunk to drive safely" isn't, they just made testing binary for the law's sake. It's possible to be under the limit for a

Breathalyzing is binary but "being too drunk to drive safely" isn't, they just made testing binary for the law's sake. It's possible to be under the limit for a test yet still have enough alcohol in you to slightly impair your driving if you're not used to drinking, it's also possible to be able to drink quite a bit over the limit and still be sober enough to drive safely, but the law just looks at averages and picks a spot to make it binary.