I get the idea that a single (BAC) number is the line and you're on one side of that line or the other, and the action for the officer to take is clear. However, the specific BAC at which an individual is more of a risk than otherwise is personal to them. Some people can't handle a single drink, other people can toss 'em back (Geez, is this right? Seems right).

However, alcohol and driving were around long before a roadside BAC test. Remember saying your ABC's backwards or holding your arms out horizontally to your sides and bring one finger to touch your nose? Those were personalized tests to determine if an individual had capable motor function and reasoning to operate a vehicle.

I would much prefer an individualized test of reasoning (or reaction time) and motor functions. I do not know at all how (in)accurate the old method was, but it was not ideal, either. I assume leaving the decision to the subjective judgement of the officer on the scene caused it's own form of problems.