What's subjective about a field driving test? The reason is simply that it's impractical to do so. It's easier to run a quick BAC test, look at a number, and c

What's subjective about a field driving test? The reason is simply that it's impractical to do so. It's easier to run a quick BAC test, look at a number, and call it in than supervise a road test. So you can imagine there being false positives where the person fails the test but would otherwise drive safely. Don't forget the true intent of these tests are to determine impaired driving. The criteria of "being under the influence of something" has historically been a useful analogue for that, but the question is if we can do better somehow by testing for impaired driving directly and not 2nd order metrics.