You don't have measure actual driving ability. Current field sobriety tests don't do that. You can instead test coordination, balance, cognition, reaction tim

You don't have measure actual driving ability. Current field sobriety tests don't do that. You can instead test coordination, balance, cognition, reaction time, etc. These are the things we actually care about when we talk about impaired driving. So why don't we test them instead of testing for one or two specific substances?