The problem here is that such a test would probably find that many stone cold sober people should not be operating motor vehicles - that's why such a test has not been developed.  This is what we need, however - a way to determine if someone's reflexes and attention is sufficient to operate a motor vehicle safely, regardless of what may or may not be in their blood at the time.  It removes the "sinner" aspect and gets to the heart of the matter which should be safety.