You do know that people don't stay high for as long as the THC stays in the bloodstream, right?
So can I take it you're pro gun control because someone might use a gun in a dangerous manner? Personally, I don't want to be on the road with someone with a gun and road rage.
Or should we have psychological profiling so we make sure that people who get road rage aren't allowed to drive at all? I don't want to be on the road with them either.
Yes, I have been pretty clear that the high doesn't last as long as the substance can be detected in bloodstream/urine. But while a person will test negative for alcohol after so many hours and you know that person is sober, no such test exists for marijuana. There is no way to know whether the person who tests positive for marijuana is impaired or not. So a zero tolerance policy is the practical one.
Road rage and hand guns are separate issues and are red herrings in this discussion. Just because there are other reasons a person should not be driving is no reason to say that oh well then, we shouldn't be concerned if he or she is high on pot.