What is needed is a skill test to see who's fit to drive but then you have the daily drunks who blow a .10 and would pass the skill test and some sober folks who flunk
How about the cops pull out a card with jokes written on them. The jokes are rated from lame to hilarious with a corresponding rating of "highness". The reaction of the suspect is noted after each joke is read. If the suspect laughs at the lame, stupid, joke then it is obvious he or she is too baked to drive and, if he or she doesn't laugh at any of them, then they are too "mellow" to drive since their reactions are also too mellow.
Now what would the pot scale be? Here's a suggestion open for rearrangement, addition or subtraction.
Mellow
Buzzed
High
Baked Sh*t-faced