I am against "per se" limits for all drugs/alcohol for the very reasons you mention. Look into the history of the development of a per se limit on alcohol (and breath testing of it) sometime and all the assumptions, roundings and kinda-abouts in developing the limit. Don't worry, only a few people will be statistically convicted of a crime they shouldn't have. Of course, when ANY amount is illegal (Drivers under 21 having zero tolerance statute.), we're not talking about if a person has been impaired but if the measuring is accurate within statistical error.