There is no set formula. If you are a heavy marijuana user, you may have high levels of the active THC metabolite some 12 hours after smoking even though you will not be impaired at all. The NHTSA and leading scientific studies have stated that it is impossible to determine whether someone was using marijuana at or near the time of driving based on the presence of the active THC metabolite in the blood. I would simply recommend a “Drug Recognition Expert.” Just a police officer who took some drug classes taught by other another police officer. Because the tests are notoriously inaccurate. And a lawyer who regularly handles DUI marijuana cases will understand how to use federal guidelines to impeach the credibility of this “drug recognition expert.”
Good point Skip...Brain waves ie. the thought process has a foggy and foggier way of working the more stoned you get... Blood levels of THC can be measured just like booze. Yet the tolerance level goes up over time... Maybe a combo: test blood level and questioning would work...I know of jobs were you can't even have tabacco in your urine...!
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