Presence of THC metabolite in your urine does not prove current intoxication unless there are other signs of intoxicated behavior to go with it. In fact it is my understanding that if you are clean to start, the metabolite may not show up in your urine for a couple of hours. Now for those with CDL's or in businesses like I am in, any positive test will get you fired/cause loss of licence so be careful, but a positive test alone for metabolite is not proof of present intoxication and courts from even conservative states such as Arizona agree.
I'm going to use Washington state here for my example. To test to see if a driver is impaired they will need to do a blood draw. This is the only way I know to conclusively prove someone is actively under the influence of THC. Washington's standard is 5 Ng THC /ml of blood. This seems reasonable to me as as the studies out there show that most peoples' risk of accident goes up significantly after crossing the 4Ng level in the blood. Though at the 5 level the risk is still less than a couple of alcohol drinks. And at levels under 4 the risk is pretty much the same as sober. Probably because cannabis tends to makes you drive slower because of the heightened senses, whereas a couple of shots of whiskey tends to make you think you can do anything while reducing your judgement.
This test is required by state law to be done within 2 hours of suspicion of intoxication.
It was a while ago that I was researching this so if anyone wants to add in or correct me, feel free.
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