It is well enough understood to know there are complications when it comes to determining impairment. It seems the legalized states are going for some sort of 5 nanograms per milliliter rule which on not sure what that means as far as how 5 nanograms make you feel (like 3 beers (or roughly .08) feels like a good buzz). I'm not sure what my numbers meant when my positive tests came back a gas chromatography – mass spectrometry -- only info I remembered was the number of nanograms. I can't remember the details around the LOL (it was almost 8 years ago) but the one that was almost 200 I was sober a good (the 4 or 5 days before that--a different story) 2 days and of course that one after that, over a week. Don't know what it means but can tell you it doesn't mean I was high. I realize blood tests are different.
Even then, the limit used is unreliable. There are so many studies on the issues and they all pretty much come back the same way.
NY Times Article
THC levels must be measured from blood or urine samples, which are typically taken hours after an arrest. Urine tests, which look for a metabolite of THC rather than the drug itself, return a positive result days or weeks after someone has actually smoked. Yet most states have laws that equate any detectable level of THC metabolite in urine with detectable levels of actual THC in blood, and criminalize both. Only six states have set legal limits for THC concentration in the blood. In Colorado and Washington, where recreational use has been legalized, that limit is five nanograms per milliliter of blood, or five parts per billion.
The problem, Dr. Huestis said, was that studies from Europe suggested that this limit was far too high. Ninety percent of impaired-driving cases in Sweden would be missed at that level, she said.
http://www.nytimes.com/2014/02/18/health/driving-under-the-influence-of-marijuana.html?_r=0
http://www.popsci.com/science/article/2013-05/now-recreational-marijuana-now-legal-two-states-how-will-police-regulate-stoned-driving?page=0%2C1
The issue is actual impairment. Someone who uses regularly isn't as impaired as a new smoker at the same levels, also depends on the person's mind & body -- could cause panic attacks for instance and not a good drug if you have schizophrenia or history of or a predisposition. Also indica would probably cause more impairment than sativa.
In may case, I agree you're still technically high more than a few hours after but the initial "awareness" has faded and you're basically sober. In a first timers case it is likely different, if you expect and learn the effects it doesn't get as high as much anymore which is why I touch it once in a blue moon these days.
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