There's a very simple way to know. None. That's the only way to know. The question that I've received from the general public and other people over the years that I've been doing this kind of work has been, “How much can I drink and still drive?” We can do calculations and we can look at time and sex and those kinds of things and sort of say, “Yeah, probably keep it under two, maybe three you're okay, and you'll be under the limit.” I didn't say you weren't impaired, but you'd probably be under the limit.
I dread the day somebody comes to me and says, “How much cannabis can I smoke and still drive”, because the only answer to that question is none. I don't believe that there's any amount of cannabis that you can smoke, and at some point not be above two. Because the level of THC rises so quickly, you will rise above two at some point, and you will come back down at some point, too. There really is no amount of cannabis that we can recommend that a person could smoke and still be below the limit and be unimpaired.