Let us provide a rational answer to a nonsensical question. It is a nonsensical question because blood is never impaired by THC. Never. Alcohol doesn’t impair blood either. These drugs only impair the brain, not the blood. We can only test for drug content in the brain by means of an autopsy, something most drivers would reasonably object to. We test blood as a surrogate for what’s in the brain. For alcohol, blood is a very good surrogate. Alcohol is a tiny, water-soluble molecule that rapidly crosses the blood-brain barrier and quickly establishes and maintains an equilibrium concentration between what’s in the blood and what’s in the brain.
Blood is a terrible surrogate for learning the amount of THC in the brain. It’s used because we blindly follow the precedence set by alcohol, perhaps even believing the pot lobby’s mantra that marijuana should be regulated like alcohol. It’s also used because we haven’t proven anything else that’s any better. Oral fluid likely is somewhat better, but that may only be because it can be collected more quickly at the roadside.
Perhaps this explains why researchers agree that marijuana impairs driving, but none claim there is a good correlation between blood levels of THC and impairment.
The fact is that there is no level of THC above which, everyone is impaired, and below which, no one is impaired.
The same is true of alcohol. In spite of common belief, the .08 BAC limit wasn’t determined by science. It can’t be, due to the reality of biological variability. The .08 BAC limit was determined by politicians, using scientific input as well as societal input. That explains why the alcohol per se limit varies from .02 to .08 gm/dl in various developed countries of the world, and those countries based their decision all on the same science! It’s other societal inputs such as risk tolerance and desire for freedom that come into play to make that decision.
None of this proves it’s safe to drive after smoking pot. It’s not. It simply explains why a defined per se limit of THC in blood that proves someone is impaired can never be supported by science.
This also may explain why the preferred means to deal with drug impaired driving is not to establish per se limits, but rather to establish a zero tolerance policy for mind altering drugs in a driver that has been shown to be impaired.