My preferred way to wade through a political research topic like this is to look at the aviation industry. If a pilot can not use a medical substance, then it is very likely that there is some thing there. Pilots are generally fairly high investment, and they are also fairly international in research and standards. All nations with an airforce tend also be interested in such research, regardless of current political flavor.
From glancing at it, it seems that TCH impair pilots ability. Here is such study (done with flight sims). https://pubmed.ncbi.nlm.nih.gov/1849400/
"The results support our preliminary study and suggest that very complex human/machine performance can be impaired as long as 24h after smoking a moderate social dose of marijuana, and that the user may be unaware of the drug's influence. "
I think that aviation has a lot of factors that bias its decisions conservatively. The “cost” of being conservative can be pretty easily borne in aviation. As an example, Zyrtec is grounding for 48 hours after taking. I think most Zyrtec users drive daily and likely quite safely, but aviation can afford to ground pilots at a lower level of risk than probably makes sense for drivers.
(I have very little doubt that THC is impairing; that pilots can’t legally use it is only very loosely related to that likely linkage.)
I think you miss the point of the comment you replied to.
> The law's assumption that this level of THC is evidence of impairment seems to be invalid.
I think most will recognize that THC causes impairment. The question that (AFAIK) is unanswered is if it can be measured simply by looking at the concentration of THC in the blood.
In fact, if you look into the mechanisms for alcohol tolerance vs THC tolerance. What you'll find is that alcohol tolerance is a result of the body developing fast paths for breaking down ethanol. Meaning the same BAC will have the same intoxication level, the body just works harder to keep the BAC down.
THC tolerance, on the other hand, appears to be the THC receptors becoming desensitized to THC. Which means the body doesn't appear to metabolize THC faster as tolerance builds.
That's where a blood test might not be a good indicator of THC impairment.
What the aviation study show is that the impair can continue for a very long time after a single small use. A blood test might be a terrible indicator how how much the impairment is, but the question then is how much tolerance you need in order to have zero impairment under some levels of THC. It also but a bit of doubt that even after 24hrs the impairment can be noticed, long past where the user subjectively feel any effect.
Alcohol tolerance might even be a positive here, since a drinker can drink a glass of beer 24hrs before a flight and be fairly certain that the ethanol has been broken down, regardless of tolerance. If THC metabolize slower as the body builds tolerance, then the impairment may continue for a longer time at a lower intensity even for a small dose, increasing the period of uncertainty.
The question then becomes "what level of impairment is tolerable".
Driving doesn't require perfect cognition, just good enough. If we went for perfect then anyone over 65 would be banned from operating a car.
And I think that's the hard thing with THC. Yes you may be impaired 24 hours later, but how impaired and how does that compare to age related impairment.