>This kind of test seems silly. It's going to be far too hard to remove the confounding variables. Much easier just to give people different levels of weed and have them do driving tests. Directly measure their driving skill instead of doing it by shitty proxy like this.

Given that differing levels of THC impact people differently both because of potential "tolerance" in frequent users as compared with occasional users and individual responses to cannabis (and even different cannabis strains with varied chemical profiles). There may well be other confounding factors as well.

Cannabis does not affect everyone the same way. It doesn't even affect the same people in the same way every time.

As such, while the testing you suggest may well be useful over the long term, it will require large populations and repeated testing at varying levels of both subjective intoxication and THC levels in the blood over extended periods to get good data about how THC use (both in temporal proximity and overall usage patterns) causes impairment.

As anecdata, I can absolutely say that lower levels of THC consumption results in much more impairment if cannabis hasn't been used recently and higher levels result in less impairment if there has been recent use.

That's not to say that driving (or any high-risk activity) is appropriate while actually high. It is not. Driving while impaired (by anything) is a terrible idea.