This is it exactly.
If an accident happens, the drug test is going to show THC in the bloodstream. It may be from 2 weeks ago, or it may be from 2 hours ago. It depends on so many factors: how regular of a user are they? how fast is their metabolism? what is their bmi (the way I understand it, it is stored in fat??)?
What is an acceptable level of THC? That’s what courts are going to have to decide.
The few times I’ve used it since college, I felt like I was in a fog for a day or two after. Reactions slowed, wasn’t thinking quite as clearly. It was like a hangover but without the pain. I just felt slow. You could make an argument that I was the definition of impaired and I would have a hard time disagreeing with you.
That said, was I more impaired than when I had been up for 36-40 straight hours and driving company vehicles and attending vessels and terminals on the clock? Probably not. But nobody cares about that, and I know my (ex)profession wasn’t unique in that regard.