Zero-tolerance for THC doesn’t make sense to me because of how it lingers in the system. You definitely shouldn’t smoke and drive, but there’s a difference between having a smoke and driving a few hours later versus smoking right before getting in the car.
Cannabis also affects people differently; people new to it or returning to it after a long hiatus are going to be completely incapacitated if they smoked the amount that I’m used to smoking.
In my opinion, alcohol is still a much greater risk when it comes to impaired driving.
This is no way justifying people who drive while high, it’s just that we need to have a better way of measuring who’s actually high or not, and I don’t know if that’s possible, yet.