The first question I have is whether driving high on cannabis is less safe than sober. Then there is a question of levels of impairment. I wouldn't be surprised that someone lightly stoned is actually safer than sober due to being more patient with performance degrading the more stoned they are. Treating cannabis like alcohol is not the correct approach. I sold find it better to think of it like coffee. Some coffee before driving is fine but large jitter inducing amounts is not.