A car operated by a person who is high out of his mind will probably arrive safely since Marijuana does not impair your judgment. If you start to drive like an idiot, you can tell what you are doing and fix the situation. I've been on the highway doing 130km/h, hitting my pipe, while steering a broken down pedovan with broken handling. I also used to hotbox the shit out of the car while parked, and drive away with the windows open so I can see the smoke billow out. I've never driven drunk, but I've been in cars with people who shouldn't have been driving, and know the difference. Drive with anyone "high out of their mind", and the worst case scenario would be you probably drive way to slow, and need to make constant drink stops.
If you are so goddamn high that you are acting like a drunk driver, then you pull over for 5-10 minutes, relax, and your fine. Worst case scenario, you pull over for 30 minutes, and wait for it to wear off, because as much as most people would like to think, Alcohol isn't the same thing as pot.
I've seen people who actually drive infinitely better when stoned. Sober, the asshole is a maniac cutting people off tearing down the street, and totaled 3 cars in 7 years. Stoned, he goes 5-10 under the limit, and is so worried about getting arrested he takes extra care into every little maneuver.