Lots of marijuana activists claim that driving while high does not impair your driving ability. Anyone who's ever been extremely stoned knows that even basic tasks like walking or talking are difficult, let alone driving.

I don't really mind a nearly-zero-tolerance policy for driving while on any kind of drug. Some people are way more tolerant to weed than others, and may be nearly sober even with a THC concentration that makes someone else couchlocked, but I think it's better to err on the side of caution.