As I have posted before the main reason pot is illegal relates to have a sound standard for impairment (e.g. a .08 standard) and a way to measure this quickly (e.g. during a police traffic stop).

Just having a law that says it is illegal to drive while 'stoned' means nothing unless there is a legal way to define 'stoned' and a way an easy and cheap way to determine if someone is stoned.

Until these two points can be worked out I don't see pot being legal.