Just like alcohol, different amounts/types affect people in different ways. I can drink one beer then go drive myself home with no impairment. If my wife drinks 1/4 of a beer, she'll fall asleep within 10 minutes.
One of my best friends was a frequent smoker during college. He said it actually made his driving better because he felt like he was more careful. Granted he's a shitty driver to begin with, so his statement may be true. However I've been around enough "high" people to know their quick decision making skills are definitely impaired and I would not want them driving me home. I think they are less dangerous than a drunk guy on the road.. but that's not legal.