the day that i spent with him, he had been smoking all day long. >> do you feel impaired at all? >> no, i don't. i feel normal. >> turns out when you test people who have a lot of experience with cannabis, you don't see many disruptions, but if you test people who have sort of limited history with cannabis, you can see some clear pronounced disruptions. >> of course, no one thinks that driving when using marijuana is a good idea, but what scientists can't answer is if there is a safe legal limit and if people who use marijuana daily as a medicine should be able to drive. how impaired are they? what is more clear, though, is the effect of marijuana on the young brain. >> what we see is a very big difference in people who begin to smoke prior to the age of 16 and those who smoke after age 16. what we call early versus later onset. >> gruber's brain scans show the white matter, those are the highways that help the brain