Cannabis remains in the system for a long time but there is little research to say how long it will impede someone's ability to drive.
Some research has even shown that cannabis smoking makes you more cautious and more aware of dangers on the road!
Prescribed sedatives and painkillers are more likely to make you a danger to other road users.
http://www.washingtonpost.com/news/w...ral-data-show/
I don't smoke it myself but I do think more research is needed.