"It's not clear why traffic deaths might drop when medical marijuana becomes legal, and the study can only show an association."

Well, DUH! Who do you want coming at you in the other lane: a stoner doing 35 with a white knuckle grip on the wheel or a drunk doing 80 with NO hands on the wheel? Having said that, combine both substances - all bets are off. Further, throw in a cell phone and someone is gonna die.