How many people have killed others while driving under the influence of alcohol?
It's pretty damned irresponsible to drive while stoned. In that sense, it's not much different than drinking and driving. A responsible person arranges for some other (safe) way to get where they are going. It doesn't say much about the legality question, however, because then we would be talking about reviving Prohibition, because there is no doubt that a great many people are killed every year thanks to drunks behind the wheel.
I just don't think that the "Should marijuana be legal?" debate comes down to driving safety. That's not to say that it's of no consequence at all, but I don't see DUI enforcement being relaxed at all anytime soon. If anything, I would expect even more DUI arrests and fines in the future, after it is decriminalized everywhere.
So what did the marijuana do?
Probably didn't roll down the windows =/