Every peer reviewed, replicated study of the effect of Marijuana on driving ability essentially shows the same thing: You are insignificantly more impaired than a “Straight” driver, and considerably more in control of your vehicle than under the influence of alcohol.
That said, personal responsibility must always play a role. The National Organization for the Reform of Marijuana Laws (NORML, our parent organization) has long taken a position that one shouldn’t drive while stoned. If you’re a good driver normally, you probably won’t be different when you’re stoned. If you’re not that great of a driver, being stoned isn’t going to help.
At the time of this writing, there is no good, accurate testing method that tells one is currently impaired. Most drug tests measure metabolites of Marijuana…by products…of use. These metabolites stay in the body’s fatty tissue for, on average, 30 days, though in heavy users with a lot of body fat, they may test positive for up to 90 days. Meanwhile, a Marijuana “High” might last for a few hours.
This article in the Wiley Online Library by noted Marijuana policy researcher Paul Armentano, lays out several of these studies and should answer most questions one might have on the psychomotor effects of Marijuana.