As we start to really talk about the legalization of marijuana there is a false equivalence that has to be addressed. Marijuana is an intoxicant but that does not mean it is like alcohol. Legalization advocates have had to deal with a lot of misinformation over the years about pot. The false correlation that pot itself lead to other more addictive and damaging drugs (when the reality was the correlation is about illegality of pot, not the affects of using it), the concept that it somehow makes a user less productive or permanently impairs cognitive functions.
Luckily there is new and emerging science that shows this is not the case. There was a recent study which looked at the affect of being high on marijuana and driving. The University of Iowa did a small double blind study to see if how smoking pot would affect the ability to drive.
They took a group of 85 volunteers (50 men, 35 women) and ran them through a simulator for baseline comparison. They then gave some cigarettes with THC and some without and repeated the testing. The test looked at basic driving, distracted driving and collision avoidance scenarios.
The result was that there was no significant difference between pre-pot baseline or the performance between those who were high and those who had received the placebo. The major difference was that the high drivers were more likely to slow down in situations where the straight drivers did not. Since we know that there is a correlation between average driving speed and accidents, this tends to indicate that stoned drivers would actually be safer than none stoned drivers.