Depends on the dosage. I expect that for any given amount of alcohol, there is some amount of marijuana that is equally driving-impairing; but I wouldn't be a bit surprised if the degree of driving impairment from 'typical' use of marijuana (if any) is considerably less than that for 'typical' use of alcohol.
now there is a blanced post! thanks!
and you're right, at a certain amount (from personnal experience, that amount was quite high...) it is a dangerous impairement...no doubt about it and no scientific article will say the opposit.