One study[1] has found that "THC's effects after doses up to 300 mcg / kg never exceed alcohol's at BAC's of 0.08 g %" and that "alcohol impaired performance relative to placebo but subjects did not perceive it", whereas "THC did not impair driving performance yet the subjects thought it had."

Another study[2] finds that "marijuana smokers tend to compensate effectively for their impairment by utilizing a variety of behavioral strategies such as driving more slowly, passing less, and leaving more space between themselves and cars in front of them", although it notes that the combination of marijuana and alcohol can impair even at doses that would be harmless for either drug alone.

[1] http://casr.adelaide.edu.au/t95/paper/s1p2.html

[2] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2722956/