Cannabis and alcohol acutely impair several driving-related skills in a dose-related fashion, but the effects of cannabis vary more between individuals than they do with alcohol because of tolerance, differences in smoking technique, and different absorptions of Δ9-tetrahydrocannabinol (THC), the active ingredient in marijuana.
Links
[1] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2722956/
[2] http://www.rethinkpot.org/tags/alcohol
[3] http://www.rethinkpot.org/tags/driving