A National Institute on Drug Abuse study tasked stoned participants with driving a highly sophisticated driving simulator.
Researchers found that after their blood levels of THC reached a certain point, the drivers weaved similarly to a driver with a BAC of .08
The study also found that combining cannabis and alcohol decreased motor skills even more than just one or the other
Links
[1] http://www.dailymail.co.uk/news/article-3137943/Marijuana-DOES-impair-driving-kind-comprehensive-government-study-reveals-cannabis-use-affect-motor-skills-three-drinks.html
[2] http://rethinkpot.org/tags/car%2A
[3] http://rethinkpot.org/tags/alcohol
[4] http://rethinkpot.org/tags/impaired