Cannabinol, a remnant of marijuana, was found in 12.2 percent of those deceased drivers during 2010, (up from 4.2 percent in 1999). Pot was the most common non-alcoholic drug detected by those toxicology screenings.
Links
[1] http://www.nbcnews.com/health/health-news/pot-fuels-surge-drugged-driving-deaths-n22991
[2] http://rethinkpot.org/tags/car-accident%2A
[3] http://rethinkpot.org/tags/impaired
[4] http://rethinkpot.org/tags/alcohol
[5] http://rethinkpot.org/tags/colorado-0