They reported that Colorado underwent a significant increase in the proportion of drivers in a fatal motor vehicle crash who were marijuana-positive after the commercialization of medical marijuana in the middle of 2009. The increase in Colorado was significantly greater compared to the 34 non-medical marijuana states from mid-2009 to 2011.
Links
[1] http://www.eurekalert.org/pub_releases/2014-05/uocd-mui051514.php
[2] http://www.rethinkpot.org/tags/colorado-0
[3] http://www.rethinkpot.org/tags/impaired
[4] http://www.rethinkpot.org/tags/car-accident%2A
[5] http://www.rethinkpot.org/tags/fatalities