Since the commercialization of medical marijuana in the middle of 2009, the proportion of marijuana-positive drivers involved in fatal motor vehicle crashes in Colorado has increased dramatically, according to data from the National Highway Traffic Safety Administration's Fatality Analysis Reporting System covering 1994 to 2011.

The University of Colorado School of Medicine  researchers analyzed fatal motor vehicle crashes in Colorado and in the 34 states that did not have medical marijuana laws, comparing changes over time in the proportion of drivers who were marijuana-positive and alcohol-impaired.

Results: Fatal motor vehicle crashes in Colorado involving at least one driver who tested positive for marijuana accounted for 4.5 percent in the first six months of 1994; this percentage increased to 10 percent in the last six months of 2011. 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. The researchers also reported no significant changes over time in the proportion of drivers in a fatal motor vehicle crash who were alcohol-impaired within Colorado and comparing Colorado to the 34 non-medical marijuana states.  

The results raise important concerns about the increase in the proportion of drivers in a fatal motor vehicle crash who were marijuana-positive since the commercialization of medical marijuana in Colorado, particularly in comparison to the 34 non-medical marijuana states.

While the study does not determine cause and effect relationships, such as whether marijuana-positive drivers caused or contributed to the fatal crashes, it indicates a need for better education and prevention programs to curb impaired driving.