A study published in November 2011 by a University of Denver professor found that traffic collision fatalities went down by 9 percent after medical marijuana had been legalized in 16 states, including Colorado.

The study, by DU Professor Daniel Rees, was done in conjunction with Professor Mark Anderson, of Montana State University.

Rees’ and Anderson’s research also suggested that marijuana is a regular substitute for alcohol, which could be the reason for the decrease in fatalities.

“Drivers under the influence of marijuana reduce their velocity, avoid risky maneuvers, and increase their ‘following distances,’ suggesting compensatory behavior,” the study reads. “There is unequivocal evidence…that alcohol consumption leads to an increased risk of collision.”

“Even at low doses, drivers under the influence of alcohol tend to underestimate the degree to which they are impaired, drive at faster speeds, and take more risks.”

According to the study, medical marijuana legalization led to a drop in crash fatalities by 19 percent in drivers age 20 to 29, a 17 percent drop among drivers 30 to 39, and a 12 percent drop decrease in 40- to 49-year old drivers. The study found no evidence that medical marijuana is associated with any deaths among drivers over the age of 50.

The number of crashes and deaths among drivers under the influence of alcohol dropped by nearly 12 percent per 100,000 licensed drivers, the study found, and 14 percent per 100,000 among severely intoxicated drivers.

Colorado voters approved the use of medical marijuana in 2000. California was the first state to do so, in 1996.

Recreational marijuana was legalized by Colorado voters this year with Amendment 64, but the DU research only focused on medical marijuana. So it’s not clear if the numbers in Rees’ and Anderson’s study is a prediction of Amendment 64′s effects.

The study was based on data from the Fatal Accident Report System from 1990 to 2009 and the National Highway Traffic Safety Administration. Read the study here.

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