NEW YORK (MainStreet) — According to a new study by researchers from the University of Colorado School of Medicine, the proportion of marijuana-positive drivers involved in fatal motor vehicle accidents has increased significantly since the 2009 legalization of medical marijuana.

The study, which was funded by the National Institute on Drug Abuse and the National Institute on Alcohol Abuse and Alcoholism, was conducted by Stacy Salomonsen-Sautel, Ph.D., a pharmacology postdoctoral fellow; Christian Hopfer, M.D., associate professor of psychiatry; Sung-Joon Min, Ph.D.; Joseph T. Sakai, M.D.; and Christian Thurstone, M.D. They looked at data from the National Highway Traffic Safety Administration's Fatality Analysis Reporting System for the period from 1994 to 2011.

They compared fatal motor vehicle crashes in Colorado with 34 states that did not have medical marijuana laws, then compared changes over time in the proportion of drivers who were marijuana-positive and alcohol-impaired.

The researchers found that 4.5% of fatal motor vehicle crashes in Colorado, involved at least one driver testing positive for marijuana in the first six months of 1994. This percentage was 10% in the last six months of 2011. The spike occurred after the commercialization of medical marijuana in 2009. When compared to the 34 non-medical marijuana states from mid-2009 to 2011 the increase in Colorado was a lot more. But they did not find a concomitant increase in the proportion of alcohol-impaired drivers in a fatal motor vehicle crash during this period.

Salomonsen-Sautel said the study "raises 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."

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The researchers did add that although the study does not determine causality, i.e. marijuana-positive drivers caused or contributed to the fatal crashes; it does mean there needs to be more education of the public of the role marijuana can play in impaired driving.

But some were critical of the study.

"This study only shows that an increased number of people had tested positive for marijuana," said Morgan Fox, spokesperson for the Marijuana Policy Project, a pro-legalization organization. "This does not mean that the drivers were impaired at the time of the accident, since marijuana is detectable in the body long after the effects have ceased. The study does not show that these accidents were marijuana-related."

Fox also noted that the study does not show an increase in the number of accidents. To say that it demonstrates an increase in accidents caused by marijuana impairment would be incorrect, Fox said, arguing that no causal relationship between changes in marijuana policy and an increase in traffic accidents has been established.

It should be noted that the researchers did not claim an increase in accidents caused by marijuana impairment. They simply pointed out that there was an increase in drivers involved in fatal accidents who were marijuana positive.

"It is not surprising that more people were testing positive during that time, as the number of registered patients was increasing," said Fox. He then pointed to a 2011 study by Daniel Rees, an economist at the University of Colorado in Denver, and D. Mark Anderson of Montana St. University that indicates a decrease in traffic fatalities after passage of medical marijuana laws.

"This also does not prove a causal relationship but does show a correlation," said Fox.

--Written by Michael P. Tremoglie for MainStreet