This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

AURORA, Colo. — A study by the University of Colorado School of Medicine shows the proportion of marijuana-positive drivers involved in fatal vehicle accidents in Colorado has increased dramatically since the commercialization of medical marijuana in the middle of 2009, researchers announced Thursday.

Using data from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System from 1994 to 2011, researchers looked at fatal car accidents in Colorado and the 34 states that didn’t have medical marijuana laws.

They found fatal car crashes in Colorado with at least one driver who tested positive for marijuana was 4.5 percent in the first six months of 1994. In the last six months of 2011, that percentage had jumped to 10 percent. The researchers found no major changes over the same time in the proportion of drivers in fatal crashes in which drivers were alcohol-impaired.

Lead study author Stacy Salomonsen-Sautel said the study raises concerns and shows a need for better education and prevention programs to curb impaired driving.