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In one of the first studies to look at fatal accidents after medical marijuana dispensaries became widespread, researchers at the University of Colorado School of Medicine find that fatal car crashes involving marijuana jumped dramatically in Colorado. 

The proportion of deadly accidents in which the driver tested positive for marijuana doubled between 1994 and 2011.  A significant increase came after 2009 when hundreds of medical pot shops opened

Lead researcher Stacy Salomonsen-Sautel was surprised to see, in contrast, the rate of fatal accidents in states that don't allow medical marijuana were flat.

"They do not have any increase from 1994 to 2011, their rate of the proportion of marijuana-positive drivers is relatively stable over time," Salomonsen-Saute says. 

She says Colorado policymakers should consider marijuana driving education and prevention programs.

"It shows there is concern out there, and education prevention programs do need to be implemented for Colorado drivers," Salomonsen-Saute says.

The study appears in the journal Drug and Alcohol Dependence.