After legalizing recreational use of marijuana, the states of Colorado, Oregon and Washington saw collision claim frequencies rise about 3% higher overall compared to neighboring states, according to a new analysis from the Highway Loss Data Institute (HLDI).

Washington and Colorado were the first states to legalize recreational use of marijuana for adults 21 and older, when voters approved a ballot measure in November 2012. Retail sales began in January 2014 in Colorado and in July 2014 in Washington. Oregon sales started in October 2015, after voters approved a measure in November 2014.

HLDI’s analysis relied on neighboring states as additional controls to evaluate the collision claims activity in Colorado, Oregon and Washington before and after law changes. The control states included Idaho, Montana, Nevada, Utah and Wyoming, in addition to Colorado, Oregon and Washington before the legalization of recreational cannabis use, the institute said.

Additionally, HLDI took into account loss results for each individual state and compared those numbers with loss results in adjacent states without legalized recreational marijuana use before November 2016. The data included collision claims filed from January 2012 to October 2016 for 1981 to 2017 model vehicles. HLDI noted that its analysts controlled for differences in the rated driver population, insured vehicle fleet, mix of urban versus rural exposure, unemployment, and weather and seasonality.

“The combined-state analysis shows that the first three states to legalize recreational marijuana have experienced more crashes,” said Matt Moore, senior vice president of HLDI. “The individual state analyses suggest that the size of the effect varies by state.”

According to HLDI, Colorado experienced the largest estimated jump in claim frequency, compared with its control states. After retail recreational pot sales began in the state, the collision claim frequency climbed to 14% higher than in nearby Nebraska, Utah and Wyoming. In Washington, the estimated increase in claim frequency was 6.2% higher than in Montana and Idaho. Oregon’s estimated increase was 4.5% higher than in Idaho, Montana and Nevada.

“The combined effect for the three states was smaller but still significant at 3%,” Moore said. “The combined analysis uses a bigger control group and is a good representation of the effect of marijuana legalization overall. The single-state analyses show how the effect differs by state.”

In response to the new analysis, Governors Highway Safety Association (GHSA) Executive Director Jonathan Adkins issued a statement reiterating the group’s stance that states should “consider the risk of marijuana-impaired driving as they move toward liberalizing marijuana laws.”

GHSA cited recent national data that found marijuana was present in 12.2% of all fatally injured drivers who tested for drugs.

A study earlier this year published in the American Journal of Public Health found that crash fatality rates were not statistically different three years after legalization in Washington and Colorado compared to states that have not legalized marijuana. Researchers in that study noted that future studies should be conducted looking at longer term data. 

Other statistics have also seemed to indicate either no change or a slight decrease in accidents in states with legal marijuana, according to report in Governing.com. For example, numbers from the Colorado Highway Patrol, which were not used in the HDLI study, seemed to indicate a slight decrease in the number of impaired driving crashes in the past year.

Commercial vehicle drivers are still prohibited from using marijuana by Federal law, even in states that have legalized it, according to the Department of Transportation.