Motor vehicle crashes are up 6 percent in four states with legalized recreational marijuana, compared with four neighboring states where the drug is illegal or restricted, according to Consumer Reports.

Researchers say the findings suggest that as more states legalize recreational marijuana, more will need to be done to find ways to prevent impaired driving accidents.

The findings come from a study conducted by the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI). Researchers compared insurance claims for vehicle collisions in Colorado, Nevada, Oregon and Washington — where recreational marijuana is legal — with Idaho, Montana, Utah and Wyoming — where it is not.

“The new IIHS-HLDI research on marijuana and crashes indicates that legalizing marijuana for all uses is having an impact on the safety of our roads,” said David Harkey, president of IIHS and HLDI. “States exploring legalizing marijuana should consider the highway safety impact.”