A new study finds traffic accidents are increasing in states that have legalized recreational marijuana, Bloomberg reports.

Crashes have risen by as much as 6 percent in Colorado, Nevada, Oregon and Washington, compared with neighboring states that haven’t legalized marijuana for recreational use, according to research from the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI).

“The new IIHS-HLDI research on marijuana and crashes indicates that legalizing marijuana for all uses is having a negative impact on the safety of our roads,” IIHS-HLDI President David Harkey said in a news release. “States exploring legalizing marijuana should consider this effect on highway safety.”

In a separate study, IIHS examined police-reported crashes before and after retail marijuana sales began in Colorado, Oregon and Washington. The study found the three states combined saw a 5.2 percent increase in the rate of crashes per million vehicle registrations, compared with neighboring states that did not legalize marijuana.