October 23, 2018—Car crashes increased as much as 6 percent recently in states where the recreational use of marijuana has been legalized, reported USA Today.

According to research from the Insurance Institute for Highway Safety and the Highway Loss Data Institute, the frequency of collision claims filed to insurers were higher in four states where marijuana is legal: Colorado, Nevada, Oregon and Washington. The studies were presented last week at the Combating Alcohol- and Drug-Impaired Driving summit.

According to the report, a separate study conducted by the Insurance Institute for Highway Safety focused on police-reported crashes before and after retail marijuana was allowed and found Colorado, Oregon and Washington saw a 5.2 percent increase in the rate of crashes per million vehicle registrations, compared with neighboring states.

Nine states, including Alaska, California, Colorado, Maine, Massachusetts, Nevada, Oregon, Vermont and Washington, and the District of Columbia have legalized marijuana use. Michigan and North Dakota will vote next month on whether to allow marijuana for recreational use.