Insurance claims in four states that have legalized recreational marijuana have increased 6%, while police-reported crashes have increased 5% in three of the states, according to new findings from the Insurance Institute for Highway Safety and Highway Loss Data Institute.

The findings are based on two new studies of data from Colorado, Nevada, Oregon, and Washington because these were among the first states to legalize recreational marijuana. In the first study, HLDI researchers compared crash data in the states with that of neighboring states. Retail marijuana sales began as early as 2014 in Colorado and Washington, in 2015 in Oregon, and in 2017 in Nevada. Retail sales have recently started in California.

HLDI analysts estimate that the frequency of collision claims per insured vehicle year rose a combined 6% following the start of retail sales of recreational marijuana in those four states, compared with the control states of Idaho, Montana, Utah and Wyoming. The combined-state analysis is based on collision loss data from January 2012 through October 2017.

In a separate study, IIHS examined police-reported crashes from 2012 to 2016 before and after retail sales began in Colorado, Oregon, and Washington. IIHS estimates that the three states combined experienced a 5.2% increase in the rate of crashes per million vehicle registrations, compared with neighboring states that didn't legalize marijuana sales.

The study compared Colorado data with that of Nebraska, Wyoming and Utah — where the sale of recreational marijuana is presently illegal. It compared Oregon and Washington with Idaho and Montana, also states where the drug is illegal.

Researchers at IIHS-HLDI conclude that legalizing marijuana for recreational use is having a negative impact on road safety.

The report authors note that driving under the influence of marijuana is illegal in all 50 states and Washington D.C., but determining impairment is challenging. Unlike alcohol, the amount of marijuana present in a person's body doesn't consistently relate to impairment. 

Finally, because of several factors including lack of consistent information on drug use in crash reports, marijuana's role in roadway collisions isn't as clear as the link between alcohol and crashes.