Car crashes went up by about 6% in states that have legalized recreational marijuana, a couple of new studies found.

Colorado, Nevada, Oregon, and Washington have seen an increase in insurance claims for collisions, according to separate studies by the Insurance Institute for Highway Safety and the Highway Loss Data Institute.

The institutes presented their research at the Combating Alcohol- and Drug-Impaired Driving Summit last Thursday.

The Highway Loss Data Institute focused on claims between 2012 and October 2017, comparing the results to four illegal states, including Idaho, Montana, Utah, and Wyoming. The Insurance Institute for Highway Safety observed a 5.2% increase in police-reported crashes after legalization in Colorado, Oregon, and Washington.

“States exploring legalizing marijuana should consider this effect on highway safety,” said David Harkey, president of the Insurance Institute for Highway Safety and Highway Loss Data Institute.

The findings are important as the tide is increasingly turning to favor the legalization of marijuana. Nine states and the District of Columbia have legalized marijuana, and Michigan and North Dakota are holding a vote on the issue next month. Canada has legalized it as well.

Figuring out who is or isn’t impaired by marijuana while driving is also a challenge, the institutes acknowledge. Marijuana’s active ingredient, THC, can remain detectable in a person for as long as 30 days—long after the high has worn off. Currently, there is no reliable method to determine whether someone is currently high. But according to the studies, it’s clear that marijuana has some effect on driving ability.

“Despite the difficulty of isolating the specific effects of marijuana impairment on crash risk, the evidence is growing that legalizing its use increases crashes,” Harkey said.

The Center for Medicinal Cannabis Research at the University of California San Diego is undertaking studies to better understand the effects of marijuana on driving as well as methods to more reliably detect if a driver is currently under the influence.

Methods now used by law enforcement are notoriously unreliable, and as a result field sobriety tests for marijuana cannot be used as evidence in some cases.