Legalizing marijuana for all uses is having a negative impact on the safety of U.S. roads according to new research by the Insurance Institute for Highway Safety and the Highway Loss Data Institute. States exploring legalization should consider this effect on highway safety when deciding on whether to legalize pot.

Crashes are up by as much as 6 percent in Colorado, Nevada, Oregon and Washington, compared with neighboring states that haven’t legalized marijuana for recreational use, new research from IIHS and HLDI shows. The findings come as campaigns to decriminalize marijuana gain traction with voters and legislators in the U.S., and Canada began allowing recreational use of marijuana across all of its provinces this past October.

“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,” says David Harkey, president of IIHS and HLDI. “States exploring legalizing marijuana should consider the highway safety impact.” Recreational use of marijuana by adults 21 and older won voter approval in November 2012 in Colorado and Washington. Retail sales began in January 2014 in Colorado and in July 2014 in Washington. Oregon voters approved recreational marijuana in November 2014, and sales started in October 2015. Nevada voters approved recreational marijuana in November 2016, and retail sales began in July 2017.

Alaska, California, Maine, Massachusetts, Vermont and the District of Columbia also allow recreational use of marijuana for adults 21 and older. Legalization of recreational use is pending in New Hampshire, New Jersey, New York and Pennsylvania. In November, Michigan and North Dakota held referendums on marijuana, and Missouri and Utah voters decided whether to expand medical marijuana laws in their states.

Impaired driving is a longstanding problem, with about a third of all fatally injured drivers in crashes having a blood alcohol concentration (BAC) of 0.08 percent or higher (0.08 g alcohol per 100 mL blood). All 50 states and the District of Columbia have per se laws making it a crime to drive with a BAC at or above 0.08 percent. Effective in December, Utah will lower the threshold to 0.05 percent or above.

Marijuana’s role in crashes isn’t as clear as the link between alcohol and crashes. Many states don’t include consistent information on driver drug use in crash reports, and policies and procedures for drug testing are inconsistent. More drivers in crashes are tested for alcohol than for drugs. When drivers are tested, other drugs are often found in combination with alcohol, which makes it difficult to isolate their separate effects.

Driving under the influence of marijuana is illegal in all 50 states and 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. THC, or Tetrahydrocannabinol, is the primary psychoactive component of cannabis. A positive test for THC and its active metabolite doesn’t mean the driver was impaired at the time of the crash. Habitual users of marijuana may have positive blood tests for THC days or weeks after using the drug.

Next week’s article will deal with cannabis users who drive with children.