Crashes are up by as much as 6 percent in the first states to legalize recreational marijuana, compared with neighboring states without legalized recreational use, according to new research from the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI). The findings come as campaigns to decriminalize marijuana gain traction with voters and legislators in the United States.The two new studies were presented at the Combating Alcohol- and Drug-Impaired Driving summit, hosted by IIHS and HLDI at the Vehicle Research Center. The summit brought together highway safety and law enforcement experts to discuss the prevalence of alcohol- and drug-impaired driving, as well as strategies to combat impaired driving.The frequency of collision claims rose 6 percent following the start of retail sales of recreational marijuana in Colorado, Nevada, Oregon, and Washington, compared with the control states of Idaho, Montana, Utah, and Wyoming. The analysis is based on collision data from January 2012 through October 2017 and correlate with the 5.2 percent increase in police-reported crashes during this time.Analysts controlled for differences in the driver population, insured vehicle fleet, the mix of urban versus rural exposure, unemployment, weather, and seasonal factors. The size of the effect varied by state."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," says IIHS-HLDI President David Harkey. "States exploring legalizing marijuana should consider this effect on highway safety."Marijuana is still an illegal controlled substance under federal law.In addition to the study states, Alaska, California, Maine, Massachusetts, Vermont, and the District of Columbia also allow recreational use of marijuana for adults 21 and older and medical use of marijuana. Another 22 states allow medical marijuana, while 15 more states permit the use of specific cannabis products for designated medical conditions.Legalization of recreational use is pending in New Hampshire, New Jersey, New York, and Pennsylvania. In November, Michigan and North Dakota will hold referendums on marijuana, and Missouri and Utah voters will decide whether to expand medical marijuana laws in their states.Measuring impairment is difficultDriving 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.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."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 says.