The study compared the crash rates of the 4 states with legal recreational marijuana with 4 states where it is not legal.
The IIHS released their findings Thursday.
Citing research from the Insurance Institute for Highway Safety and the Highway Loss Data Institute, the study indicates crashes are up by as much as 6 percent in Colorado, Nevada, Oregon and Washington compared to adjacent states. Crashes involving alcohol typically take place at night as that's when most drinking is done.
The Insurance Institute for Highway Safety and the Highway Loss Data Institute conducted the research. But this is now the second year in a row for the IIHS to find this troubling trend and thee is now one year left for the nonprofit group to be looked at the three states including Washington, Oregon and the Colorado.
Driving while impaired by any substance, including marijuana, is illegal in all states.
"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 added.
"States exploring legalizing marijuana should consider this effect on highway safety", he said.
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A separate 2017 study from the American Journal of Public Health found that legalized recreational marijuana did not have a significant increase on the number of fatality crashes, however.
Those in favor of relaxing cannabis laws could point out that the studies only looked at four states, while a total of nine allow recreational consumption of the drug.
"We know a lot of states are considering making recreational marijuana available", Harkey said.
Unlike alcohol, the presence of THC in the body does not necessarily mean that an individual is impaired, and a higher level of marijuana use does not necessarily mean greater impairment.
Witnesses told authorities the driver had been driving erratically for more than 15 minutes before the crash. That followed stark warnings from the National Transportation Safety Board, which on Tuesday issued several recommendations to combat drug-impaired driving.
The studies do, however, mention that the role of cannabis in these accidents isn't clear as drivers who test positive for drugs are often found with alcohol in their system as well.