A new study from the Insurance Institute for Highway Safety (IIHS) found that injury crash rates rose by about 7 percent in five states after they legalized recreational marijuana use. But there was no statistically significant increase in traffic fatality rates, and state-licensed recreational sales had no apparent impact on injury rates. Meanwhile, another IIHS study found that “drivers who used marijuana alone were no more likely to be involved in crashes than drivers who hadn’t used the drug.”
Given these mixed and counterintuitive results, the take offered by IIHS President David Harkey is misleading. “Our latest research makes it clear that legalizing marijuana for recreational use does increase overall crash rates,” Harkey said in a press release. “That’s obviously something policymakers and safety professionals will need to address as more states move to liberalize their laws—even if the way marijuana affects crash risk for individual drivers remains uncertain.”
The fear that marijuana legalization leads to more car crashes is based on the expectation that legalization increases use, resulting in more stoned drivers on the road. But if so, you would expect to see some impact from newly legal commercial distribution, and the IIHS study found no evidence of that. In fact, the researchers report a slight decrease in injury crash rates after state-licensed retailers opened, although that change was not statistically significant.
Furthermore, the effect of legalization on road safety depends crucially on “the way marijuana affects crash risk for individual drivers,” and the second IIHS study suggests that marijuana use does not increase crash risk. As the IIHS notes, that result is “consistent with a 2015 study by the National Highway Traffic Safety Administration [NHTSA],” which “found that a positive test for marijuana was not associated with increased risk of being involved in a police-reported crash.”
The IIHS researchers