I.I.I. Study Starts Ringing Alarm Bells On Marijuana Legalization and Impaired Driving

Early Evidence Suggests More Accidents Occur Where Marijuana Sales Are Legal


For immediate release
New York Press Office: (212) 346-5500; media@iii.org  

NEW YORK, March 13, 2019 –Given that 10 states allow the sale of recreational marijuana, U.S. auto insurers are examining how the drug is impacting motorist behavior and claims-filing patterns, according to a new Insurance Information Institute (I.I.I.) white paper.

A Rocky Road So Far: Recreational Marijuana and Impaired Driving explores marijuana impairment’s impact on a motorist’s driving abilities, how states deal with marijuana-impaired motorists, and marijuana’s broader implications for U.S. roadway safety.

Early results aren’t promising, the I.I.I. found. There are signs legalizing recreational marijuana pushes accident rates higher, while it can be difficult to detect whether a driver is high on marijuana.

Percent change in collision claim frequency following recreational marijuana legalization.
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“There is currently no ‘breathalyzer’ equivalent for marijuana impairment,” write James Lynch, the I.I.I.’s Chief Actuary, and Lucian McMahon, the I.I.I.’s Senior Research Specialist, the white paper’s co-authors, referring to the device which detects alcohol-impaired motorists. 

Some U.S. states hold that a motorist is driving while marijuana-impaired if the THC concentration detected in their bodies exceeds a certain limit, such as 5 nanograms per milliliter (ng/ml).  THC is the main psychoactive compound found in marijuana. Other states prohibit altogether motorists from driving with any trace of THC in their bodies.

Map of states with marijuana-impaired driving laws.
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“Early evidence suggests that recreational marijuana legalization is associated with an increase in traffic accidents,” the paper states, citing a 2018 Highway Loss Data Institute (HLDI) study and police-reported crashes in states with legal recreational marijuana. HLDI found collision claim frequency was significantly higher in Colorado (+12.5 percent) and Washington (+9.7 percent) than in nearby states without recreational marijuana, although the increase in Oregon (+1 percent), the other state the HLDI study covered, was deemed statistically insignificant. 

In addition to those three states, recreational marijuana is legal in seven others—Alaska, California, Maine, Massachusetts, Michigan, Nevada, and Vermont—as well as the District of Columbia.

Current marijuana laws by state.
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“When a state legalizes marijuana, more people use the drug. More people using marijuana is associated with more people driving with THC in their systems,” the paper states. “The standard personal auto policy does not address driving under the influence of any drug, including alcohol and marijuana. However, auto insurance rates may be affected by the spread of marijuana legalization, particularly if such legalization is associated with an increase in impaired driving and related accidents.”

Moreover, when a motorist consumes both alcohol and marijuana, and then drives, that motorist is 25 times more likely than a sober one to be involved in a fatal accident, the I.I.I.’s white paper notes, pointing to a 2017 study in Injury Epidemiology.

Just the Facts

  • Marijuana affects its users differently but the drug generally impairs cognitive and motor skills.
  • Marijuana impairment increases the risk of culpability for a car crash. And mixing marijuana and alcohol heightens driving risks.
  • Marijuana use could increase after recreational marijuana legalization — and the number of THC-positive drivers could increase, as well.
  • Recreational marijuana legalization is associated with an increase in collision claim frequency, according to the Highway Loss Data Institute (HLDI).



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Insurance Information Institute, 110 William Street, New York, NY 10038; (212) 346-5500; www.iii.org

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