Traffic Deaths Related To Cannabis Impaired Drivers Doubled After Legalization Interview with:

Mr. Brian C. Tefft Senior Research Associate AAA Foundation for Traffic Safety

Mr. Brian Tefft

Mr. Brian C. Tefft
Senior Research Associate
AAA Foundation for Traffic Safety What is the background for this study? What are the main findings?

Response: In December 2012, a new law took effect in Washington state that effectively legalized the possession and use of small amounts of marijuana by adults aged 21 years and older for recreational purposes, and also created a legal limit for driving under the influence of marijuana such that having a concentration of 5.00 nanograms or greater of delta-9-tetrahydrocannabinol (THC, the main psychoactive chemical in marijuana) per milliliter of whole blood while driving in the state of Washington is per se driving under the influence.

Data from population-based surveys indicate that the proportion of Washington state residents who report using marijuana increased after this law took effect; however, not much was known about the impact of this new law on traffic safety in the state. To investigate the traffic safety impact of the new law, we examined drug test results from drivers involved in motor vehicle crashes that occurred in years 2010 – 2014 in Washington and resulted in the death of at least one person within 30 days of the crash. Specifically, we looked at the proportion of all drivers involved in fatal crashes who had detectable THC in their blood at or shortly after the time of the crash, which generally suggests that the driver had used marijuana within the past few hours.

Results showed that from 2010 through 2013, approximately 8-9% of drivers in fatal crashes each year were positive for THC, and that proportion was basically flat from 2010 through 2013. In 2014, the proportion basically doubled, to 17%. Our modelling suggests that an increasing trend in the proportion of drivers who were positive for THC began in late 2013, about 9-10 months after the new law took effect. What should readers take away from your report?

Response: Several months after Washington state legalized recreational marijuana use for adults, we started seeing a strong increasing trend in the proportion of drivers involved in fatal crashes who had detectable THC in their blood, which suggests that they had likely used marijuana within a few hours prior to the crash. We can’t say that any specific driver was necessarily impaired or that impairment from marijuana was the cause of a given crash—the data that we had available to us did not allow us to pinpoint whether any particular driver was actually impaired or “at fault” in a given crash, and a person who uses marijuana frequently might still have detectable THC in their system even if they last consumed cannabis days ago and are not impaired at all. However, the growth in the proportion of drivers in fatal crashes who had THC in their blood was substantially greater than the growth in marijuana use post-legalization as reflected by surveys, suggesting that the prevalence of driving within a few hours after using marijuana increased in Washington.

Finally, while we can’t say that it was because of marijuana—fatal crashes almost always involve numerous contributing factors without any one of which the crash wouldn’t have happened—the total number of traffic fatalities in Washington increased by 6% from 2013 to 2014, concurrent with the large jump in marijuana involvement in fatal crashes, while at the same time the nation as a whole and more than half of all states individually were experiencing decreases, not increases, in the number of fatalities on their roads. What recommendations do you have for future research as a result of this study?

Response: Given our findings, it will be very important for other states that have legalized or that might in the future legalize marijuana to examine the effects of such legislation on traffic safety and to identify effective countermeasures to reduce the prevalence of impaired driving. We have decades of research on the impairing effects of alcohol and on countermeasures to combat drinking and driving, but comparatively little research on drug-impaired driving, what drugs are impairing, how impairing they are, how prevalent they are, how or whether drug test results correlate with impairment for specific drugs, and what the medical community and the traffic safety community can do to reduce drug-impaired driving. Is there anything else you would like to add?

Response: We wanted to study marijuana involvement in fatal crashes in Washington because Washington was one of the first states to legalize marijuana not only for therapeutic purposes but also for recreational purposes.  However, we should also note, the state of Washington has been a real leader in collecting high-quality data that researchers can use to examine the relationship between marijuana and traffic safety. The Washington Traffic Safety Commission compiled data on the drug test results of drivers in fatal crashes that goes well beyond what is reported in federal databases maintained by the US Department of Transportation. Without the efforts of the Washington Traffic Safety Commission to compile these data, this study would not have been possible. Other states that value evidence-based policy should follow Washington’s lead in developing the kind of data that researchers need to be able to examine the relationship between marijuana and traffic safety. Thank you for your contribution to the community.


AAA report:

Fatal Road Crashes Involving Marijuana Double After State Legalizes Drug 

Tefft BC, Arnold LS, Grabowski JG. 2016. Prevalence of Marijuana Involvement in Fatal Crashes, Washington, 2010 – 2014. AAA Foundation for Traffic Safety. Washington, DC.

Note: Content is Not intended as medical advice. Please consult your health care provider regarding your specific medical condition and questions.

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Last Updated on May 11, 2016 by Marie Benz MD FAAD