Learn About Cannabis

Science-based information for the general public.

Driving Under the Influence of Cannabis

Learn About Cannabis 5 Health Risks 5 Driving Under the Influence of Cannabis

Driving under the influence of cannabis (DUIC) is a significant public safety concern among Washington residents and voters.  A 2019-2022 study looking at cannabis-related health beliefs among 9,215 Washington state residents found that 74% of them believed it is “dangerous to drive or operate machinery after using marijuana.” In contrast, only 57% believed that “it is harmful to use marijuana when pregnant or breastfeeding” and even less (53%) believed that “marijuana can be addictive.”7

The following summarizes some of the current research findings around this important topic.

Cannabis use impairs peoples' ability to safely control a vehicle and increases the risk of crashing.
Research studies found that the psychoactive ingredient in cannabis, delta-9-tetrahydracanabidiol (THC) impairs memory, attention tracking, information processing, executive functioning, reaction time, fine motor function, and sustained attention.10

These effects on mental abilities can significantly impact driving. People are more likely to drive slower, weave in their lane,18and have slower reaction times (e.g.,  people weaving out of their lane for longer times before returning) when driving within 2-3 hours of using moderate amounts of cannabis.1,4,11,16

THC has also been shown to decrease visual function, particularly at night when the glare from bright lights can lead to night blindness.12

A review of 21 studies measuring crash risk associated with driving under the influence of cannabis showed that being high on THC increased the risk of being in a car crash by 20-30%.17 According to the National Highway Traffic Safety Administration, cannabis is the second most frequently found drug, after alcohol, in drivers involved in a crash.2

DEFINITIONS

Memory impairment: Issues with remembering things that were previously recalled.

Attention tracking: Being able to concentrate on multiple tasks over time.

Information processing: Being able to understand and use various information such as sounds and sights.

Executive functioning: Mental skills that help us plan, organize, prioritize, and execute tasks. It also helps us regulate our emotions, control our impulses, and make decisions.

Reaction time: How fast your brain can process information and react.

Fine motor function: The coordination of small muscles in the movement with the eyes, hands, and fingers.

Sustained attention: Being able to maintain focus on a single continuous and repetitive task over a period of time.

Most Washingtonians who use cannabis don’t drive under the influence of cannabis. Those who do drive after using cannabis are more likely to believe that driving under the influence of cannabis is not dangerous.

A recent study about Washington state drivers’ behaviors and beliefs found that among drivers who had used cannabis within the past year (42%):

  • 85% did not drive within 2 hours of consuming cannabis
  • 91% did not drive within 2hrs. of consuming cannabis and alcohol
  • 81-83% had a negative attitude about driving within 2 hours after using cannabis and alcohol and found it unacceptable13

Another recent study evaluating young adults’ (18-25 years old) perceptions of driving under the influence of cannabis found that one third of respondents believe that DUIC and riding with a driver under the influence of cannabis (RWI-C) is totally unacceptable. At the same time, three quarters of participants view driving under the influence of alcohol and riding with a driver under the influence of alcohol as totally unacceptable.8 Overall, the frequency of young adults who report driving under the influence of cannabis (12%)8 has not changed since the opening of cannabis retail stores but has decreased (45% to 33%) among those who reported cannabis use in the past month.9

Young adults (YA) who find DUIC or RWI-C more acceptable are more likely to engage in these behaviors themselves. Washington state young adults with more favorable attitudes toward DUIC had 11.9 times greater chance of driving under the influence of cannabis  than those who see it as totally unacceptable. Similarly, those with more favorable attitudes of riding with a driver under the influence of cannabis were 7.73 times more likely to do so.8 Compared to YA females, YA males had greater odds of DUIC and driving under the influence of cannabis and alcohol (DUICA). As they get older, they’re less likely to DUIC.9

The number of THC-positive drivers in fatal crashes in Washington state has increased.

The number of THC-positive drivers in fatal crashes in Washington state has increased with the largest increase in 2014, the year cannabis stores opened.Figure 1, 15

THC-positive drivers represented about 10 percent of all drivers in fatal crashes in Washington state (WTSC Report). State law mandates that drivers and pedestrians killed in an auto accident have their blood analyzed within four hours of the crash. While all are tested for alcohol, only 48% are tested for THC.14 A police officer generally needs a warrant to require a blood test in non-fatal accidents. For this reason, about 75% of blood tests are from individuals who died.14

Figure 1.(14)

The combination of alcohol and cannabis is more detrimental to driving performance than either substance in isolation.
Data shows that combining alcohol and cannabis increases impairment (16) and risk of crash more than either substance alone.18,5 In Washington State, about 44 percent of delta-9 THC-positive drivers involved in fatal crashes also had a Blood Alcohol Concentration (BAC) of 0.08 or greater.Figure 2, 19

This finding contrasts with beliefs of those WA-based drivers who reported driving within 2 hours of using cannabis and alcohol within the past year (9% of total drivers). Those drivers believed that using cannabis after having too much to drink would calm them down and sober them up.13

Figure 2 (19)

Higher levels of THC increase crash risk.

Many states, including Washington, have set a THC blood level limit of 5ng/mL or more of active delta-9 THC. One study of non-fatally injured motor vehicle drivers in British Columbia, Canada, found no evidence of increased crash risk in drivers with THC blood levels below 5ng/ml, and a slight increased risk of crash in drivers with THC blood levels at or above 5ng/ml (compared to those who test negative).3 A combination of both a positive toxicology result and field sobriety test observations provide the most accurate determination of whether person is driving impaired under the influence of THC.6

Washington state’s DUI prevention education efforts have made a positive impact since legalization, given that the majority of people believe it is dangerous to drive under the influence of cannabis.

 

WA state law defines impairment as:
  • 5ng/mL or more of active delta-9 tetrahydrocannabinol (THC) in the blood within 2 hours of driving (RCW 46.61.506), and/or,
  • Detailed observation notes, field test results, witness observations, or Drug Recognition Expert assessments demonstrate impairment.

References

  1. Arkell, T. R., Vinckenbosch, F., Kevin, R. C., Theunissen, E. L., McGregor, I. S., & Ramaekers, J. G. (2020). Effect of Cannabidiol and Δ9-Tetrahydrocannabinol on Driving Performance: A Randomized Clinical Trial. JAMA, 324(21), 2177–2186. https://doi.org/10.1001/jama.2020.21218
  2. Berning, A., Compton, R., & Wochinger, K. (2015, February). Results of the 2013–2014 National Roadside Survey of alcohol and drug use by drivers. (Traffic Safety Facts Research Note. Report No. DOT HS 812 118). Washington, DC: National Highway Traffic Safety Administration.
  3. Brubacher, J. R., Chan, H., Erdelyi, S., Macdonald, S., Asbridge, M., Mann, R. E., Eppler, J., Lund, A., MacPherson, A., Martz, W., Schreiber, W. E., Brant, R., & Purssell, R. A. (2019). Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study. Addiction (Abingdon, England), 114(9), 1616–1626. https://doi.org/10.1111/add.14663
  4. Compton, R. (2017, July). Marijuana-Impaired Driving – A Report to Congress. (DOT HS 812 440). Washington, DC: National Highway Traffic Safety Administration.
  5. Downey, L. A., King, R., Papafotiou, K., Swann, P., Ogden, E., Boorman, M., & Stough, C. (2013). The effects of cannabis and alcohol on simulated driving: Influences of dose and experience. Accident; analysis and prevention, 50, 879–886. https://doi.org/10.1016/j.aap.2012.07.016
  6. Fitzgerald, R. L., Umlauf, A., Hubbard, J. A., Hoffman, M. A., Sobolesky, P. M., Ellis, S. E., Grelotti, D. J., Suhandynata, R. T., Huestis, M. A., Grant, I., & Marcotte, T. D. (2023). Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests. Clinical chemistry, 69(7), 724–733. https://doi.org/10.1093/clinchem/hvad054
  7. Hammond, D., Corsetti, D., Fataar, F., Iraniparast, M., Danh Hong, D., Burkhalter, R. International Cannabis Policy Study – Washington 2022 Summary. May 2023.
  8. Hultgren, B. A., Guttmannova, K., Cadigan, J. M., Kilmer, J. R., Delawalla, M. L. M., Lee, C. M., & Larimer, M. E. (2023). Injunctive Norms and Driving Under the Influence and Riding With an Impaired Driver Among Young Adults in Washington State. The Journal of adolescent health : official publication of the Society for Adolescent Medicine, 73(5), 852–858. https://doi.org/10.1016/j.jadohealth.2023.06.010
  9. Hultgren, B. A., Calhoun, B. H., Fleming, C. B., Lyons, V. H., Rhew, I. C., Larimer, M. E., Kilmer, J. R., & Guttmannova, K. (2024). Young Adult Alcohol and Cannabis Impaired Driving After the Opening of Cannabis Retail Stores in Washington State. Prevention science : the official journal of the Society for Prevention Research, 10.1007/s11121-024-01679-6. Advance online publication. https://doi.org/10.1007/s11121-024-01679-6
  10. McCartney, D., Arkell, T. R., Irwin, C., & McGregor, I. S. (2021). Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review. Neuroscience and biobehavioral reviews, 126, 175–193. https://doi.org/10.1016/j.neubiorev.2021.01.003
  11. Miller, R. E., Brown, T. L., Lee, S., Tibrewal, I., Gaffney, G. G., Milavetz, G., Hartman, R. L., Gorelick, D. A., Compton, R., & Huestis, M. A. (2020). Impact of cannabis and low alcohol concentration on divided attention tasks during driving. Traffic injury prevention, 21(sup1), S123–S129. https://doi.org/10.1080/15389588.2020.1814956
  12. Ortiz-Peregrina, S., Casares-López, M., Ortiz, C., Castro-Torres, J. J., Martino, F., & Jiménez, J. R. (2022). Comparison of the effects of alcohol and cannabis on visual function and driving performance. Does the visual impairment affect driving?. Drug and alcohol dependence, 237, 109538. https://doi.org/10.1016/j.drugalcdep.2022.109538
  13. Otto, J., Ward, N., Finley, K., Baldwin, S., & Grondel, D. (2021). The Culture of Driving under the Influence of Cannabis and Alcohol in Washington State. Journal of Applied Social Science, 15(1), 29-46. https://doi.org/10.1177/1936724420980405
  14. Rashid, A. (2023). Technical report—Licensed cannabis retail access and traffic fatalities
  15. (Document Number 23-09-3202). Olympia: Washington State Institute for Public Policy.
  16. Ramaekers, J. G., Robbe, H. W. J., & O’Hanlon, J. F. (2000). Marijuana, alcohol and actual driving performance. Human Psychopharmacology, 15(7), 551–558. https://doi.org/10.1002/1099-1077(200010)15:7<551::AID-HUP236>3.0.CO;2-P
  17. Rogeberg, O., & Elvik, R. (2016). The effects of cannabis intoxication on motor vehicle collision revisited and revised. Addiction (Abingdon, England), 111(8), 1348–1359. https://doi.org/10.1111/add.13347
  18. Simmons, S. M., Caird, J. K., Sterzer, F., & Asbridge, M. (2022). The effects of cannabis and alcohol on driving performance and driver behaviour: a systematic review and meta-analysis. Addiction (Abingdon, England), 117(7), 1843–1856. https://doi.org/10.1111/add.15770
  19. Washington Traffic Safety Commission. (2024). Delta-9 THC Involvement Among Drivers in Fatal Crashes (Brief No. 5). 05_Cannabis-Involvement-in-Fatal-Crashes_March-2024.pdf (wa.gov)

Last updated June 2024.