Clear-Headed Facts: Seven Myths About Cannabis and Driving
As cannabis has become legal in a growing number of places, a dangerous idea has come along with it: that legal means safe to drive on. The two are unrelated. Legality settles what a person may buy and possess. It says nothing about what the drug does to the brain of someone behind the wheel.
Here are seven common beliefs about cannabis and driving, and what the research says about each.
Myth 1: “Cannabis doesn’t really impair driving.”
Any substance that acts on the central nervous system can impair driving, and cannabis does. Driving is a demanding mental task. It requires taking in information continuously, interpreting it, predicting what happens next, and turning decisions into precise movements within fractions of a second.
The crash data point the same way. A systematic review and meta-analysis of nine studies found that driving under the influence of cannabis was associated with significantly higher collision risk, with an odds ratio of 1.92 (95% CI 1.35–2.73), though results varied considerably between studies (Asbridge et al., 2012). A later analysis of 13 culpability studies, built specifically to avoid overstating the risk, estimated the average increase in crash risk at about 28 percent (Røgeberg, 2019).
The size of the effect depends on the study. Individual studies range from no measurable increase to a tenfold one (White & Burns, 2021). In a review of higher-quality studies, the estimate for fatal crashes was 2.1 (95% CI 1.31–3.36), while the non-fatal estimate was too imprecise to be statistically significant (Preuss et al., 2021). Still, the pooled results consistently land above 1.0, not at or below it. A recent review concluded that meta-analyses and culpability studies indicate a small but statistically significant increase in crash risk after acute cannabis use (Preuss et al., 2021). Dose matters too: in a French study of more than 10,000 drivers involved in fatal crashes, the odds of being responsible for the crash rose with blood THC concentration (Preuss et al., 2021).
Myth 2: “It’s not like alcohol, so it doesn’t affect you the same way.”
Cannabis and alcohol do affect people differently, and that difference is often used to argue that cannabis is harmless behind the wheel. It doesn’t follow. Two drugs can impair in different ways, and both can still impair. Cannabis impairment can look different from alcohol impairment, but different does not mean smaller.
Nor is this a niche problem. In NHTSA’s national roadside survey, the share of weekend nighttime drivers testing positive for THC rose from 8.6 percent in 2007 to 12.6 percent in 2013–2014, a 48 percent increase (Berning et al., 2015). The survey’s authors caution that drug presence does not necessarily imply impairment, but the trend shows how many drivers share the road with the question.
The fatality data show a similar shift. Using federal crash records from 2000 to 2018, researchers found that the share of crash deaths involving a driver who tested positive for a cannabinoid rose from 9.0 percent to 21.5 percent, while deaths involving both cannabis and alcohol rose from 4.8 percent to 10.3 percent (Lira et al., 2021). A positive test is not proof of impairment at the time of the crash. But the direction and size of the trend deserve attention.
Myth 3: “Cannabis only affects how you feel, not how you think.”
The high is what people notice, but the effects on thinking go deeper. A meta-analysis of 52 experimental studies of acute THC exposure found that verbal learning and working memory were the most affected areas. It reported moderate deficits in working memory (effect size about 0.5) and smaller but meaningful deficits in processing speed (about 0.4) (Zhornitsky et al., 2021). Both matter for driving:
Processing speed. A driver must notice a hazard, recognize it, decide, and act, often in well under a second. Slower processing lengthens that chain. At highway speeds, even a small delay means many feet traveled before the driver reacts.
Working memory. This is the brain’s short-term workspace. It lets you track the car that just passed on your left, remember that your exit is half a mile ahead, and anticipate that the truck in front may change lanes.
Visual working memory. This is the ability to hold a recent visual scene in mind, such as where nearby vehicles were a moment ago, so you can judge how they are moving. Driving is overwhelmingly a visual task.
Physical coordination. Cannabis can also affect the fine motor control behind steering, braking, and lane keeping. In one simulator study, inhaled THC reduced both vigilance and driving performance (Hartley et al., 2019).
Together, these functions underlie nearly every part of the driving task.
Myth 4: “Regular users build a tolerance, so they can drive just fine.”
This is the most persuasive-sounding myth, because part of it is true. Tolerance is real, and it shows up in the data. In a controlled study, 15 chronic users (one to two joints a day) and 15 occasional users (one to two joints a week) inhaled placebo, 10 mg, or 30 mg of THC. The decline in vigilance and simulated driving performance was more pronounced and lasted longer in the occasional users (Hartley et al., 2019). Anyone claiming a daily user is affected exactly like a novice overstates the case.
But less affected is not the same as unaffected, and the evidence on tolerance and driving is not settled. The cognitive functions that matter most for driving are also the ones that show lasting effects in heavier users. A 2025 meta-analysis of studies of people with cannabis use disorder found moderate residual deficits in speed of processing, working memory, and verbal learning and memory, roughly the same domains impaired during acute intoxication (Pilon et al., 2025). That finding applies to people with cannabis use disorder, not to every frequent user, and it measures cognition rather than driving. But it points in a consistent direction: the abilities driving relies on are not fully spared by tolerance.
The takeaway is not that every regular user is dangerously impaired at every moment. It is that tolerance blunts some effects and leaves others, and a lack of the “high” feeling is not proof that the underlying abilities are intact.
Myth 5: “I drive more carefully when I’m high.”
Some people say cannabis makes them slow down and drive more cautiously. Even taking that at face value, caution has limits. Driving slowly does not restore working memory or speed up information processing. A slower driver still reacts late to surprises, and driving is full of surprises: a child running into the street, a sudden stop, a car drifting into the lane. Cautious habits help in routine conditions and fail in the emergencies where fast, accurate responses matter most.
Myth 6: “If I’m not feeling high anymore, I’m okay to drive.”
Timing is one of the trickiest parts of this topic. How long impairment lasts depends on dose, potency, how the drug was consumed, and the individual. Effects from smoking or vaping tend to come on quickly, while edibles take longer to start and can last longer, which makes them easy to misjudge.
Research does show that the effect is tied to recent use. In a University of California San Diego study, 191 regular users completed a 25-minute driving simulation after at least 48 hours of abstinence. Their performance showed no relationship to how much they used, how many days they had abstained, or their cannabinoid levels, and a small pilot comparison found no difference between 18 of the most frequent users and 12 non-users (Mastropietro et al., 2026). The lesson is not that impairment lingers indefinitely. It is that the hours after use are the risk window, and no one can precisely mark its end by feel. Treat cannabis like any other impairing substance: don’t drive after using it, and arrange a sober driver, a rideshare, or another way home in advance.
Myth 7: “There’s a blood level for THC like there is for alcohol, so we can tell who’s impaired.”
This myth is worth including because it trips up people on both sides of the debate. For alcohol, blood concentration tracks impairment well enough to support a numeric legal limit. THC is different. NHTSA’s report to Congress states that THC concentration does not correlate well with impairment and that no chemical test for cannabis impairment exists comparable to a blood or breath alcohol test (Compton, 2017). A study of drivers arrested for DUI, commissioned by the AAA Foundation, found no correlation between blood THC concentration and performance on impairment indicators, and concluded that a numeric per se threshold cannot be scientifically supported (Logan et al., 2016). A comprehensive review reached a similar conclusion: some studies link high THC concentrations to higher crash risk, but no scientifically supported cutoff can be derived (Preuss et al., 2021).
The reasons are practical. Peak impairment does not necessarily coincide with peak blood levels, and THC can remain detectable after impairing effects have passed, while impairing effects can be measurable when the drug can’t be detected (Berning et al., 2015). Tolerance complicates it further: in the study above, peak blood THC was about twice as high in chronic users as in occasional users at the same dose, even though their performance declined less (Hartley et al., 2019).
Whatever legal standard a jurisdiction uses, the underlying question is the same: is this person’s ability to drive safely affected? Cannabis can affect it, and no number in a lab report replaces that question.
What the evidence adds up to
Pooled crash studies find elevated risk. Experimental studies show moderate effects on working memory and smaller effects on processing speed. Regular users appear less affected than occasional ones, but the residual cognitive effects seen in heavier users involve the same abilities driving depends on. And because THC levels don’t track impairment, neither a feeling nor a lab number can settle whether a driver is fit.
None of this requires a position on whether cannabis should be legal or whether adults should use it. The point is narrower: cannabis use and driving are a bad combination, and whether a substance is legal, natural, medically prescribed, or familiar has no bearing on what it does to the brain.
Practical takeaways
• Don’t rely on how you feel. There is no reliable test, at the roadside or in the mirror.
• Plan ahead. Decide how you’ll get home before you use.
• Be especially careful with edibles. Delayed onset and long duration make them easy to misjudge.
• Don’t treat tolerance as protection. It may reduce some effects without restoring what driving demands.
• Speak up. Offering a friend a ride or a couch can prevent a tragedy.
Every driver shares the road with people who never agreed to take the risk. That is the strongest reason to get this right.
References
Asbridge, M., Hayden, J. A., & Cartwright, J. L. (2012). Acute cannabis consumption and motor vehicle collision risk: Systematic review of observational studies and meta-analysis. BMJ, 344, Article e536. https://doi.org/10.1136/bmj.e536
Berning, A., Compton, R., & Wochinger, K. (2015). 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). National Highway Traffic Safety Administration. https://www.nhtsa.gov/sites/nhtsa.gov/files/812118-roadside_survey_2014.pdf
Compton, R. (2017). Marijuana-impaired driving: A report to Congress (Report No. DOT HS 812 440). National Highway Traffic Safety Administration. https://www.nhtsa.gov/sites/nhtsa.gov/files/documents/812440-marijuana-impaired-driving-report-to-congress.pdf
Hartley, S., Simon, N., Larabi, A., Vaugier, I., Barbot, F., Quera-Salva, M.-A., & Alvarez, J. C. (2019). Effect of smoked cannabis on vigilance and accident risk using simulated driving in occasional and chronic users and the pharmacokinetic–pharmacodynamic relationship. Clinical Chemistry, 65(5), 684–693. https://doi.org/10.1373/clinchem.2018.299727
Lira, M. C., Heeren, T. C., Buczek, M., Blanchette, J. G., Smart, R., Pacula, R. L., & Naimi, T. S. (2021). Trends in cannabis involvement and risk of alcohol involvement in motor vehicle crash fatalities in the United States, 2000–2018. American Journal of Public Health, 111(11), 1976–1985. https://doi.org/10.2105/AJPH.2021.306466
Logan, B., Kacinko, S. L., & Beirness, D. J. (2016). An evaluation of data from drivers arrested for driving under the influence in relation to per se limits for cannabis [Technical report]. AAA Foundation for Traffic Safety. https://aaafoundation.org/research/evaluation-data-drivers-arrested-driving-influence-relation-per-se-limits-cannabis/
Mastropietro, K. F., Rattigan, J. A., Umlauf, A., Grelotti, D. J., Huestis, M. A., Suhandynata, R. T., Grant, I., Fitzgerald, R. L., & Marcotte, T. D. (2026). Short-term residual effects of smoked cannabis on simulated driving performance. Psychopharmacology, 243(4), 793–804. https://doi.org/10.1007/s00213-025-06880-1
Pilon, F., Dumais, A., Giguère, C.-É., & Potvin, S. (2025). The effects of cannabis use disorder on cognitive functions: A meta-analysis. Addictive Behaviors, 170, Article 108434. https://doi.org/10.1016/j.addbeh.2025.108434
Preuss, U. W., Huestis, M. A., Schneider, M., Hermann, D., Lutz, B., Hasan, A., Kambeitz, J., Wong, J. W. M., & Hoch, E. (2021). Cannabis use and car crashes: A review. Frontiers in Psychiatry, 12, Article 643315. https://doi.org/10.3389/fpsyt.2021.643315
Røgeberg, O. (2019). A meta-analysis of the crash risk of cannabis-positive drivers in culpability studies—Avoiding interpretational bias. Accident Analysis & Prevention, 123, 69–78. https://doi.org/10.1016/j.aap.2018.11.011
White, M. A., & Burns, N. R. (2021). The risk of being culpable for or involved in a road crash after using cannabis: A systematic review and meta-analyses. Drug Science, Policy and Law, 7, Article 20503245211055381. https://doi.org/10.1177/20503245211055381
Zhornitsky, S., Pelletier, J., Assaf, R., Giroux, S., Li, C.-S. R., & Potvin, S. (2021). Acute effects of partial CB1 receptor agonists on cognition: A meta-analysis of human studies. Progress in Neuro-Psychopharmacology and Biological Psychiatry, 104, Article 110063. https://doi.org/10.1016/j.pnpbp.2020.110063
