Marijuana and Psychomotor Impairment

Operating a motor vehicle under the influence of cannabis is a criminal offense in every state, irrespective of cannabis' legal status under the law.

  • Acute cannabis intoxication may influence in a dose-related manner certain psychomotor skills, such as reaction time, necessary to operate a motor vehicle safely. However, these effects tend relatively short-lived and are far less dramatic than changes in psychomotor performance associated with drivers under the influence of alcohol. In studies of either on-road or simulated driving behavior, subjects under the influence of cannabis tend to drive in a more cautious and compensatory manner — such as by reducing speed and engaging in fewer lane changes — while subjects under the influence of alcohol tend to drive more recklessly.

  • RESOURCES: NORML's state-by-state summary of drugged driving laws | National Conference of State Legislatures summary of marijuana-impaired driving laws | National Conference of State Legislature's summary of drugged driving per se laws
  • "The compensatory behavior exhibited by cannabis-influenced drivers distinctly contrasts with an alcohol-induced higher risk behavior, evidenced by greater percent speed." (Cannabis effects on driving longitudinal control with and without alcohol, Drug and Alcohol Dependence, 2016)
  • "Subjects seemed to be aware of their impairment after THC intake and tried to compensate by driving slower, alcohol seemed to make them overly confident and caused them drive faster than in the control sessions." (Effects of THC on driving performance, physiological state and subjective feelings relative to alcohol, Accident Analysis and Prevention, 2008)
  • "Experimental research on the effects of cannabis ... indicate ... that any effects dissipate quickly under one hour. Furthermore, while drivers feel high, they actually tend to compensate for their feelings." (US National Highway Traffic Safety Administration, State of Knowledge of Drug-Impaired driving: FINAL REPORT, 2003
  • "THC's effects differ qualitatively from many other drugs, especially alcohol. For example, subjects drive faster after drinking alcohol and slower after smoking marijuana. ... Very importantly, our city driving studied showed that drivers who drank alcohol over-estimated their performance quality whereas those who smoked marijuana under-estimated it. ... "[S]ubjects in the marijuana group were not only aware of their intoxicated condition, but were ... attempting to compensate for it. These ... findings ... support ... the common belief that drivers become overconfident after drinking alcohol and ... that they become more cautious and self-critical after consuming low doses of THC, as smoked marijuana." (US National Highway Traffic Safety Administration, Marijuana and Actual Driving Performance, 1993)

In assessments of actual on-road driving performance, subjects typically demonstrate only modest changes in psychomotor performance following THC administration

  • "Most marijuana-intoxicated drivers show only modest impairments on actual road tests. ... Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect." (The effect of cannabis compared with alcohol on driving, The American Journal on Addictions, 2009)

The combined administration of cannabis and alcohol typically has an additive influence upon psychomotor performance, which can lead to significantly reduced performance and increased odds of accident

  • "Relative to drivers testing negative for both alcohol and marijuana, the adjusted odds ratios of fatal crash initiation were 5.37 for those testing positive for alcohol and negative for marijuana, 1.62 for those testing positive for marijuana and negative for alcohol, and 6.39 for those testing positive for both alcohol and marijuana." (Role of alcohol and marijuana use in the initiation of fatal two-vehicle crashes, Annals of Epidemiology, 2017)
  • "Relative to drivers who tested negative for both alcohol and marijuana, the estimated odds of fatal crash involvement increased 16 fold for those testing positive for alcohol and negative for marijuana, 1.5 fold for those testing negative for alcohol and positive for marijuana, and over 25 fold for those testing positive for both alcohol and marijuana." (Interaction of marijuana and alcohol on fatal motor vehicle crash risk: a case-control study, Injury Epidemiology, 2017)
  • "Significantly higher blood THC and 11-OH-THC Cmax values with alcohol possibly explain increased performance impairment observed from cannabis-alcohol combinations." (Controlled cannabis vaporizer administration: Blood and plasma cannabinoids with and without alcohol, Clinical Chemistry, 2015)
  • "When alcohol and THC were combined the odds of an unsafe driving action increased by approximately 8 to 10 percent for each 0.01 BAC unit increase over alcohol or THC alone." (The combined effects of alcohol and cannabis on driving: Impact on crash risk, Forensic Science International, 2015)
  • "Driver culpability exhibited the expected positive association with alcohol use (OR 13.7) and with combined alcohol and cannabis use (OR 6.9,). There was only a weak positive association between cannabis use (with no other drug) and culpability (OR 1.3)." (The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs, Accident Analysis and Prevention, 2014)
  • "The effect of ethanol and cannabis taken simultaneously is additive. ... When THC and ethanol were detected together the risk of being judged impaired was increased markedly." (Impairment due to cannabis and ethanol: clinical signs and additive effects, Addiction, 2010)

By contrast, THC positive drivers, absent the presence of alcohol, typically possess a low — or even no — risk of motor vehicle accident compared to drug-negative drivers.

  • "Acute cannabis intoxication is associated with a statistically significant increase in motor vehicle crash risk. The increase is of low to medium magnitude (OR between 1.2 and 1.4)." (The effects of cannabis intoxication on motor vehicle collision revisited and revised, Addiction, 2016)
  • "Adjusted odds ratios between drug class use and crash risk, adjusted for demographic variables: age, gender and race/ethnicity: THC = 1.05" (US National Highway Traffic Safety Administration, Drug and Alcohol Crash Risk, 2015)
  • "For both sober and drinking drivers, being positive for a drug was found to increase the risk of being fatally injured. When the drug-positive variable was separated into marijuana and other drugs, only the latter was found to contribute significantly to crash risk." (Drugs and Alcohol: Their Relative Crash Risk, Journal of Studies on Alcohol and Drugs, 2014)
  • "Summary estimates of relative risk of accident involvement associated with the use of various drugs. Based on meta-analysis: Cannabis and auto injury: best estimate adjusted for publication bias, OR = 1.10" (Risk of road accident associated with the use of drugs: a systematic review and meta-analysis of evidence from epidemiological studies, Accident Analysis and Prevention, 2013)
  • "The highest risk of the driver being severely injured was associated with driving positive for high concentrations of alcohol (≥0.8 g/L), alone or in combination with other psychoactive substances. For alcohol, risk increased exponentially with blood alcohol concentration (BAC). The second most risky category contained various drug-drug combinations, amphetamines and medicinal opioids. Medium increased risk was associated with medium sized BACs (at or above 0.5 g/L, below 0.8 g/L) and benzoylecgonine. The least risky drug seemed to be cannabis and benzodiazepines and Z-drugs." (Risk of severe driver injury by driving with psychoactive substances, Accident Analysis and Prevention, 2013)
  • "The study concludes that drug use, especially alcohol, benzodiazepines and multiple drug use and drug–alcohol combinations, among vehicle drivers increases the risk for a road trauma accident requiring hospitalization. ... No increased risk for road trauma was found for drivers exposed to cannabis." (Psychoactive substance use and the risk of motor vehicle accidents, Accident Analysis and Prevention, 2004)

By comparison, operating a vehicle with multiple passengers or after consuming even slight amounts of alcohol are behaviors associated with greater risk of motor vehicle accident

  • "[Tobacco] smokers had a 1.5-fold increase in risk for motor vehicle crash over non-smokers. Also, an increased tendency to smoke while driving was associated with greater risk of motor vehicle crash.";
  • "When the drivers texted, their collision risk was 23 times greater than when not texting";
  • "Significant increased risk of motor vehicle accidents was found in subjects taking antidepressants within 1 month (adjusted odds ratio (AOR) 1.73, 95% confidence interval (CI) 1.34, 2.22), 1 week (AOR 1.71, 95% CI 1.29, 2.26), and 1 day (AOR 1.70, 95% CI 1.26, 2.29) before MVAs occurred."

Data has not substantiated claims of an alleged uptick in marijuana-induced fatal accidents in states that have regulated the use of cannabis for either medical or recreational purposes. In fact, some studies have identified a decrease in motor vehicle accidents following legalization

  • "[O]n average, medical marijuana law states had lower traffic fatality rates than non-MML states. .... Medical marijuana laws are associated with reductions in traffic fatalities, particularly pronounced among those aged 25 to 44 years. ... It is possible that this is related to lower alcohol-impaired driving behavior in MML-states." (US Traffic Fatalities, 1985–2014, and Their Relationship to Medical Marijuana Laws, Journal of the American Public Health Association, 2016)
  • "State-specific estimates indicated a reduction in opioid positivity for most states after implementation of an operational MML. ... Operational MMLs are associated with reductions in opioid positivity among 21- to 40-year-old fatally injured drivers and may reduce opioid use and overdose." (State medical marijuana laws and the prevalence of opioids detected among fatally injured drivers, American Journal of Public Health, 2016)
  • "In Colorado, despite limited traffic data, the Department of Public Safety reports the following: [T]he number of summons issued for Driving Under the Influence [DUI] in which marijuana or marijuana-in-combination[ 138] with other drugs [was recorded] decreased 1% between 2014 and 2015 (674 to 665)." ((The Marijuana Policy Gap and the Path Forward, Congressional Research Service, 2017)
  • "In monitoring the impacts of recreational marijuana legalization in Washington State, government researchers report that there was no trend identified in the percentage of drivers testing positive for marijuana (either marijuana only or marijuana in combination with other drugs/alcohol) for those involved in traffic fatalities and who were tested for drugs or alcohol (The Marijuana Policy Gap and the Path Forward, Congressional Research Service, 2017)
  • "To this point, as a result of legalization, we haven't seen a large spike or epidemic of ... THC driving [in Oregon]." (Marijuana Legalization Hasn't Increased Traffic Fatalities In Oregon, Oregon Public Broadcasting, March 9, 2017)
  • "[Marijuana] legalization is associated with a nearly nine percent decrease in traffic fatalities, most likely to due to its impact on alcohol consumption." (Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption, Institute for the Study of Labor, 2011)

Proposed per se thresholds for THC are not evidence-based and may result in inadvertently criminalizing adults who previously consumed cannabis several days earlier but are no longer under the influence