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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."

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

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

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)."
  • "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

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

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