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Marijuana and Psychomotor Performance

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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

  • "Although laboratory studies have shown that marijuana consumption can affect a person's response times and motor performance, studies of the impact of marijuana consumption on a driver's risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage. Levels of impairment that can be identified in laboratory settings may not have a significant impact in real world settings, where many variables affect the likelihood of a crash occurring."
  • "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 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.

  • "In this multi-site observational study of non-fatally injured drivers, we found no increase in crash risk, after adjustment for age, sex, and use of other impairing substances, in drivers with THC <5ng/mL. For drivers with THC≥5ng/mL there may be an increased risk of crash responsibility, but this result was statistically non-significant and further study is required. ... There was significantly increased risk in drivers who had used alcohol, sedating medications, or recreational drugs other than cannabis. ... Our findings ... suggest that the impact of cannabis on road safety is relatively small at present time."
  • "As noted above, even if cannabis impairment is present, it creates (unless combined with alcohol or other drugs) only a fraction of the risks associated with driving at the legal 0.08 BAC threshold, let alone the much higher risks associated with higher levels of alcohol. ... The maximum risk for cannabis intoxication alone, unmixed with alcohol or other drugs, appears to be more comparable to risks such as talking on a hands-free cellphone (legal in all states) than to driving with a BAC above 0.08, let alone the rapidly-rising risks at higher BACs."

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

Data has not substantiated claims of an uptick in marijuana-induced fatal accidents in states that have regulated the use of cannabis for medical purposes, and some data has identified a decrease in motor vehicle accidents. Adult-use regulations have also largely not been associated with statistically significant increases in traffic fatalities, though researchers are still assessing longer-term trends.  

  • "In the five years after legalization, fatal crash rates increased more in Colorado and Washington than would be expected had they continued to parallel crash rates in the control states (+1.2 crashes/billion vehicle miles traveled, but not significantly so. The effect was more pronounced and statistically significant after the opening of commercial dispensaries ... [This finding]... stands in contrast to earlier studies finding decreases in traffic fatalities following medical marijuana legalization. ... [T]hese unexpected findings raise the possibility that legalization of medical and recreational marijuana represent two distinct policy exposures rather than 'escalating doses' of the same exposure and pose very different risks. This is an area in need of further study."

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