Psychomotor Skills

Psychomotor Skills of persons under the influence of cannabis, in particular, the psychomotor skills required to operate a motor vehicle, have been studied more thoroughly than most of the potential medical benefits known to science. As Harvard Professor Lester Grinspoon has pointed out, the government’s continuous attempts to determine the dangers of marijuana impairment have proven precisely the opposite. In this example, numerous attempts have been made to blame cannabis for a large number of highway fatalities and other auto accidents, but a thorough analysis of available data supports the contention that these critical psychomotor skills are not significantly impaired by typical doses of marijuana.


In 1990, the US National Transportation Safety Board (NTSB) reported that 12.8% of those involved in fatal truck accidents showed signs of cannabis use in postmortem examination. However, that statistic is an unreliable indicator of the effects of cannabis on driving performance in a majority of those cases. A much larger NTSB study published in 1988 found that those drivers using only cannabis accounted for 2.2% of fatal accidents. That report concluded, “THC plays a relatively minor role in fatal traffic accidents as compared with alcohol.”[1]


In 1992, a study released by the National Highway Transportation Safety Administration (NHTSA) indicated that alcohol is by far the leading cause of drug-related traffic accidents, while marijuana poses a negligible danger, except when combined with alcohol. In an analysis of blood samples from 1,882 drivers killed in vehicular accidents in seven states, alcohol was found in over 51%. Marijuana was a distant second to alcohol, at just 6.7%. Because two-thirds of those deceased marijuana users were also under the influence of alcohol at the time of death, the actual number of drivers who tested positive for cannabis was 2.2%, the exact number determined by the NTSB in 1988. The US government’s 1992 NHTSA research, considered the most comprehensive study of cannabis use among driving fatalities, was suppressed for almost two years because it contradicted America’s “just say no” anti-pot propaganda.[2]


The irrationality of marijuana prohibition becomes plain upon realization that drunk drivers are more than seven and one-half times more likely to die in auto accidents than are marijuana users who don’t drink and drive. Then again, even that statistic is misleading because testing positive for marijuana use does not prove that the subject was inebriated or impaired at the time of the accident. While alcohol leaves the body relatively quickly over the course of a few hours, cannabinoids are stored in fat cells of the body and may be detected up to five weeks following a single exposure. Cannabis users continue to test positive long past the time of intoxication. There is no evidence that any of those drivers who died at the wheel with cannabinoids in their systems were actually under the influence of marijuana at the time of death. Considering that drinking and driving is the leading cause of death in 18 to 24-year-old Americans, the contrast between “acceptable limits” of alcohol inebriation and “zero tolerance” of marijuana use shows a serious flaw in drug prohibition rationale. Those policies tell young people that it’s okay to skirt with disaster by driving slightly impaired, but a far less dangerous episode involving the mild effects of pot can result in very severe punishment.


In 1996, the National Highway Traffic Safety Administration published results of the University of Lindberg study in the Netherlands under the title, “Marijuana and Actual Driving Performance.”[3] In the introduction of that paper, a review of previously published data on traffic accidents involving cannabis use was again declared inconclusive due to the prevalence of other substances; 60-80% of drivers who were found positive for cannabis also tested positive for alcohol. As these statistics are an unreliable indicator of the affect of cannabis intoxication on motor skills, author Hindrik Robbe and his team of researchers conducted exhaustive testing of actual performance of drivers while they were under the influence of cannabis. Drivers were tested both on highways and in urban traffic. Driving performance was rated by licensed driving instructors. The US National Institute on Drug Abuse supplied marijuana cigarettes and non-intoxicating placebo “joints.” Comprehensive urine analysis was conducted to determine actual levels of THC intoxication in two dozen 21 to 40-year-old subjects. The well-controlled scientific study concluded,


This program of research has shown that marijuana produces only a moderate degree of driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol. Drivers under the influence of marijuana retain insight into their performance and will compensate when they can (e.g. by increasing distance between vehicles or increasing effort). As a consequence, THC’s adverse effects on driving performance appeared relatively small in the tests employed in this program.[4]


Most studies of actual driving ability do show that psychomotor skills are mildly impaired at high doses of delta-9 THC, especially when it is taken by inexperienced subjects. Subjects familiar with the effects of cannabis use generally show less impairment, and in some cases, experienced marijuana users have shown a documented increase in psychomotor skills while under the influence of THC.[5] All major research on the subject indicates that the effects of cannabis and synthetic delta-9 THC have minimal effects on psychomotor skills as compared to the effects of alcohol, barbiturates, opiate compounds, and some over-the-counter medications.


The US NTHSA has determined that the only major statistical outcome associated with marijuana use in driving is a mild degree of speed reduction, a finding noted by Hindrik Robbe in the Netherlands. Speed reduction is a classic symptom of cannabis intoxication, which also reported in the United Nations World Health Organization report on the probable health effects of cannabis. In the suppressed UN report of 1997, the World Health Organization concluded that, “No controlled epidemiological studies have established that cannabis users are at increased risk of motor vehicle accidents.”[6]


Also in 1997, a psychiatrist named Lehman, with over 60 years of medical practice under his belt, testified before a Canadian court in the case of Ontario hemp store owner Chris Clay. In that doctor’s opinion, “A driver talking on a cellular phone is more of a menace on the road than one who has just smoked a marijuana joint.”[7]


Related sections: Cerebral Effects, Psychoactivity.



[1] Nelson, “A critical review of the research literature concerning some biological and psychological effects of cannabis.” Advisory Committee on Illicit Drugs, Cannabis and the Law in Queensland: Criminal Justice Commission of Queensland, Australia, 1993

[2] Terhune, et. al., “The incidence and role of drugs in fatally injured drivers.” Accident Research Group, Buffalo, NY, Report #DOT-HS-808-065

[3] Rosenthal, Geiringer, Mikuriya, MD, Marijuana Medical Handbook: A Guide to Therapeutic Use. San Francisco: Quick American Archives, 1997

[4] Robbe, Marijuana Use and Driving. Institute for Human Psychopharmacology, University of Limberg, 1994

[5] Klonoff, “Marijuana and driving in real life situations.” Science, Vol. 186, pp. 312-24, 1974

[6] World Health Organization Project on Health Implications of Cannabis Use, 1997

[7] London Free Press (Canada), May 8, 1997