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Marihuana, A Signal of Misunderstanding - Table of Contents

National Commission on Marihuana and Drug Abuse

Marihuana: A Signal of Misunderstanding

Chapter III

Social Impact of marihuana use


Within the context of public safety another issue which merits attention is the extent to which drivers under the influence of marihuana constitute a hazard on the nation's streets and highways. Although in recent years increasing attention has been given to this issue, at present little empirical evidence exists to inform discussion.

To assess the actual and potential impact of marihuana on traffic safety, a number of basic research questions must be answered.

the extent to which marihuana, users actually drive while under the influence of the drug

the extent to which marihuana users driving while "high" commit traffic violations and are involved in traffic accidents

the amounts of marihuana consumed immediately prior to the commission of traffic violations or the involvement in traffic accidents and the drug's role in these events

the nature and extent to which marihuana actually impairs psychomotor skills, judgment and driving performance

To date, the generalizations made concerning the effects of marihuana on driving behavior have generally been based on statistical studies of traffic violations and accidents and inferences drawn from more general studies of the physiological and psychological consequences of marihuana use, such as changes in pulse rate, reaction time, neuromuscular coordination, time estimation and spatial perceptions.

Such studies pose serious limitations in the nature, reliability and validity of the data. The basic problems derive from difficulties in identifying and attributing cause. A major obstacle in such retrospective analysis is the inability to separate the effects of marihuana from those possibly engendered by the use of other drugs, such as alcohol, tranquilizers and amphetamines. Finally, conclusive analysis is impossible until a reliable technique is developed for measuring the level of marihuana present in the body of the driver at the time of his violation or accident.

Prospective experimental studies of actual reactions to road conditions and traffic emergency situations would undoubtedly provide the most reliable and valid data, but such studies would themselves endanger the public and have not been undertaken. Researchers have relied, therefore, on controlled laboratory simulator studies and direct interviews with those who have admitted to driving while under the influence of marihuana.

With respect to the simulator studies , the available evidence suggests that while, in some cases, marihuana has produced interference with certain motor or mental abilities which affect driving behavior, these effects were generally believed to be readily overcome by the exercise of extreme caution by the driver and a significant reduction in speed.

The few driving simulator tests completed to date have generally revealed no significant correlations between marihuana use and driving disabilities. Comparison of the simulator scores of users and nonusers, however, did reveal small but nonsignificant differences in the number of speedometer errors made.

These simulator studies also examined the comparative effects of alcohol and marihuana on driving scores. The findings of one study, though controversial, suggested that intoxication resulting from low doses of marihuana was less detrimental to driving performance than was the presence of alcohol at the legally prohibited blood level of .10%.

The methodological limitations of the study raise serious questions about the reliability and validity of the findings. As one critic has noted, "It does not follow automatically that lack of effect of a drug on the simulated task will correlate with lack of effect on the actual task." Further , the use of dissimilar doses of alcohol and marihuana has led another critic to assert that "finding that a heavy dose of alcohol caused more impairment than a mild dose of marihuana is neither surprising nor helpful in assessing the relative effects of the two drugs in the relative doses in which they are normally used."

Recent research has not yet proven that marihuana use significantly impairs driving ability or performance. The Commission believes, nonetheless, that driving while under the influence of any psychoactive drug is a serious risk to public safety; the acute effects of marihuana intoxication, spatial and time distortion and slowed reflexes may impair driving performance. That the risk of injury may be greater for alcohol than for marihuana matters little.

Obviously, Much more research needs to be undertaken in this area. Hopefully, recent studies sponsored by the National Institute of Mental Health and other agencies will soon provide the concrete information that is needed.


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