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Marihuana, A Signal of Misunderstanding - Table of Contents

The Report of the National Commission on Marihuana and Drug Abuse

Marihuana and Driving

Experimental Studies

Controlled laboratory experiments have attempted to measure these and other effects on driving with greater specificity. Manno and his associates (1970) found that there are significant impairments of motor and mental performance attributable to marihuana.

In an experiment conducted by Frank and his colleagues (1971), marihuana smokers demonstrated "a marked and very consistent increase in the amount of time required to recover from glare . . . This ranged up to four times as long with a mean peak of almost twice as long (171%) after smoking marihuana. . . . Furthermore, this increase in glare recovery time persisted for several hours" (p. 9) and did not seem to be dose related.

These researchers did not find significant differences between marihuana users and nonusers, however, in pulse, rate, time estimation or dilation of the pupils.

With respect to marihuana's effect on emotional reactions, Dr. S. E. Miller (1959) suggested that "these drugs (Including marihuana) have similar abilities for changing normal emotional reactions, even causing individuals to become oblivious or indifferent to their surroundings" (p. 864). Klein and his colleagues (1971), however, urge against generalizing these findings from the laboratory situation to the complex task of actual driving.

Crancer and his coworkers (1969) conducted a study designed to determine the effects of a "normal social marihuana high" on simulated driving performance among 36 experienced marihuana smokers and compared the effects they discovered with those occasioned by alcohol use.

The researchers found that experienced marihuana users under conditions of a "normal social marihuana high" (from two cigarettes totalling 1.7 grams of THC) accumulated significantly more speedometer errors than under control conditions. No significant differences were found, however, relative to accelerator, brake, signal, steering or total errors in simulator scores.

Comparing the effects of alcohol (at a blood level of 0.10%-the legal limit of intoxication) and marihuana intoxication (1.7 grams THC), the researchers concluded that moderate intoxication by marihuana was less detrimental to simulated driving performance than was the presence of alcohol at the 0.10% blood level. The mean error scores were 84.46 for the control group, 84.49 for the marihuana group and 97.44 for the alcohol group (p. 6).

There have been several criticisms leveled at this study, however, and several researchers have cited contradictory findings. Frank and his associates (1971) have pointed out the fact that the subjects did not have, complete control over their simulated drive.

Kalant (1969) noted that "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" (p. 640). He also criticized the use of dissimilar doses of marihuana and alcohol for the comparison and in this regard stated that "the 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" (p. 640).

Lastly, Kalant pointed out that Crancer and his colleagues failed to indicate if any measures were actually taken to ensure effective absorption of the doses by the experimental subjects. This may be important, if the findings of dose dependent impairments observed by Dagirmanjian and Boyd (1962) and Isbell and his associates (1967) are valid.

Although Crancer and his fellow researchers did not feel that the impairments found were related to either dosage level or experience with the drug, this latter finding is also subject to question in view of the earlier findings of the Mayor's Committee on Marihuana (1944) and Weil and his colleagues (1968) that the performance of drug-naive subject, was more impaired than was that of experienced marihuana users.

In a more recent simulator study, attention was directed to the effects of marihuana on risk acceptance (Dott, 1971). The experimental situation involved the placement of 12 experienced marihuana users under four conditions (non-smoking, placebo, low dose, and high dose) in order to compare subjects' reactions to various passing situations, some of which required an immediate response to an emergency.

The data show that even though more accidents occurred under the two marihuana conditions than under the placebo (and normal) condition, the differences were not significant. Significant differences were found in the number of passes completed and in the time needed to make pass decisions. Not only did the marihuana smokers complete fewer passes (174 in placebo condition, 153 in low dose condition, and 133 in the high dose condition), but they took more time to make the pass decisions while under the influence of the drug. The researcher concluded that "marihuana appears to make the subject less willing to accept risk, and it delays elective decision reaction time. Effects were most noted in situations which did not have a high attention demanding value. . . . In those situations which were of an emergency nature and which demanded immediate attention and response . . . . no drug effects were noted" (p. 28).

Dott also found, in contrast to Frank and his associates (1971) that pulse rate did seem to be affected by marihuana intoxication.

Based upon a comparison with the same experiment performed years before in relation to alcohol (Light and Kelper, 1969), Dott stated that "the effects of marihuana on driving behavior are more subtle and less hazardous than the effects of alcohol."

McGlothlin (1971) is presently conducting another simulator study and is attempting to measure the effects of marihuana, methadone and alcohol on simulated driving, attention, information processing abilities and other measures related to driving efficiency such is peripheral vision, depth perception and glare recovery. At this time, five experiments have been conducted and the researcher has stated that:

concentrated and divided attention for both auditory and visual modalities are significantly impaired by smoked marihuana containing 15 mg. THC.... In general, where impairment was found for the marihuana treatments, it appears to be equivalent to that resulting from a blood alcohol level of about 0.01%. Frequent marihuana users (one or two times per week) typically showed less impairment than those using less frequently (p. 22.).

The inconclusive and controversial nature of the research to date suggests that there is enough of a potential risk involved to both the individual and the public safety to recommend strongly against driving while intoxicated-no matter what the intoxicant. Although marihuana does not seem to produce serious impairments of driving skills or performance, to say that the drug does not at all adversely affect driving behavior or that it may not be a factor in traffic violations or accidents is to misrepresent the current state of knowledge. As the National Institute of Mental Health (1972) has noted, "obviously, more research is needed in elucidating the role of various drugs on highway accidents . . ." (p. 220).


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