The Report of the National Commission on Marihuana and Drug Abuse

Marihuana and Driving

The United States Department of Transportation has estimated that in 1970, more than 111 million Americans were licensed drivers and that there were approximately 109 million vehicles registered and regularly traversing America's streets and highways. The Commission-sponsored National Survey (Abelson, et al., 1972) has revealed that approximately 21 million or 15% of all American adults 18 years and over have tried marihuana and that about 6.9 million adults currently use the drug.

At the present time, there is no reliable estimate of the proportion of marihuana users who drive while "high," but America's tragic experience with highway accidents and fatalities involving persons driving under the influence of alcohol raises serious questions about the extent to which marihuana impairs driving skills and performance and thereby constitutes a public safety hazard on this nation's thoroughfares.

In recent years, public safety experts, along with the medical and scientific communities, have devoted increasing attention to the effects of any mind-altering drug on driving, but there is as yet little evidence to inform discussion. As part of its more general concern with the impact of marihuana on public safety, the Commission has reviewed the available research and has concluded that the evidence which presently exists is, at best, inconclusive.

In view of this finding and prior to a rather brief summary of the knowledge we now have, the Commission feels compelled to urge the public to consider these findings as only tentative, to adopt an extremely cautious attitude about the effects of marihuana on driving skill and performance, and perhaps most importantly, to avoid driving at all while under the influence of any mind-altering drug or intoxicant.

THE CURRENT STATE OF KNOWLEDGE

The literature on drugs and driving contains several references to the role of drugs, including marihuana, in traffic violations, accidents and fatalities. They differ considerably, however, in the nature and extent to which they influence or affect driving behavior (Waller, 1965; Chetta, 1967, Commission on Narcotic Drugs, 1968; Third Triannual Congress on Traffic Accidents, 1969; Waller, 1970; Klein, Davis and Blackbourne, 1971 National Institute of Mental Health, 1972).

The studies examine the relationship between marihuana and driving and tend to be either statistical and enumerative with regard to traffic accidents or experimental with respect to the physiological and psychological effects of marihuana use deemed to be related to driving skill and performance. All of them suffer to one degree or another from an overabundance or dearth of extraneous variables and have, therefore, precluded generalization of the results and the production of conclusive, and valid findings.

The difficulty in interpreting the data gathered from statistical or enumerative studies of traffic accidents derives primarily from the inability to isolate precise cause. There are no tissue, urine or blood analysis methods currently and systematically in use outside the laboratory, similar to those available for alcohol, for determining the presence of marihuana in the bodily fluids of drivers.

Statistical Studies

The statistical or enumerative studies generally can be characterized as two types. In the first type, samples of arrested marihuana, or other drug law violators are selected and their traffic violations or accident rates are compared with those in the general population. In the second type, samples are drawn initially from lists of persons known to have committed traffic violations or been involved in traffic accidents. The samples are then divided into persons who in some way are identified as marihuana users and those who are not and the incidence of traffic violations or accidents in the two groups is then compared.5

5 Dr. E. J. Woodhouse, a chemist, is now in the process of developing a marihuana testing method. At the present time he reports being able to detect the presence of marihuana by analyzing urine samples (Polak, 1971).

Waller (1965), compared the crash rates per unit miles of driving of known marihuana users with those of other drivers of similar age distribution. He found that the crash risk was not increased by the use of marihuana.

A similar study yielding similar results was performed in the State of Washington (Crancer and Quiring, 1968). There were no significant differences in the crash rates per 100 drivers between those who did use marihuana and those who did not. The researchers compared the driving records of persons arrested for illegal drug use (100 narcotics users, 123 dangerous drug users and 79 marihuana users) with those of 687,228 licensed drivers living in the same general driving environment (King's County, Washington). Comparisons were made of the number of accidents, the number of violations and the type of violations accumulated between January 1, 1961 and October 1, 1967.

All three drug using groups had significantly higher accident and violation rates than did the comparison group matched for age and sex; the accident rate for the marihuana users was 39% higher; for the narcotics users, 29% higher; and for the dangerous drug users, 57% higher. The violation rates were 180 % higher, 149 % higher and 16% higher, respectively.

Table 9 below shows the percentage of marihuana users and county drivers, by sex, with none and 10 or more traffic violations and accidents between January 1, 1961 and October 1, 1967.

Since the majority of the users were first arrested for their illegal drug use in 1964, the researchers compared their violation rates before and after that time. The data show a violation rate of 1.78 per marihuana driver prior to 1964 (January 1, 1961 to June 30, 1964) ; the rate increased to 3.44 per driver between July 1, 1964 and October 1, 1967. For the same time periods, the county group's violation rate per driver increased from 0.4-4 to 0.53.

The violation rates for both reckless and hit-and run driving were significantly higher for the marihuana users than for the county comparison group.

In short, these data suggest that marihuana users are much more likely to have many violations and accidents and are much less likely to have clear accident and violation records than are a comparison group of drivers drawn from the same general population. Them findings are difficult to interpret more precisely, however, because Such variables as the number of miles driven and overall driving experience were not taken into account.

Table 9.-MARIHUANA USERS AND COUNTY DRIVERS HAVING NONE AND TEN OR MORE TRAFFIC VIOLATIONS AND ACCIDENTS, BY SEX, JANUARY 1, 1961OCTOBER 1, 1967

(Figures in Percentages)

Marihuana users County drivers

Male Female Male Female

None... 10.0 33.3 41.1 67.7

10..... 22.9 11.1 2.2 0.1

Source: Crancer and Quiring, 1968:9.

A survey of 12,453 Virginia high school students was conducted during 1970 by the Virginia Highway Safety Division (Ferguson and Howard, 1971). The objectives of the survey were to determine the extent of drug use in the Virginia high school population and to assess the number of traffic crashes which could be caused by drug-impaired drivers.

The data, show that 2.9% of the sample reported experiencing, either as a passenger or driver, at least one traffic crash in which drug use "could have been a causal factor." The data also show that the students were more likely to attribute the crashes to marihuana (54%) than to other drugs (46%). Use of marihuana was found to be slightly more common than was the use of other drugs among drivers involved in non-fatal collisions but was used with equal frequency in those drivers involved in fatal crashes (p. 31).

The researchers suggest as an explanation for their findings the fact that marihuana. usage was greater among this population (12.3%) than was the use of other drugs (7.7%). This explanation is not completely adequate, however, in that the survey ignores both the possible presence of alcohol along with marihuana and other drugs, either separately or in combination. As such, it cannot be said that marihuana causes more accidents than do other drugs, including alcohol.

Klein, Davis and Blackbourne (1971) surveyed students at four academic institutions in Florida in an effort to assess the role of marihuana in traffic involvements. Respondents were divided into five groups of marihuana users: (a) non-users, (b) previous users, (c) using less than four times a month, (d) using four to eight times per month, and (e) using more than eight times per month. For each group information was obtained relative to the frequency of alcohol and tobacco use; respondents' ability to judge speed, time and reaction time; traffic involvements; and license revocations. In addition, respondents were asked their opinion about whether or not persons under the influence of marihuana should be permitted to operate aircraft and various other vehicles, including taxis and private automobiles.

With respect to traffic involvements, the data show that 18% of the infrequent users and 53% of the frequent users reported having been stopped by the police while under the influence of marihuana. The data also show that as the frequency of use increases, so too does the number of license revocations.

As Nichols (1971) has pointed out, however:

Apparently no attempt was made to compare the number of times they were stopped while under the influence of marihuana as opposed to the number of times they were stopped while not under the influence. Thus, the data do not give any indication of whether the violations were due to the effects of marihuana or whether they were the result of poor driving habits in the first place (pp. 28-29).

Both experimental and quasi-experimental approaches to assessing the effects of marihuana on driving suffer from methodological shortcominogs and inconsistent results. The primary deficiency in the experimental investigations lies in their inability to hold constant the numerous external variables which actually affect driving behavior and which, therefore, precludes valid comparisons of actual driving performance with that simulated in a controlled but unrealistic environment. In the quasi-experimental studies using interviewing techniques, the accuracy of self-reports becomes a question which must be raised with respect to the, effects of marihuana on driving skills and performance.

The data derived front these studies, however, suggest that marihuana does interfere, at least in some users, with the ability to judge time, speed and distance ; with reaction time; and with the ability to control the vehicle and respond to an emergency situation (Zinberg and Well, 1969; James, 1970- Hochman and Brill, 1971; Klein, et al., 1971).

Data from a survey of 10% of the undergraduate students at UCLA (Hochman and Brill, 1971) show that one-third of all marihuana users drive, occasionally while "high." Of the chronic users, 42% drive frequently while high and 10% always drive while "stoned" (p. 22).

According to the researchers, "both (marihuana) users and non-users were universally of the opinion that (marihuana) 'intoxication' affected driving, but users thought, that they compensated by being more cautious, driving more slowly, and concentrating on the driving," (p. 22). The researchers also report that as usage of marihuana becomes more chronic, "fear" and avoidance of driving while intoxicated decreases and the ability to compensate for the drug's effects increases.

With respect to traffic violations, the data show that 4% of occasional and chronic marihuana users had received traffic tickets when they were intoxicated by the drug but that none had been discovered to be intoxicated at the time.

These findings corroborate those, from an earlier study (Zinberg and Weil, 1969) in which the researchers stated that "users appear to be able to compensate 100% for the nonspecific effects of ordinary doses of marihuana on ordinary psychological performance" (p. 39). In another report Weil (1969) wrote that "it appears that once a person becomes accustomed to the effects of cannabis, he, can compensate fully for the drug's influence on performance of tasks of ordinary complexity." (p. 6).

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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Marihuana, A Signal of Misunderstanding