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Major Studies of Drugs and Drug Policy
Marihuana, A Signal of Misunderstanding - Table of Contents

The Report of the National Commission on Marihuana and Drug Abuse

Marihuana and Driving

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.


(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).


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