Marijuana use and driving
Hindrik W.J. Robbe
Institute for Human Psychopharmacology, University of Limburg,
P.O. Box 616, 6200 MD Maastricht, The Netherlands
Robbe, Hindrik W.J. , 1994. Marijuana use and driving. Journal of the
International Hemp Association 1: 44-48.
This article concerns the effects of marijuana smoking on actual driving
performance. It presents the major results of one laboratory and three
on-road driving studies. The latter were conducted on a closed section
of a primary highway, on a highway in the presence of other traffic and
urban traffic, respectively. 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
where 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.
After alcohol, delta-9-tetrahydrocannabinol (THC), marijuana's major
psychoactive constituent, is the drug which is found most often in the
of drivers involved in road accidents. With some exceptions, epidemiological
studies indicate the presence of THC in roughly 4-12% of drivers
injured or killed in traffic accidents: for example, 10% in New York
(Terhune 1982), 7% in New South Wales (Chesher and Starmer 1983), 8% in
North Carolina (Mason and McBay 1984), 11% in Dčsseldorf (Daldrup et
al. 1987), 10% in Tasmania (McLean 1987) and 11% in Ontario
(Cimbura et al. 1990). The most recent data regarding the incidence
of drugs in fatally injured drivers in the United States are available
nationwide study conducted in 1990 and 1991 (Terhune et al. 1992):
THC was found in only 4% of the drivers. This relatively low percentage
indicate a declining trend in the incidence rate of THC in fatally
injured drivers in the United States, explainable by the declining prevalence
marijuana use throughout the 1980s (Johnston et al. 1992).
These numbers are, however, inconclusive regarding THC's contribution
to accidents because alcohol has been a severe confounding factor in all
surveys of injured or killed drivers: generally 60-80% of drivers who
were found positive for THC also showed the presence of alcohol. Another
problem of these surveys is the common lack of sound control groups.
To determine whether drivers under the influence of THC are
overrepresented in accidents, the THC incidence in accident victims
should be compared to the THC incidence in randomly selected drivers
passing the same accident site at the same times and days of the week.
This has been done for alcohol (Borkenstein et al. 1974), but not for THC.
The lack of separate control groups has been circumvented by the use
of a 'culpability index'. This index is the ratio of the percentage of
with detectable drug levels and deemed responsible for a traffic accident
to that of drug-free drivers from the same sample who were likewise
culpable. But results from this approach have not been consistent:
three studies yielded a culpability index of about 1.7 (Warren et al. 1981,
Terhune 1982, Donelson et al. 1985), two other studies failed to find
a significantly elevated culpability index for marijuana users (Williams
1985, Terhune et al. 1992). For this and other reasons given above,
the independent contribution of THC to accidents remains exceedingly obscure.
Several literature reviews, the most recent by Robbe (1994), have shown
that the results from driving simulator and closed-course tests indicate
that THC in inhaled doses up to 250 µg/kg has relatively minor
effects on driving performance, certainly less than blood alcohol concentrations
(BACs) in the range 0.08-0.10 g%. In contrast to this, laboratory studies
have repeatedly shown performance impairment occurring after inhaled
doses as low as about 40 µg/kg. These became large and persistent
after 100-200 µg/kg doses. Tracking, divided attention and vigilance
performance were particularly vulnerable to THC's effects. Assuming
that both sets of results are valid for the particular circumstances under
which they were obtained, they demonstrate that performance decrements
obtained under the artificial and non-life threatening conditions in the
laboratory do not automatically predict similar decrements in driving
simulations that are closer to real-world driving. These conflicting results
led, however, to opposing opinions regarding marijuana's effects on
driving performance. Real-world driving studies were therefore warranted.
Only one study (Klonoff 1974) had been conducted in actual traffic
before the present research program started. In a city driving study, Klonoff
assessed the effects of two THC doses, 4.9 and 8.4 mg, which are equivalent
to 70 and 120 µg/kg for a 70 kg person. Aspects of subjects' driving
performance were scored by a professional examiner. The results showed
that subjects performed less competently when under the influence of
the highest, but not the lowest dose. In particular, they scored lower
on judgment and concentration scales. Several investigators, however,
criticized the method used by Klonoff for measuring driving performance
on the grounds that the examiners' reliability was never determined and
that the scoring instrument had never been shown to provide measures
related to driving safety (Moskowitz 1985, Smiley 1986). Furthermore,
Klonoff administered relatively low THC doses to his subjects. The
effects of high doses of THC on driving in real traffic still needed to
The studies reported in this article were conducted to escape these
limitations. First, the effects of low, moderate and high THC doses on
driving were determined, both in the absence and presence of other
traffic. Second, Klonoff's city driving study was replicated, with some
modifications with regards to the employed procedures and with the
addition of another group of subjects who undertook the same driving test
then under the influence of a low dose of alcohol.
An important practical objective of the program was to determine whether
degrees of driving impairment can be accurately predicted from either
measured concentrations of THC in plasma or performance measured in
potential roadside 'sobriety' tests of tracking ability or hand and posture
stability. The results (Robbe 1994), like many reported before, indicate
that none of these measures accurately predicts changes in actual driving
performance under the influence of THC.
Subjects in all studies were recreational users of marijuana or hashish,
i.e., smoking the drug more than once a month, but not daily. They were
healthy, between 21 and 40 years of age, had normal weight and binocular
acuity, and were licensed to drive an automobile. Furthermore, law
enforcement authorities were contacted, with the volunteers' consent,
to verify that they had no previous arrests or convictions for drunken
or drug trafficking.
Each subject was required to submit a urine sample immediately upon
arrival at the test site. Samples were assayed qualitatively for the following
common 'street drugs' (or metabolites): cannabinoids, benzodiazepines,
opiates, cocaine, amphetamines and barbiturates. In addition, a breath
sample was analyzed for the presence of alcohol using a Lion-SD3 breath-analyzer.
Subjects were accompanied during every driving test by an licensed
driving instructor. A redundant control system in the test vehicle was
for controlling the car, should emergency situations arise.
Marijuana and placebo marijuana cigarettes were supplied by the U.S.
National Institute on Drug Abuse. The lowest and highest THC
concentrations in the marijuana cigarettes used in the studies were
1.75% and 3.57%, respectively.
The pilot study was conducted in a hospital under strict medical supervision
to identify THC doses that recreational marijuana users were likely to
consume before driving. Twenty-four subjects, twelve males and twelve
females, participated. They were allowed to smoke part or all of the THC
content in three cigarettes until achieving the desired psychological
effect. Cigarettes were smoked through a plastic holder in a manner determined
by the subjects. The only requirement was to smoke for a period not
exceeding 15 minutes. When subjects voluntarily stopped smoking, cigarettes
were carefully extinguished and retained for subsequent gravimetric
estimation of the amount of THC consumed.
Six subjects consumed one cigarette, thirteen smoked two and four smoked
three. The average amount of THC consumed was 20.8 mg, after
adjustment for body weight, 308 µg/kg. It should be noted that
the amounts of THC consumed represent both the inhaled dose and the portion
was lost through pyrolysis and side-stream smoke during the smoking
process. There were no significant differences between males and females,
nor between frequent and infrequent users, with respect to the weight
adjusted preferred dose. It was decided that the maximum dose for
subsequent driving studies would be 300 µg/kg. This is considerably
higher than doses that have usually been administered to subjects in
experimental studies (typically, 100200 µg/kg THC).
Study 1: Driving on a restricted highway
The first driving study was conducted on a highway closed to other
traffic. One objective of this study was to determine whether it would
to repeat the study on a normal highway in the presence of other traffic.
The second objective was to define the dose-effect relationship between
inhaled THC dose and driving performance. The same twelve men and twelve
women who participated in the laboratory study served again as the
subjects. They were treated on separate occasions with marijuana cigarettes
containing THC doses of 0 (placebo), 100, 200, and 300 µg/kg.
Treatments were administered double-blind and in a counterbalanced
order. On each occasion, subjects performed a road-tracking test beginning
40 minutes after initiation of smoking and repeated one hour later.
The test, developed and standardized by O'Hanlon et al. (1982, 1986), involved
maintaining a constant speed at 90 km/h and a steady lateral position
between the delineated boundaries of the traffic lane. Subjects drove 22
on a primary highway and were accompanied by a licensed driving instructor.
The primary dependent variable was the standard deviation of
lateral position (SDLP), which has been shown to be both highly reliable
and very sensitive to the influence of sedative drugs and alcohol. Other
dependent variables were mean speed, and standard deviation of speed
and steering wheel angle.
All subjects were willing and able to finish the driving tests without
great difficulty. The study demonstrated that marijuana impairs driving
performance as measured by an increase in SDLP; all three THC doses
significantly affected SDLP relative to placebo. The driving performance
decrement after smoking marijuana persisted almost undiminished for
two hours after smoking. Marijuana's effects on SDLP were compared to
those of alcohol obtained in a very similar study by Louwerens et al.
(1985, 1987). It appeared that the effects of the various administered
THC (100300 µg/kg) on SDLP were equivalent to those associated
with BACs in the range of 0.03-0.07 g%. Other driving performance measures
were not significantly affected by THC. Both the observed degree of
driving impairment, and what subjects said and did, indicated that normal
safeguards would be sufficient for ensuring safety in further testing.
Hence, the final conclusion was to repeat this study on a normal highway
the presence of other traffic.
Study 2: Driving on a normal highway in traffic
The second driving study was conducted on a highway in the presence
of other traffic and involved both a road-tracking and a car-following
A new group of sixteen subjects, equally comprised of men and women,
participated in this study. A conservative approach was chosen in
designing the present study in order to satisfy the strictest safety
requirements. That is, the study was conducted according to an ascending
series design where both active drug and placebo conditions were administered,
double-blind, at each of three THC dose levels. THC doses were
the same as those used in the previous study, namely 100, 200, and
300 µg/kg. Cigarettes appeared identical at each level of treatment
any subject had reacted in an unacceptable manner to a lower dose,
he/she would not have been permitted to receive a higher dose.
The subjects began the car-following test 45 minutes after smoking.
The test was performed on a 16Êkm stretch of the highway and lasted
minutes. After the conclusion of this test, subjects performed a 64-km
road-tracking test on the same highway which lasted about 50 minutes. At
the conclusion of this test, they participated again in the car-following
The road-tracking test was the same as in the previous study except
for its duration and the presence of other traffic. Subjects were instructed
maintain a constant speed of 95Êkm/h and a steady lateral position
between lane boundaries in the right traffic lane. They were allowed to
from this only if it became necessary to pass a slower vehicle in the
same lane. Data from the standard test were analyzed to yield the same
performance measures as in the previous study.
The car-following test involved attempting to match velocity with,
and maintain a constant distance from a preceding vehicle as it executed
of deceleration/acceleration maneuvers. The preceding vehicle's speed
would vary between 80 and 100 km/h and the subject was instructed to
maintain a 50 m distance however the preceding vehicle's speed might
vary. The duration of one deceleration and acceleration maneuver was
approximately 50 seconds and six to eight of these maneuvers were executed
during one test, depending upon traffic density. The subject's average
reaction time to the movements of the preceding vehicle, mean distance
and coefficient of variation of distance during maneuvers were taken as
dependent variables from this.
All subjects were able to complete the series without suffering any
untoward reaction while driving. Road-tracking performance in the standard
test was impaired in a dose-related manner by THC and confirmed the
results obtained in the previous closed highway study. The 100 µg/kg
produced a slight elevation in mean SDLP, albeit not statistically
significant. The 200 µg/kg dose produced a significant elevation,
practical relevance. The 300 µg/kg dose produced a highly significant
elevation which may be viewed as practically relevant but unexceptional
comparison with similarly measured effects of many medicinal drugs.
After marijuana smoking, subjects drove with an average speed that was
slightly lower than after a placebo and very close to the prescribed
In the car-following test, subjects maintained a distance of 4550
m while driving in the successive placebo conditions. They lengthened mean
distance by 8, 6 and 2 m in the corresponding THC conditions after
100, 200 and 300 µg/kg, respectively. The initially large drug-placebo
difference and its subsequent decline is a surprising result. Our explanation
for this observation is that the subjects' caution was greatest the first
time they undertook the test under the influence of THC and progressively
less thereafter. The reaction time of the subjects to changes in the
preceding vehicle's speed increased following THC treatment, relative
to placebo. The administered THC dose was inversely related to the change
in reaction time, as it was to distance. However, increased reaction
times were partly due to longer distance. Statistical adjustment for this
confounding variable resulted in smaller and non-significant increases
in reaction time following marijuana treatment, the greatest impairment
(0.32 s) being observed in the first test following the lowest THC
dose. Distance variability followed a similar pattern as mean distance
reaction time; the greatest impairment was found following the lowest
Study 3: Driving in urban traffic
The program proceeded to the third driving study, which involved tests
conducted in high-density urban traffic. There were logical and safety
reasons for restricting the THC dose to 100 µg/kg. It was given
to a group of regular marijuana (or hashish) users, along with a placebo.
comparative purposes, another group of regular alcohol users was treated
with a modest dose of its members' preferred recreational drug, ethanol,
or a placebo, before undertaking the same city driving test. Two groups
participated, each composed of sixteen new subjects comprising equal
numbers of men and women. Subjects in the alcohol group were regular
users of alcohol, but not marijuana. Both groups were treated on separate
occasions with the active drug and a placebo. Marijuana was administered
to deliver 100Êµg/kg THC. The driving test commenced 30 minutes
smoking. The alcohol dose was chosen to yield a BAC approaching 0.05
g% when the driving test commenced 45 minutes after onset of drinking.
Active drug and placebo conditions were administered double-blind and
in a counterbalanced order in each group.
Driving tests were conducted in daylight over a constant 17.5 km route
within the city limits of Maastricht. Subjects drove their placebo and
active-drug rides through heavy, medium and low density traffic on
the same day of the week, and at the same time of day. Two scoring methods
were employed in the present study. The first, a 'molar' approach,
required the driving instructor acting as the safety controller during
the tests to
rate the driver's performance retrospectively using a standard scale.
The second, a more 'molecular' approach, involved the employment of a
specially trained observer who applied simple and strict criteria for
recording when the driver made or failed to make each in a series of observable
responses at predetermined points along a chosen route.
The study showed that a modest dose of alcohol (BAC = 0.034 g%) produced
a significant impairment in city driving, as measured by the molar
approach, relative to a placebo. More specifically, alcohol impaired
both vehicle handling and traffic maneuvers. Marijuana, administered in
dose of 100Êµg/kg THC, on the other hand, did not significantly
change mean driving performance as measured by this approach. Neither alcohol
nor marijuana significantly affected driving performance measures obtained
by the molecular approach, indicating that it may be relatively
insensitive to drug-induced changes.
Driving quality, as rated by the subjects, contrasted with observer
ratings. Alcohol impaired driving performance according to the driving
instructor, but subjects did not perceive it; marijuana did not impair
driving performance, but the subjects themselves perceived their driving
performance as such. Both groups reported about the same amount of
effort in accomplishing the driving test following a placebo. Yet only
subjects in the marijuana group reported significantly higher levels
of invested effort following the active drug. Thus there is evidence that
in the marijuana group were not only aware of their intoxicated condition,
but were also attempting to compensate for it. These seem to be
important findings. They support both the common belief that drivers
become overconfident after drinking alcohol and investigators' suspicions
that they become more cautious and self-critical after consuming low
doses of THC, as smoked marijuana.
The results of the studies corroborate those of previous driving simulator
and closed-course tests by indicating that THC in inhaled doses up to
300 µg/kg has significant, yet not dramatic, dose-related impairing
effects on driving performance. Standard deviation of lateral position
road-tracking test was the most sensitive measure for revealing THC's
adverse effects. This is because road-tracking is primarily controlled
automatic information processing system which operates outside of conscious
control. The process is relatively impervious to environmental
changes, but highly vulnerable to internal factors that retard the
flow of information through the system. THC and many other drugs are among
these factors. When they interfere with the process that restricts
SDLP, there is little the afflicted individual can do by way of compensation
restore the situation. Car-following and, to a greater extent, city
driving performance depend more on controlled information processing and
therefore more accessible for compensatory mechanisms that reduce the
decrements or abolish them entirely.
THC's effects after doses up to 300 µg/kg never exceeded alcohol's
at BACs of 0.08 g% and were in no way unusual compared to many medicinal
drugs (Robbe 1994). Yet THC's effects differ qualitatively from many
other drugs, especially alcohol. Evidence from the present and previous
studies strongly suggests that alcohol encourages risky driving whereas
THC encourages greater caution, at least in experiments. Another way THC
seems to differ qualitatively from many other drugs is that the former's
users seem better able to compensate for its adverse effects while driving
under the influence.
Although THC's adverse effects on driving performance appeared relatively
small in the tests employed in this program, one can still easily imagine
situations where the influence of marijuana smoking might have a dangerous
effect; i.e., emergency situations which put high demands on the
driver's information processing capacity, prolonged monotonous driving,
and after THC has been taken with other drugs, especially alcohol.
Because these possibilities are real, the results of the present studies
should not be considered as the final word. They should, however, serve
point of departure for subsequent studies that will ultimately complete
the picture of THC's effects on driving performance.
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Ach ja, es sind ein paar Umlaute drin, die nicht richtig wiedergegen
1. soll glaub ich microg/kg heissen.
2. vor dem km/h kann ich selbst im Orginal nicht richtig deuten, da
anscheinend mein Netscape auch meint es nicht richtig darstellen zu muessen.
Netscape stellt ein E mit einem Dach oben drauf (ist glaub ich irgendso
Leider weiss ich nicht mehr woher ich das Ding aus dem Internet habe!
Es schien aber vertrauenswuerdig zu sein und so weit ich mich erinnern
konnte gabs dort noch mehr von dem Zeug...