are in Research / Driving
And Actual Driving Performance
on behalf of: U.S. Department of Transportation,
National Highway Traffic
(DOT HS 808 078), Final Report, November 1993
Conducted by: HWJ Robbe Institute for Human Psychopharmacology,
of Maastricht, P.O. Box 616, NL-6200 MD,<
Maastricht, The Netherlands
Marijuana's effects on actual driving performance were assessed in a series of
three studies wherein dose-effect relationships were measured in actual driving
situations that progressively approached reality. The first was conducted on a
highway closed to other traffic. Subjects (24) were treated on separate occasions
with THC 100, 200 and 300 g/kg, and placebo. They performed a 22-km road tracking
test beginning 30 and 90 minutes after smoking. Their lateral position variability
increased significantly after each THC dose relative to placebo in a dose-dependent
manner for two hours after smoking. The second study was conducted on a highway
in the presence of other traffic. Subjects (16) were treated with the same THC
doses as before. They performed a 64-km road tracking test preceded and followed
by 16-km car following tests. Results confirmed those of the previous study. Car
following performance was only slightly impaired. The third study was conducted
in high-density urban traffic. Separate groups of 16 subjects were treated with
100 g/kg THC and placebo; and, ethanol (mean BAC .034 g%) and placebo. Alcohol
impaired performance relative to placebo but subjects did not perceive it. THC
did not impair driving performance yet the subjects thought it had. These studies
show that THC in single inhaled doses up to 300 g/kg has significant, yet not
dramatic, dose-related impairing effects on driving performance.
article describes the results of a research program that was set up to determine
the dose-response relationship between marijuana and objectively and subjectively
measured aspects of real world driving; and to determine whether it is possible
to correlate driving performance impairment with plasma concentrations of the
drug or a metabolite. The program consisted of three driving studies in which
a variety of driving tasks were employed, including: maintenance of a constant
speed and lateral position during uninterrupted highway travel, following a leading
car with varying speed on a highway, and city driving. A laboratory study preceded
the driving studies for identifying the highest THC dose to be administered in
the subsequent studies.
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 all 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 driving 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. Blood samples were repeatedly taken after smoking by venepuncture. Quantitative
analysis of THC and THC-COOH in plasma was performed by gas chromatography/mass
spectrometry (gc/ms) using deuterated cannabinoids as internal standards.
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. Subjects smoked
the administered cigarettes through a plastic holder in their customary fashion.
Subjects were accompanied
during every driving test by a licensed driving instructor. A redundant control
system in the test vehicle was available for controlling the car, should emergency
In each study, subjects repeatedly performed certain simple laboratory tests (e.g.
critical instability tracking, hand and posture stability), estimated their levels
of intoxication and indicated their willingness to drive under several specified
conditions of urgency. In addition, heart rate and blood pressure were measured.
Results of these measurements are reported elsewhere (Robbe, 1994).
subjects, equally comprised of men and women, participated in this study. They
were allowed to smoke part or all of the THC content in three cigarettes until
achieving the desired psychological effect. 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.
consumed one cigarette, thirteen smoked two and four smoked three (data from one
male subject were excluded from the results because no drug was found in his plasma
after smoking). The average amount of THC consumed was 20.8 mg, after adjustment
for body weight, 308 g/kg. It should be noted that these amounts of THC represent
both the inhaled dose and the portion that 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 g/kg.
STUDY 1: DRIVING ON A RESTRICTED HIGHWAY
driving study was conducted on a highway closed to other traffic. 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 g/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 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 km 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 medicinal drugs and alcohol. Other dependent variables
were mean speed, and standard deviations of speed and steering wheel angle. Blood
samples were taken 10 minutes before the driving tests (i.e. 30 and 90 minutes
after initiation of smoking, respectively).
were willing and able to finish the driving tests without great difficulty. Data
from one male subject were excluded from the results because no drug was found
in his plasma after smoking.
Figure 1 demonstrates that marijuana impairs driving performance as measured by
an increase in lateral position variability: all three THC doses significantly
affected sdlp relative to placebo (p<.012, .001 & .001, for the 100, 200 & 300
g/kg conditions, respectively. The Dose by Time effect was not significant indicating
that impairment after marijuana was the same in both trials. Marijuana's effects
on sdlp were compared to those of alcohol obtained in a very similar study by
Louwerens et al. (1987). It appeared that the effects of the various administered
THC doses (100-300 g/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. Plasma concentrations of the drug were clearly related to the
administered dose and time of blood sampling but unrelated to driving performance
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 test. 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
dose 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 g/kg. Cigarettes appeared
identical at each level of treatment conditions. If any subject would have 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 16 km segment of the highway and lasted about 15 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 test. Blood samples were taken both
before the first and after the last driving test (i.e. 35 and 190 minutes after
initiation of smoking, respectively).
The road-tracking test was the same as in the previous study except for its duration
and the presence of other traffic. The car-following test involved attempting
to match velocity with, and maintain a constant distance from a preceding vehicle
as it executed a series 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 the dependent variables from this.
All subjects were
able to complete the series without suffering any untoward reaction while driving.
Data from one female subject were excluded from the results because no drug was
found in her plasma after smoking.
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 (Figure
2). The 100 g/kg dose produced a slight elevation in mean sdlp, albeit not statistically
significant (p<.13). The 200 g/kg dose produced a significant (p<.023) elevation,
of dubious practical relevance. The 300 g/kg dose produced a highly significant
(p<.007) elevation which may be viewed as practically relevant. After marijuana
smoking, subjects drove with an average speed that was only slightly lower than
after placebo and very close to the prescribed level.
In the car-following test, subjects maintained a distance of 45-50 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 g/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 (i.e. the longer the distance to the preceding vehicle, the more difficult
it is to perceive changes in its speed). 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 (Figure 3). Distance variability followed a
similar pattern as mean distance and reaction time; the greatest impairment was
found following the lowest dose. As in the previous study, plasma concentrations
of the drug were not related to driving impairment.
3: DRIVING IN URBAN TRAFFIC
The program proceeded
into 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 g/kg. It was given to a new group of 16 regular marijuana (or hashish) users,
along with a placebo. For comparative purposes, another group of 16 regular users
of alcohol, but not marijuana, were treated with a modest dose of their preferred
recreational drug, ethanol, and again placebo, before undertaking the same city
driving test. Both groups were equally comprised of men and women.
was administered to deliver 100 g/kg THC. The driving test commenced 30 minutes
after 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. Blood samples were taken immediately prior to and following
all placebo and drug driving tests (i.e. 20 and 80 minutes after initiation of
smoking, or 35 and 95 minutes after initiation of drinking).
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 'molecular' approach adopted from Jones (1978), 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 second, a 'molar' approach,
required the driving instructor acting as the safety controller during the tests
to retrospectively rate the driver's performance using a shortened version of
the Royal Dutch Tourist Association's Driving Proficiency Test. In total, 108
items were dichotomously scored, as either pass or fail. Total test performance
was measured by the percentage items scored as 'pass'. Subscores were calculated
for vehicle checks, vehicle handling, traffic maneuvers, observation and understanding
of traffic, and turning'. This method has been applied previously to show the
impairing effects of alcohol and diazepam (De Gier, 1979; De Gier et al., 1981).
from two male subjects in the marijuana group were excluded from the results because
neither THC nor THC-COOH was found in their plasma after smoking.
alcohol nor marijuana significantly affected driving performance measures obtained
by the molecular approach, indicating that it may be relatively insensitive to
drug-induced changes. The molar approach was more sensitive. Table 1 shows that
a modest dose of alcohol (bac=0.034 g%) produced a significant impairment in city
driving, relative to placebo. More specifically, alcohol impaired both vehicle
handling and traffic maneuvers. Marijuana, administered in a dose of 100 g/kg
THC, on the other hand, did not significantly change mean driving performance
as measured by this approach.
ratings of driving quality and effort to accomplish the task were strikingly different
from the driving instructor's ratings. Both groups rated their driving performance
following placebo as somewhat better than 'normal'. Following the active drug,
ratings were significantly lower (35%, p<.009) in the marijuana, but not (5%,
ns) in the alcohol group. Perceived effort to accomplish the driving test was
about the same in both groups following placebo. Following the active drug, a
significant (p<.033) increase in perceived effort was reported by the marijuana,
but not the alcohol group.
Thus, there is evidence that subjects 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.
Drug plasma concentrations were neither related to absolute driving performance
scores nor to the changes that occurred from placebo to drug conditions. With
respect to THC, these results confirm the findings in previous studies. They are
somewhat surprising for alcohol but may be due to the restricted range of ethanol
concentrations in the plasma of different subjects.
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 g/kg has significant, yet not
dramatic, dose-related impairing effects on driving performance (cf. Smiley, 1986).
Standard deviation of lateral position in the road-tracking test was the most
sensitive measure for revealing THC's adverse effects. This is because road-tracking
is primarily controlled by an 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 road-tracking error, there is little
the afflicted individual can do by way of compensation to restore the situation.
Car-following and, to a greater extent, city driving performance depend more on
controlled information processing and are therefore more accessible for compensatory
mechanisms that reduce the decrements or abolish them entirely.
effects on road-tracking after doses up to 300 g/kg never exceeded alcohol's at
bacs of 0.08 g%; and, were in no way unusual compared to many medicinal drugs'
(Robbe, 1994; Robbe and O'Hanlon, 1995; O'Hanlon et al., 1995). 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
correlations between plasma concentrations of the drug and driving performance
after every dose were essentially nil, partly due to the peculiar kinetics of
THC. It enters the brain relatively rapidly, although with a perceptible delay
relative to plasma concentrations. Once there, it remains even at a time when
plasma concentrations approach or reach zero. As a result, performance may still
be impaired at the time that plasma concentrations of the drug are near the detection
limit. This is exactly what happened in the first driving study. Therefore an
important practical implications of the study is that is not possible to conclude
anything about a driver's impairment on the basis of his/her plasma concentrations
of THC and THC-COOH determined in a single sample.
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 as the
point of departure for subsequent studies that will ultimately complete the picture
of THC's effects on driving performance.
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