MARIJUANA AND ACTUAL DRIVING PERFORMANCE

NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

BY HINDRIK W.J. ROBBE AND JAMES F. O'HANLON

Institute for Human Psychopharmacology
University of Limburg
Abstract 2A
6211 LS Maastricht -- The Netherlands

Sponsoring Agency:
U.S. Department of Transportation
National Highway Traffic Safety Administration
400 Seventh Street, S.W.
Washington, DC 20590

Published November, 1993

(Government disclaimer)

This publication is distributed by the U.S. Department of Transportation,
National Highway Traffic Safety Administration, in the interest of
information exchange. The opinions, findings and conclusions expressed in
this publication are those of the author(s) and not necessarily those of
the Department of Transportation or the National Highway Traffic Safety
Administration. [No Kidding] The United States Government assumes no
liability for its contents or use thereof. If trade or manufacturers' name
or products are mentioned, it is because they are considered essential to
the object of the publication and should not be construed as an
endorsement. The United States Government does not endorse products or
manufacturers.

Abstract:
This report concerns the effects of marijuana smoking on actual
driving performance. It presents the results of one pilot and three actual
driving studies. The pilot study's major purpose was to establish the THC
dose current marijuana users smoke to achieve their desired "high". From
these results it was decided that the maximum THC dose for subsequent
driving studies would be 300 mcg / kg (0.3 mg / kg). The first driving
study was conducted on a closed section of a primary highway. After
smoking marijuana delivering THC doses of 0, 100, 200, and 300 mcg / kg,
subjects drove a car while maintaining a constant speed and lateral
position. This study was replicated with a new group of subjects, but now
in the presence of other traffic. In addition, a car following test was
executed. The third driving study compared the effects of a modest dose of
THC (100 mcg / kg) and alcohol )BAC of 0.04 g %) on city driving
performance. This program of research has shown that marijuana, when taken
alone, produces 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 in their performance and will compensate where they can, for
example, by slowing down or increasing effort. As a consequence, THC's
adverse effects on driving performance appear relatively small.

EXECUTIVE SUMMARY

This report concerns the effects of marijuana smoking on actual driving
performance. It presents the results of one pilot and three actual driving
studies which were conducted between April 1990 and March 1992. The
program was funded by the U.S. National Highway Traffic Safety
Administration (NHTSA), with the exception of the alcohol part of the city
driving study which was sponsored by the Dutch Ministry of Transport and
Public Works. The project was conducted by the Institute for Drugs, Safety
and Behavior of the University of Limburg, Maastricht, The Netherlands.
The major objectives of the program were to determine the dose-response
relationship between delta-9-tetrahydrocannabinol (THC), marijuana's main
constituent, 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. A variety of driving tests 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. The purpose of applying different tests was to determine
whether similar changes in performance under the influence of THC occur in
all, thereby indicating a general drug effect on driving ability.

Chapter One provides background information about the drug, its
pharmacological properties, the prevalence of its use, and a review of
marijuana smoking and traffic safety. THC's effects on the ability of
drivers to operate safely in traffic situations have traditionally been
determined in two ways: from epidemiological surveys of users' involvement
in traffic accidents and from empirical studies to measure the drug's
influence on skills related to driving, or driving itself. Epidemiology
shows that people drive after marijuana use and that drivers involved in
accidents often show the drug's presence. The results are, however,
inconclusive because of the high proportion of cases which also involve
alcohol use and the lack of proper control groups. There, the extent
marijuana contributes to traffic accident causality remains obscure.
Results from driving simulator and closed-course tests show that THC in
single inhaled doses up to about 250 mcg / kg has relatively minor effects
on driving performance, certainly less than blood alcohol concentrations
(BAC's) in the range of 0.08-0.10 g %.

Chapter Two describes the studies of the program and certain
procedures that were common to all. These were subject recruiting,
compliance with ethical and legal standards, screening for the presence of
other illicit drugs and alcohol, blood sampling procedures and quantitative
analyses. Subjects in all studies were recreational users of cannabis,
i.e. smoking marijuana or hashish 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. Subjects
were accompanied in every driving test by a licensed driving instructor,
experienced in supervising subjects who operated under the influence of
medicinal drugs in previous studies. Redundant control system in the test
vehicle was available for controlling the car if emergency situations
should arise. Marijuana and placebo marijuana cigarettes were supplied by
the U.S. National Institute on Drug Abuse (NIDA). [Isn't it curious that
a study done in the Netherlands used NIDA supplied marijuana? It is nearly
IMPOSSIBLE to get NIDA supplied marijuana in the U.S. for these kind of
studies or medical studies.]

Chapter Three presents the results of the pilot study. It 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 continuously for a perod
not exceeding 15 minutes. When subjects voluntarily stopped smoking,
cigarettes were carefully extinguished and retained for subsequent
gravimetric estimation 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 mcg / kg
[of body weight]. There was no significant difference between males and
females with respect to the weight adjusted preferred dose. It was decided
that the maximum dose for subsequent driving studies would be 300 mcg / kg.
This is considerably higher than doses that have usually been administered
to subjects in experimental studies (typically, 100-200 mcg / kg THC).

The study provided the opportunity for obtaining valuable
information about THC's pharmacokinetics and its pharmacodynamic effects
after marijuana smoking. Blood samples were repeatedly taken for measuring
plasma concentrations of THC and its major inactive metabolite, THC-COOH.
The subjects repeatedly performed certain simple laboratory tests,
estimated their levels of intoxication and indicated their willingnes to
drive under several specified conditions of urgency. Heart rate was
measured at these times. The secondary purpose of the pilot study was that
of specifying relationships between (THC) and (THC-COOH) with changes in
the other physiological, performance or subjective variables. Other
results from the pilot study showed that perceived "high" and heart rate
are very sensitive measures of marijuana intoxication which confirms prior
findings. Impairments in laboratory tests performance were found at the
time of peak subjective feelings but generally, objective impairment
dissipated more rapidly than lthe feelings themselves.

The first driving study, described in Chapter Four, was conducted
on a highway closed to other traffic. One objective of the study was to
determine whether it would be safe 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 pilot study
served again as the subjects. They were treated on separate occasions with
THC doses of 0, 100, 200, 300 mcg / 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 (56 mph) and a steady lateral position between the delineated
boundaries of the traffic lane. Subjects drove 22 km (13.6 mi) on a
primary highway and were accompanied by a licensed driving instructor. The
latter was charged with responsibility for ensuring safety at all times and
was able to intervene, if necessary, using redundant vehicular controls.
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. Blood samples were taken prior to each driving test; and,
performance in critical tracking and hand steadiness tests, heart rate, and
blood pressure were measured after its termination. Questionnaires were
repeatedly administered to estimate the "high" and other subjective
feelings.

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 while drug plasma concentrations, perceived
"high" and heart rate elevation had decreased. 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 THC's effects on SDLP
were equivalent to those associated with BAC's in the range of 0.03-0.07 g
%. Other driving performance measures were not significantly affected by
THC. Intersubject correlations between plasma concentrations of the drug
and driving performance after every dose were essentially nil. Thus,
driving impairment cannot be predicted by prevailing plasma concentrations
of THC or THC-COOH. Driving impairment was also not related to performance
in the laboratory tests. 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 in the presence of
other traffic.

The second driving study, described in Chapter Five, was conducted
to come a step closer to driving reality than its predecessor. Driving
tests were now conducted on a highway in the presence of other traffic.
The major objective of this study was to confirm the relationship between
inhaled THC dose and lateral position variability in the context of a
standard road tracking test. A secondary objective was to measure
performance in another actual driving test, i.e. car following. The third
objective was to continue efforts to correlate plasma concentrations of THC
and THC-COOH with driving performance impairment as measured in both tests.

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 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 mcg / kg. Cigarettes appeared
identical at each level of treatment conditions and were smoked through a
plastic holder in a fashion determined by the subject within a time limit
of 10 minutes. 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.

Two subjects at a time commenced smoking. Thirty minutes after
onset of smoking the subjects performed a battery of laboratory tests
(tracking, hand steadiness and body sway), yielded a blood sample, and
rated their "high" and other subjective feelings. They were then
transported to a primary highway where the driving tests were performed.
Two instrumented vehicles were employed. The subjects performed the car
following test on a 16 km (9.9 mi) segment of the highway for about twelve
minutes. After conclusion of the car following test, both subjects then
commenced the road tracking test in separate instrumented vehicles. The
highway was the same as for the car following test. Subjects drove 64 km
(40 mi) without stopping in about 50 minutes. At the conclusion of this
test, both subjects participated again in the car following test. Subjects
were then transported back to the laboratory where they rated subjective
feelings, yielded a blood sample, and repeated the test battery. The
subjects' heart rate was registered continuously during both driving tests.

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 to maintain a constant speed of 95 km / h (59 mph) and a steady
lateral position between lane boundaries in the right traffic lane. They
were allowed to deviate from this only if it would become necessary to pass
a slower vehicle in the same lane. Data from the standart test were
analyzed to yield the same performance measures as in the previous study;
i.e. SDLP, mean and standard deviation of speed, and standard deviation of
steering wheel angle. The car following test measures drivers' ability to
perceive changes in a preceding vehicle's speed and to react in a manner
maintaining a constant headway. It began as the preceding and the
following vehicle, respectively driven by one of the driving instructors
and the subject, operated in tandem on the slower traffic lane while
travelling at a speed of 100 km / h (62 mph). The subject was instructed
to maintain a 50 m (164 ft) headway however the preceding vehicle's speed
might vary. After driving in this manner for about one minute, the
operator of the preceding vehicle released the accelerator pedal allowing
its speed to fall to 80 km / h (50 mph). Immediately thereafter, the
operator of the preceding vehicle accelerated to 100 km / (62 mph). The
duration of one deceleration and acceleration maneuver was approximately 50
seconds and six to eight, depending upon traffic density, were executed
during one test. The subject's average reaction time to the movements of
the leading vehicle, mean headway and coefficient of variation of headway
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. 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 mcg / kg dose
produced a slight elevation in mean SDLP, albeit nearly significant. The
200 mcg / kg dose produced a significant elevation, of dubious practical
relevance. The 300 mcg / kg dose produced a highly significant elevation
which may be viewed as practically relevant but unexceptional in comparison
with similarly measured effects of many medicinal drugs. Following
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 headway of 45-50 m
(148-164 ft) while driving in the successive placebo conditions. They
lengthened mean headway by 8, 6 and 2 m (26.2, 19.7 and 6.6 ft) in the
corresponding THC conditions after 100, 200 and 300 mcg / 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. Reaction time 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 headway. However, increased
reaction times were partly due to longer headway. Statistical adjustment
for this confounding 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. Headway
variability followed a similar pattern as mean headway and reaction time;
the greatest impairment was found following the lowest dose.

An important practical objective of this study 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, like many reported before, indicate that none of
these measures accurately predicts changes in actual driving performance
under the influence of THC.

The program then proceeded into the third driving study, presented
in Chapter Six, which involved tests conducted in high-density urban
traffic. There were logical and safety reasons for restricting the THC
dose to 100 mcg / kg. It was given to a group of regular cannabis users,
along with placebo. For comparative purposes another group of regular
alcohol users were treated with a modest dose of their preferred
recreational drug, and again placebo, before undertaking the same city
driving test. Two groups of sixteen new subjects apiece, equally comprised
of men and women, participated. Subjects in the alcohol group were regular
users of alcohol but not marijuana. Both groups were treated on separate
occasions with active drug and placebo. Active marijuana was administered
to deliver 100 mcg / 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.

Driving tests were conducted in daylight over a constant 17.5 km
(10.9 mi) 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, "molar" approach,
required the driving instructor acting as the safety controller during the
tests to retrospectively rate the driver's performance 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.
Immediately prior to and following the driving tests subjects performed
hand steadiness and time perception tests, yielded a blood sample, and were
administered the same subjective questionnaires used in the previous
studies.

The study showed that a modest dose of alcohol (BAC = 0.04 g %)
produced a significant impairment in city driving as measured by the molar
approach, relative to placebo. More specifically, alcohol impaired vehicle
handling and traffic maneuvers. Marijuana, administered in a dose of 100
mcg / kg THC, on the other hand, did not signifcantly 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 placebo Yet only subjects in
the ;marijuana group reported significantly higher levels of invested
effort following the active drug. Thus, there was 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 THC doses
by smoking marijuana.

The laboratory performance tests also discriminated between the
drugs' effects. Hand steadiness was impaired following THC and improved
following alcohol, relative to placebo. The difference between the drugs'
effects was significant, both before and after the driving test.
Impairment after THC was about as much as that produced by the same dose in
the previous study, indicating equivalent sensitivities of the present and
previous groups. Production of time intervals was not affected by alcohol,
but THC significantly shortened interval production, relative to placebo.

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.

Chapter Seven concludes the report with a general discussion of the
results of the program and ends with list of conclusions and
recommendations. It starts with a discussion of the THC dose which
marijuana users actually prefer for achieving their desired "high."
Several questions are raised and discussed, such as: how do people
regulate their THC consumption, what role plays familiarization with the
drug, and what would the preferred dose have been if marijuana of much
higher potency were smoked. The discussion then continues with a
description of the differences between the driving tests in terms of the
type of information processing each requires, automatic vs controlled, and
the relevance of each to traffic safety.

Attention is further focussed on the effects of THC on driving
performance. The results of the studies corroborate those of previous
driving simulator and closed-course tests by indicating that THC in single
inhaled doses up to 300 mcg /kg has significant, yet not dramatic,
dose-related impairing effects on driving performance. 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 if 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 to restore the situation. Car following and, to a greater
extent, city driving performance depend more on controlled information
processing and are therefore more accesible for compensatory mechanisms
that reduce the decrements or abolish them entirely.

It appears that performance is more affected by THC in laboratory
than actual driving tests. Several reasons that may account for the
apparent discrepancy are discussed. First, laboratory tests are
experimentally controlled by drastic simplification which may affect a
subjects motivation to perform the test by making it appear "unreal."
Secondly, the restriction of response options in laboratory performance
tests leave fewer possibilities for compensation. In real life, drivers
always apply numerous skills in parallel and series. Should one become
deficient, they are often able to compensate in a number of ways to achieve
a satisfactory level of proficiency. Finally, after learning to drive,
subjects possess such skills in abundance and one can only demonstrate how
they vary with drug effects in the real task or a very close approximation
thereof. Profound drug impairment constituting an obvious traffic safety
hazard could as easily be demonstrated in a laboratory performance test as
anywhere else. But THC is not a profoundly impairing drug. It does affect
automatic information processing, even after low doses, but not to any
great extent after high doses. It apparently affects controlled
information processing in a variety of laboratory tests, but not to the
extent which is beyond the individual's ability to control when he is
motivated and permitted to do so in real driving.

Marijuana's effects on driving performance were compared to those
of many other drugs. It was concluded that THC's effects after doses up to
300 mcg / kg never exceed alcohol's at BAC's of 0.08 g %; and were in no
way unusual compared to many medicinal drugs'. Yet THC's effects differ
qualitatively from many other drugs, especially alcohol. Evidence from the
present and previous studies stronly 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. Still one can easily imagine situations
where the influence of marijuana smoking might have an exceedingly
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

Finally, the relation between driving impairment following
marijuana smoking and plasma concentrations of THC and THC-COOH is
discussed. It appears 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.

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