U.S. Department of Transportation,
National Highway Traffic Safety Administration
(DOT HS 808 078), Final Report, November 1993:
"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."
ABSTRACT
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.
INTRODUCTION
This 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.
GENERAL PROCEDURES
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 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 situations arise.
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).
LABORATORY STUDY
Methods
Twenty-four 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.
Results
Six subjects 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
Methods
The first 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).
Results
All subjects 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 [absent here] 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<.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
impairment. STUDY 2: DRIVING ON A NORMAL
HIGHWAY IN TRAFFIC Methods 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. Results 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. 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 (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.
STUDY 3: DRIVING IN URBAN TRAFFIC Methods
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.
Marijuana 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). 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 '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).
Results Data 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. 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. 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. Subjects' 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. DISCUSSION 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. THC's 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 the influence. Inter-subject
correlation's 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. 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 as the point of departure for subsequent
studies that will ultimately complete the
picture of THC's effects on driving performance. REFERENCES De
Gier JJ (1979) A subjective measurement of
the influence of ethyl/alcohol in moderate doses on real driving
performances. Blutalkohol, 16, 363-370. De
Gier JJ, 't Hart BJ, Nelemans FA and Bergman H (1981)
Psychomotor performance and real driving performance
of outpatients receiving diazepam.
Psychopharmacology, 73, 340-347. Jones MH
(1978) Driver Performance Measures for the Safe
Performance Curriculum. Traffic Safety Center,
Institute of Safety and Systems Management, University of
South California, Los Angeles, CA (DOT HS
803 461). Louwerens JW, Gloerich ABM, de Vries G,
Brookhuis KA and O'Hanlon JF (1987). The relationship
between drivers' blood alcohol concentration (bac)
and actual driving performance during high
speed travel. Pages 183-192 in PC Noordzij and R Roszbach,
eds., Alcohol, Drugs and Traffic Safety. Proceedings
of the 10th International Conference on Alcohol, Drugs
and Traffic Safety. Excerpta Medica, Amsterdam.
O'Hanlon JF, Vermeeren A, Uiterwijk MMC, van Veggel
LMA and Swijgman HF (1995) Anxiolytics' effects
on the actual driving performance of patients and healthy
volunteers in a standardized test: an integration
of three studies. Neuropsychobiology, 31:81-88. Robbe HWJ
(1994). Influence of Marijuana on Driving.
PhD thesis,Institute for Human Psychopharmacology, University
of Limburg, Maastricht. Robbe HWJ and O'Hanlon
JF (1995) Acute and subchronic effects of paroxetine and
amitriptyline on actual driving, psychomotor
performance and subjective assessments in healthy volunteers.
European Neuropsychopharmacology, 5:35-42
Smiley AM (1986). Marijuana: On-road and driving simulator
studies. Alcohol, Drugs and Driving: Abstracts
and Reviews 2: 121-134. Exposing Marijuana Myths Claim
#12 : Marijuana is a Major Cause of Highway
Accidents: By Lynn Zimmer, Associate Professor of
Sociology, Queens College, & John P Morgan,
Professor of Pharmacology, City University of New York
Medical School: The Lindesmith Center, 1995
The detrimental impact of alcohol on highway safety has been
well documented. Marijuana's opponents claim
that it, too, causes significant impairment and that any
increase in use will lead to increased highway
accidents and fatalities. THE FACTS In high doses marijuana
probably produces driving impairment in most
people. However, there is no evidence that marijuana, in
current consumption patters, contributes substantially
to the rate of vehicular accidents in America. A
number of studies have looked for evidence
of drugs in blood or urine of drivers involved in fatal crashes. All
have found alcohol present in 50 percent or
more. Marijuana has been found much less often. Furthermore,
in the majority of cases where marijuana has
been detected, alcohol has been detected as well. (*1) For
example, a recent study sponsored by the US
National Highway Traffic Safety Administration (NHTSA)
involving analysis of nearly 2000 fatal accident
cases, found 6.7 percent of drivers positive for marijuana. In
more than two-thirds of those, alcohol was
present and may have been the primary contributor to the fatal
outcome (*2) To accurately access marijuana's
contribution to fatal crashed, the positive rate among
deceased drivers would have to be compared
to the positive rate from a random sample of drivers not
involved in fatal accidents. Since the rate
of the past-month marijuana use for Americans above the legal
driving age is about 12 percent, on any given
day a substantial proportion of all drivers would test positive,
particularly since marijuana's metabolites
remain in blood and urine long after its psychoactive effects are
finished. A recent study found that one-third
of those stopped for "bad driving" between the hours of 7 p.m
and 2 a.m - mostly young males - tested positive
for marijuana only. (*3). To be meaningful, these test results
would have to be compared to those from a
matched control group of drivers. A number of driving simulator
studies have shown that marijuana does not
produce the kind of psychomotor impairment evident with
moderate doses of alcohol (*4). In fact, in
a recent NHTSA study, the only statistically significant outcome
associated with marijuana was speed reduction.
(*5) A recent study of actual driving ability under the
influence of cannabis - employing the same
protocol used to test impairment-potential of medicinal drugs -
evaluated the impact of placebo and three
active THC doses in three driving trials, including one in
high-density urban traffic. Dose-related impairment
was observed in drivers' ability to maintain steady lateral
position. However, even with the highest dose
of THC, impairment was relatively minor - similar to that
observed with blood-alcohol concentrations
between 0.03 and 0.07 percent and many legal medications.
Drivers under the influence of marijuana also
tended to drive more slowly and approach other cars more
cautiously While recognising some limitations
of this study, the authors conclude that "THC is not a
profoundly impairing drug." (*6) Refs: *1
: McBay AJ and Owens SM., "Marijuana and Driving", pp 257-63
in L.S.Harris (ed) Problems of Drug Dependence
1980, Washington, DC: U.S. Government Printing Office
(1981): Teale, JD et al., "The Incidmnece
of Cannabinoids in Fatally Impaired Drivers: An Investigation by
Radioimmunoassay and High Pressure Liquid
Chromatography," Journal of the Forensic Science Society 17:
177-83 (1978) *2 Terhune, KW et al., The Incidence
and Role of Drugs in Fatally Injured Drivers,
Washington DC: Department of Transportation
(1994) *3 Brookoff, D et al., "Testing Reckless Drivers for
Cocaine and Marijuana", New England Journal
of Medicine 331: 518-22 (1994) *4 Kv'alseth, TO, "Effects
of Marijuana on Human Reaction Time and Motor
Control", Perceptual and Motor Skills 45: 935-39 (1977):
Hansteen, RW, et al, "Effects of Cannabis
and Alcohol on Automobile Driving and Psychomotor Tracking,"
Annals of New York Academy of Science 282:
240-56 (1976): Moskowitz, H et al., " Marijuana: Effects on
Simulated Driving Performance," Accident Analysis
and Prevention 8: 45-50 (1976); Moskowitz H et al.,
"Visual Search Behaviour While Viewing Driving
Scenes Under the Influence of Alcohol and Marijuana",
Human Factors 18: 417-31 (1976) *5 Stein,
AC et al., A Simulator Study of the Combined Effects of Alcohol
and Marijuana on Driving Behavior-Phase II,
Washington DC: Department of Transportation (1983)
Cannabis makes you a better driver - more
evidence Source: 1 The AGE 21 October 1998 pA5; 2.
CANBERRA TIMES 21 October 1998 p4 The largest
study ever done linking road accidents with drugs and
alcohol has found drivers with cannabis in
their blood were no more at risk than those who were drug-free. In
fact, the findings by a pharmacology team
from the University of Adelaide and Transport SA showed drivers
who had smoked marijuana were marginally less
likely to have an accident than those who were drug-free. A
study spokesman, Dr Jason White, said the
difference was not great enough to be statistically significant but
could be explained by anecdotal evidence that
marijuana smokers were more cautious and drove more slowly
because of altered time perception. The study
of 2,500 accidents, which matched the blood alcohol levels of
injured drivers with details from police reports,
found drug-free drivers caused the accidents in 53.5 per cent
of cases. Injured drivers with a blood-alcohol
concentration of more than 0.05 per cent were culpable in
nearly 90 per cent of accidents they were
involved in. Drivers with cannabis in their blood were less likely to
cause an accident, with a culpability rate
of 50.6 per cent. The study has policy implications for those who
argue drug detection should be a new focus
for road safety. Dr White said the study showed the importance
of concentrating efforts on alcohol rather
than other drugs. This information was posted by the library of The
Alcohol and other Drugs Council of Australia
(ADCA). Requests for copies of newsclips can be directed to
the library by phone 02 62811002, fax 02 6282
7364 or e-mail library@adca.org.au. To subscribe to this
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