NOTE: This is an archive site - we now have a new site.
Cannabis and Driving - the facts
Posted by CLCIA on December 08, 1997
From "MARIJUANA AND ACTUAL DRIVING PERFORMANCE" 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, or 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. T hey 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.
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 hose associated with bacs in the range of 0.03-0.07 g%. Other driving performance measures were not significantly affected by THC. Plasma oncentrations 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.
Table 1
Mean (sed) changes in driving performance scores measured by the molar
approach for the marijuana (N=14) and alcohol (N=16) group; and, the
significance of each change and difference between changes.
Dependent Variable Marijuana Group Alcohol Group Marijuana v Alcohol Delta p
Total score -0.7 (2.7) ns -6.8 (1.8) .002 .065
Vehicle checks -0.6 (1.5) ns +0.5 (1.3) ns ns
Vehicle handling +3.7 (2.8) ns -8.4 (2.2) .002 .002
Traffic maneuvers -2.7 (3.1) ns -8.4 (2.3) .003 ns
Observation and understanding of traffic +1.8 (8.7) ns -6.3 (7.0) ns
Turning -1.8 (4.9) ns +3.1 (7.5) ns ns
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 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.
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 ord.
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.
So, the evidence which comes from actual tests on drivers suggests that
there is very little problem indeed. Any tests on drivers which are looking
simply for the presence of cannabinoids have nothing to do with driving
skills; they are just another weapon in the War against Hemp.
CLCIA Cannabis Campaigners' Guide
|
