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U.S.
DEPARTMENT OF TRANSPORTATION NATIONAL
HIGHWAY TRAFFIC SAFETY ADMINISTRATION MARIJUANA AND ACTUAL DRIVING PERFORMANCE EFFECTS OF THC ON DRIVING PERFORMANCE |
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Part 1 One of the issues addressed by the first
driving study was whether it would be safe to continue using the same
approach for subsequent on-road studies in traffic. The first group
complied with all instructions, even after high doses of THC. Changes in
mood were often reported but changes in personality were never observed.
Most importantly, the subjects were always able to complete every ride
without major interventions by the driving instructors and their safety
was never compromised. The same occurred in the subsequent studies
showing that it is possible to safely study marijuana's effects on
actual driving performance in the presence of other traffic. In this
respect, the drug is no different from many others studied by the same
investigators and their colleagues. The
standard test measured the subjects' ability to maintain a constant
speed and a steady lateral position between the lane boundaries.
Standard deviation of lateral position, SDLP, increased after marijuana
smoking in a dose-related manner. The lowest dose, i.e. 100 ug/kg THC,
produced a slight elevation in mean SDLP, albeit significant in the
first driving study. The intermediate dose, i.e. 200 ug/kg THC,
increased SDLP moderately; and, the highest, i.e. 300 ug/kg THC,
substantially. It is remarkable how well the changes in SDLP following
THC in the first driving study were replicated in the second, in spite
of the many differences in the ways they were designed. The replication
of THC's effects on SDLP substantiates the generality of these results.
Other objective measures obtained by this test were much less affected
by THC. Mean speed was somewhat reduced following the higher THC doses,
but the effects were relatively small (max. 1.1 km/hr or 0.7 mph). Standard deviations of speed and steering wheel movements
were unaffected by the drug. Subjective ratings of perceived
driving quality followed a similar pattern as SDLP indicating that the
subjects were well aware of their diminished ability to control the
vehicle after marijuana smoking. The
car following test measured the subjects' ability to follow a leading
car with varying speed at a constant distance. All THC doses increased
mean headway, but according to an inverse dose-response relationship.
This type of relationship was unexpected and probably due to the
particular design of the second driving study, i.e. the ascending dose
series. It means that subjects were very cautious the first time they
undertook the test under the influence of THC (i.e. after the lowest
dose) and progressively less thereafter. As a consequence of this
phenomenon, mean reaction time to changes in the preceding car's speed
also followed an inverse dose-response relationship. Statistical
adjustment for this confounding by analysis of covariance indicated that
reaction times would not have increased significantly if the mean
headway were constant. Coefficient of headway variation increased
slightly following THC. Together, these data indicate that there is no
more than a slight tendency towards impairment in car following
performance after marijuana smoking. They also show that subjects try to
compensate for anticipated adverse effects of the drug buy increasing
headway, especially when they are uncertain of what these might be. As
in the standard test, subjects' ratings of driving quality corresponded
to the objective changes in their performance. The
city driving study measured the subjects' ability to operate a vehicle
in urban traffic. for reasons mentioned in the respective chapter the
THC dose in that study was restricted to 100 ug/kg. For comparative
purposes another group of subjects was treated with a modest dose of
alcohol, producing a mean BAC of about 0.04g%. Results
of the study showed that the modest dose of alcohol, but not THC,
produced a significant impairment in driving performance, relative to
placebo. Alcohol impaired driving performance but subjects did not
perceive it. THC did not impair driving performance yet the subjects
thought it had. After alcohol, there was a tendency towards faster
driving and after THC, slower. The
results of these studies corroborate those of previous driving simulator
and closed-course tests by indicating that THC in single inhaled doses
up to 300 ug/kg has significant, yet not dramatic, dose-related
impairing effects on driving performance. They contrast with results
from many laboratory tests, reviewed by Moskowitz (1985), which show
that even low doses of THC impair skills deemed important for driving,
such as perception, coordination, tracking and vigilance. The
present studies also demonstrated that marijuana can have greater
effects in laboratory than driving tests. The last study, for
example, showed a highly significant effect of THC on hand unsteadiness
but not on driving in urban traffic. It
is a natural question why the effects of marijuana on actual driving
performance appear to be so small. As in many previous investigations,
subjects attempted to compensate for anticipated adverse effects of
marijuana smoking. Our subjects were aware of the impairing effects of
THC as shown by lower ratings of perceived driving quality.
Consequently, they invested more effort to accomplish the driving tests
following THC than placebo. Furthermore, in the car following test, they
drove at a greater headway after marijuana smoking; and, in both road
tracking and city driving tests, they slightly reduced their driving
speed. yet despite their effort, subjects were unable to fully
compensate for THC's adverse effects on lateral position variability.
This is because SDLP is primarily controlled by an automatic information
processing system which operates outside of conscious control. The
process is relatively impervious to environmental changes, as shown by
the high reliability of SDLP under repeated placebo conditions, but
highly vulnerable to internal factors that retard the flow of
information through the system. THC and many other drugs are among these
factors. When they interfere with the process that restricts SDLP, there
is little the afflicted individual can do by way of compensation 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. That
still leaves the question open why performance appears to be more
affected by THC in laboratory than actual driving tests. Many
researchers defend the primacy of laboratory performance tests for
measuring drug effects on skills related to driving on the basis of
superior experimental control. Certainly some control is always
necessary to reduce the confounding influence of extraneous factors that
would otherwise so increase measurement error as to totally obscure the
drug's effects. However, only some extraneous factors are truly sources
of measurement error and others either attenuate or amplify drug effects
in real driving and must be considered as relevant to a test's
predictive validity. Simply eliminating all of them, first, removes
their normal mediating influence on the drug effect, and secondly,
affects the subject's motivation to perform the test by making it appear
"unreal". Controlling the test usually involves drastic
simplification and restriction of response options. The desire in doing
this is to isolate a particular driving skill and determine how it
changes under the influence of drugs. However, 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. Thus
the demonstration of some particular skill decrement in the laboratory
in no way indicates that this would ultimately reduce driving safety in
reality.Finally there are some skills that simply can not be
measured in laboratory tests, at least not easily enough to make it a
routine matter. The acquisition of any skill which depends upon
automatic information processing requires practice over weeks or months.
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. In short, it would appear as if over-control in
laboratory performance tests has resulted in a misimpression of THC's
effect, incomplete in some respects and exaggerated in others. The
actual driving tests may provide a more realistic impression of the
drug's effects, albeit still incomplete and perhaps tending to minimize
them with respect to more complex driving situations that come closer to
"worst case". The degree of
experimental control also varied between driving tests in this series in
ways affecting the subjects' motivation. This is illustrated by a
comparison between the first and second driving study. The standard road
tracking test was applied in both, first in the absence and then in the
presence of other traffic. It was only during the former that disturbing
observations of two individual's attentional deficits caused the driving
instructor to intervene. Driving in the presence of other traffic,
subjects were always able to complete the rides without intervention.
Lateral position control, an automatic process, did not change as a
consequence of the absence or presence of other traffic. What did change
was the subjects' motivation to focus attention, a controlled process.
Motivation in the second study was very probably affected by recognition
of the increased risk of the untoward consequences of wandering
attention. This means that the intrinsic motivation produced by the reality of the
test situation is an important mediator of THC's effects on performance. Compensatory
mechanisms help the driver under the influence of marijuana to maintain
an effective level of performance but with an associated cost. If
drivers compensate for THC's adverse effects by diminishing driving
demands (e.g. by reducing speed and/or increasing headway), this will
occur without a reduction in spare capacity. But if they increase effort
as well (e.g. by focusing attention), it will occur at the expense of
spare capacity. Less capacity would be left for simultaneously
performing another task, such as conversing with passengers, using a car
telephone, or handling emergency situations. The information processing
capacity these situations demand may well go beyond the driver's spare
capacity with the result of impaired and perhaps dangerous driving.
Results of the present program show that THC increases the mental load
of driving, as shown by increased effort ratings and reduced heart rate
variability, and consequently reduces spare capacity. This corroborates
results from previous simulator and closed-course studies that with
reasonable consistency show an adverse THC effect on subsidiary task
performance (Smiley, 1986). Further research is required to determine
marijuana's effects on actual driving performance when the driver is
simultaneously performing another task or suddenly confronted with a
situation that requires a rapid adaptive response. The latter was
occasionally encountered during the city driving test, but only after a
low THC dose. The city driving test should therefore be repeated with
subjects consuming higher THC doses. Hazardous
driving can also occur in situations that demand very little of the
driver's information processing capacity. If the driving task is very
monotonous and the demand is low, wandering attention may result in
negligent monitoring with disastrous results. This is in fact what
happened twice during the driving study on the closed road. After the
highest THC dose, one subject failed to shift attention from the
prescribed task to an unexpected event (screwdriver on the road);
another failed to anticipate a normal event (end of circuit). Though
even sober experienced drivers may experience similar deficits, the fact
that it happened twice after the highest THC dose, and never after a
lower dose or placebo, strongly suggests that drivers under the
influence of THC would be unusually susceptible to attentional deficits
during prolonged and monotonous driving. How do marijuana's effects on driving performance
compare to those of alcohol? There are two sources from which one can
draw to answer the question. Information can be directly obtained from
studies comparing THC and alcohol effects in the same experiment; and,
indirectly, from studies wherein alcohol's effects were assessed using
the same methods as applied in the present THC studies. As mentioned in
Chapter 1, most closed-course studies on THC also measured alcohol's
effects (BACs between 0.04 and 0.10g%). It
was generally concluded that THC's effects were less than alcohol's
especially at BACs above 0.08g%. The city driving study in the
present program also compared the effects of modest doses of alcohol and
THC. For doses administered in that study, alcohol produced the greater
effects. Indirect evidence concerning the relative effects of THC and
alcohol can be obtained from three studies. First, the alcohol
calibration study by Louwerens et al. (1985, 1987) which resembled our
first driving study in many respects. According to their empirical equation, THC's effects on SDLP were equal to
or less than that of BAC = 0.07g%. More recently, studies by
Riedel et al. (1989) and Ramaekers et al. (1992a) measured the effects
of low doses of alcohol (BACs of 0.05 and 0.03g% respectively) on SDLP.
Both groups applied the standard test in the presence of other traffic,
as in our second driving study, but on another highway. Mean
SDLPs were respectively about 5.0 and 2.5 cm higher while driving after
alcohol than placebo. The former elevation is greater than that
produced by the highest
THC dose in our study. The latter lies between the effects of 200 and
300 ug/kg doses, which were 1.8 and 2.9 cm respectively. There was some discrepancy between alcohol's effects on SDLP in the more recent studies and those predicted by the empirical equation: the former where higher than predicted. The discrepancy appears to be related to the difference between alcohol's effects on the ascending and descending phases of its pharmacokinetic profile. Louwerens measured alcohol's effects at the time when BAC was at the ascending but Riedel and Ramaekers measured them during the descending phase. Notwithstanding methodological differences among studies, both direct and indirect evidence coverage on the conclusion that THC's effects after doses up to 300 ug/kg never exceed alcohol's at BACs of 0.08g%. |
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