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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 2 How
do marijuana's effects on driving performance compare to those of drugs
other than alcohol? No direct comparisons have ever been made, but many
studies employing the standard road tracking test were conducted for
measuring other drugs' effects on SDLP during the last decade. The
results from a few will be mentioned. Diazepam
(Valium)given for one week in a low therapeutic dose (5 mg,
thrice daily) caused anxious patients to drive with a mean SDLP about 7
cm higher than their premedication baseline (Van Laar et al.,
1992). The same drug and dose given over the same period caused healthy
volunteers to drive with a mean SDLP about 6
cm higher than placebo (Van Veggel and O'Hanlon, 1993). Lorazepam (ativan), another anxiolytic, given twice daily for one
week in a 1.5 mg dose to healthy volunteers (Volkerts et al., 1988) and
a 2 mg dose to patients (Vermeeren et al. 1993), produced an elevation
of SDLP of about 10 cm
in both cases. Amitriptyline
(Elavil), a widely prescribed antidepressant, given in a dose of
50 mg at night and 25 mg in the morning caused healthy volunteers to
drive with a mean SDLP about 6
cm higher than placebo (Robbe et al., 1989. Fluraxepam
(Dalmane)), a
hypnotic, was administered to insomniacs and its "hang-over"
effects on SDLP were measured 10-11 hours after ingestion. A 15 mg dose of flurazepam elevated mean SDLP by about
4 cm; a 30 mg
does, 7 cm. Antihistamines
Antihistamines also cause sedation and, consequently, impair road
tracking performance. Triprolidine
(actifed) increased SDLP by 3.5 cm after a single 5 mg dose
(Riedel et al., 1990); and, diphenhydramine
50 mg (Benadryl kapseals) increased SDLP by 4.5 cm (Ramaekers et
al., 1992b). This is not to say that all psychotropic drugs produce
greater elevations of SDLP than THC. Many in the same and other
experiments had less effect than THC did in our studies. These
examples are merely cited to indicate that THC's effects as measured in
the standard test were in no way unusual. In so far as its effects on
SDLP are concerned, THC was just another moderately impairing drug. The
foregoing comparisons might be misleading. THC's effects differ
qualitatively from many other drugs, especially alcohol. For example,
subjects drive faster after drinking alcohol and slower after smoking
marijuana (Hansteen et al., 1976/ Casswell, 1979; Peck et al., 1986;
Smiley et al., 1987).. Moreover, the simulator study by Ellingstad et
al. (1973) showed that subjects under the influence of marijuana were
less likely to engage in overtaking maneuvers, whereas those
under the influence of alcohol showed the opposite tendency. Very
importantly, our city driving study showed that drivers who drank
alcohol over-estimated their performance quality whereas those who
smoked marijuana under-estimated it. Perhaps as a consequence, the
former invested no special effort for accomplishing the task whereas the
latter did, and successfully. This evidence 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 users seem better able to compensate for
its adverse effects while driving under the influence. Weil et
al. (1968) were among the earliest authors who mentioned the possibility
that marijuana users can actively suppress the drug's adverse effects.
They presumed that THC's effects were confined to higher cortical
functions without any general stimulatory or depressive effect on lower
brain centers. According to them, the relative absence of neurological,
as opposed to psychiatric, symptoms in marijuana intoxication suggests
this possibility. More recently, Moskowitz (1985) concluded that the
variety of impairments found after marijuana smoking could not be
explained by decrements in sensory or motor functions which led him to
hypothesize that some important central cognitive process is impaired by
THC, without saying what it is. Identification of THC's site of action
would greatly enhance our understanding of the drug's
psychopharmacological effects. Epidemiological research has shown that THC
is infrequently detected in the blood of fatally injured drivers as the
only drug present.
In most cases alcohol is also detected. The effects of the combination
of THC and alcohol on actual driving performance have never been studied
in the presence of other traffic. Closed-course studies have shown that
the effects of both drugs, when taken in combination, are generally
additive (Atwood et al., 1981; Peck et al., 1986). This may only be so
for those behaviors that are similarly affected by both rugs given
separately. Closer examination of the combined use is warranted in those
driving situations where both drugs produce qualitatively different
effects. It may well be
so that alcohol reduces drivers' insight or motivation to the point
where they would no longer attempt to compensate for the THC effect. As
a result, the combined effects on drivers' performance could well be
greater than the sum of either drug acting separately. There
is therefore a great need for further research on marijuana and actual
driving research, but now extended to the combination of marijuana and
alcohol. In
summary, 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. Still we 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. We therefore agree with Moskowitz' conclusion that
"any situation in which safety both for self and others depends
upon alertness and capability of control of man-machine interaction
precludes the use of marijuana". However, the magnitude of
marijuana's, relative to many other drugs', effects also justify
Gieringer's (1988) conclusion that "marijuana impairment presents a
real, but secondary, safety risk; and that alcohol is the leading
drug-related accident risk factor". Of
the many psychotropic drugs, licit and illicit, that are available and
used by people who subsequently drive, marijuana
may well be among the least harmful. Campaigns to discourage the use of marijuana by drivers are
certainly warranted. But concentrating a campaign on marijuana alone may
not be in proportion to the safety problem it causes. DRUG
PLASMA CONCENTRATIONS AND DRIVING PERFORMANCE One
of the program's objectives was to determine whether it is possible to
predict driving impairment by plasma concentrations of THC and/or its
metabolite, THC-COOH, in single samples. The answer is very clear: it is
not. Plasma of drivers showing substantial impairment in these studies
contained both high and low THC concentrations; and, drivers with high
plasma concentrations showed substantial, but also no impairment, or
even some improvement. The first driving study showed that impairment in
the road tracking test was nearly the same in the first and second test,
executed between 40-60 and 100-120 minutes after initiation of smoking,
respectively. Plasma concentrations of THC and THC-COOH, however, were
not the same during the tests: both were lower during the second than
the first. The same pattern was found for ratings of perceived
"high". It has been said that behavioral signs of
intoxication, though small, outlast physiological and subjective
reactions to THC (Reeve et al. 1983; Yesavage et al., 1985). to examine
this hypothesis, future research should extend actual driving
performance measurements to 4, 8, 16 and 24 hours after smoking. If
driving impairment still occurs after THC disappears from plasma, it
could mean that previous epidemiological research has underestimated the
proportion of drivers who were driving under the influence of marijuana
at the times their accidents occurred. Mean
speed was the only measure of driving performance that was even
moderately related to plasma concentrations of the drug. Subjects with
higher THC concentrations in plasma drove slower in the standard road
tracking test (correlations varying from r = -.18 to r = -.72 between
conditions). This effect might have been even more pronounced if the
subjects had not been instructed to drive at a particular speed, and if
they had had no feedback from the speedometer. CONCLUSIONS The major conclusions from the present
program are summarized as follows: * Current users of marijuana prefer THC
doses of about 300 ug/kg to achieve their desired "high". * It is possible to safely study the effects
of marijuana on driving on highways or city streets in the presence of
other traffic. * Marijuana smoking impairs fundamental road
tracking ability with the degree if impairment increasing as a function
of the consumed THC dose. * Marijuana smoking which delivers THC
up to a 300 ug/kg dose slightly impairs the ability to maintain a
constant headway while following another car. * A low THC dose (100 ug/kg) does not
impair driving ability in urban traffic to the same extent as a blood
alcohol concentration (BAC) of 0.04g%. * Drivers under the influence of marijuana
tend to over-estimate the adverse effects of the drug on their driving
quality and compensate when they can; e.g. by increasing effort to
accomplish the task, increasing headway or slowing down, or a
combination of these. * Drivers under the influence of alcohol
tend to under-estimate the adverse effects of the drug on their driving
quality and do not invest compensatory effort. * The maximum road tracking impairment after
the highest THC dose (300 ug/kg) was within a range of effects produced
by many commonly used medicinal drugs and less than that associated with
a blood alcohol concentration (BAC) of 0.08g% in previous studies
employing the same test. |
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