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was tracking, with similar percentages showing impairment only indicates that marijuana has been used, and that the use
at BAC equal to 0.05%--significant because tracking and could be as long as a few weeks ago. The second reason is that
divided attention are inherent in almost all driving tasks. The alcohol is a singular drug with specific repeatedly demonstrated
least sensitive function was vigilance, with very few studies effects, whereas other "drugs" as a generic category include
showing impairment below BAC equal to 0.08%. Moskowitz different drugs that have different effects. These drugs are not
and Robinson concluded that although some individuals may evenly absorbed in all body tissues or even in the same brain
be more affected by small concentrations than others, "there centers; they do not necessarily have the same or similar
is no lower threshold level below which impairment does not physiological and behavioral effects and often do not exhibit
exist for alcohol." A driving simulator study by Roehers et al. a direct dose-response relationship.
(1994) demonstrated that sleepiness and low-dose ethanol
combine to impair simulated automobile driving, an impair- Finally, drugs other than alcohol are often taken in com-
ment that extends beyond the point at which breath ethanol bination (also in combination with alcohol) and depending
concentrations reach zero. on the specific drugs, the specific doses, and the user's past
experience with drugs, they have joint effects that may be addi-
Similarly, Holloway (1994) examined 155 empirical studies tive, synergistic, or antagonistic, and generally very difficult to
(19851993) to reach three conclusions. First, sensitivity predict (Shinar 2007a).
to the subjective intoxicating effects of alcohol was greater
than that for all other performance classes and appeared to
display a "threshold" with respect to BAC rather than the INFLUENCE OF CHEMICALS
linear relation evident in performance data. Second, sensitivity ON DRIVER PERFORMANCE
to performance impairment in "controlled" performance and
simulator tasks was greater than that for psychophysical The three chapters that follow provide a brief capsule view of
functions of "automatic performance." Finally, a variety of the voluminous material that could be cited to describe many
task-, subject-, and environmental-characteristics or conditions psychoactive chemical substances that occasionally may be
were found to mediate the magnitude and sensitivity to alcohol ingested by commercial drivers. More is known about the
effects, particularly at low doses. Holloway (1994) concluded effects on performance of some of these chemicals than about
that because alcohol sensitivity can vary from time to time, others. In particular, less is known regarding newer drugs
person to person, and situation to situation, the setting of a now available in the pharmaceutical marketplace, and this is
"safe" BAC will always be arbitrary, being based on low, but especially true with regard to the nutritional supplements. From
non-zero incidence of effects below that level. published research reports, short descriptions summarize a
few pertinent points about each drug or medication and focus
Unlike establishing the relationship of alcohol to perfor- on aspects most pertinent to the occupation of commercial
mance, the case for determining similar links of the presence truck and bus and motorcoach drivers. Some augmented
of other drugs to that of cognitive performance (enhancements material for each chapter is relegated to supplemental coverage
or decrements) is not so straightforward. Different sampling in the appendixes.
techniques and different residuals of the same drug have very
different implications for the presence of drug impairment. Selections of particular experimental studies and their
For example, marijuana [with the active ingredient Tetrahydro- results were made by the synthesis authors with the expectation
cannabinol (THC)] is absorbed in fatty tissues and is then that those cited provide reasonable explanations of what
released back into the blood and urine as a metabolite that the general trends in the literature portend. In particular, the
has no psychoactive effects (THC-COOH). Thus, detection selections demonstrate the significance of several data gaps
of THC in the blood is indicative of recent ingestion; but in our knowledge base about the effects of psychoactive sub-
detection of marijuana metabolites in the urine or the blood stances on driving performance.
