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11 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.