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10 of use, and the ranges of drug concentrations produced in Many individuals have experienced alcohol-impaired different individuals ingesting the same size dose. Thus, performance (even while driving), and thus they can although concentrations of drugs and metabolites in body more readily relate to comparisons employed to explain fluids can be determined, unfortunately the concentrations of the effects of other chemical substances researched in most drugs and their correlation with impairment or improve- experiments (Shinar 2007a). ment of driving are not readily known (McBay 1997). Alcohol effects are so pervasive and consistent that the In 1983, a panel of medical experts reached a consensus World Health Organization recommended that alcohol-related concerning drug concentrations and driving impairment impairment serve as a benchmark for other impairments (which was reaffirmed in 1989). The panel reported that: "In (Willette and Walsh 1983). order to establish that use of a drug results in impairment of driving skills and to justify a testing program to respond to In his book, Traffic Safety and Human Behavior, Shinar this hazard, certain facts must be available: wrote: "Despite the numerous studies on the effects of drugs on driving-related skills, on driving, and on crashes; and in 1. The drug can be demonstrated in laboratory studies contrast to the role of alcohol in driving and highway safety, to produce a dose-related impairment of skills asso- we are amazingly ignorant of the role of drugs other than ciated either with driving or with related psychomotor alcohol in driving and safety" (Shinar 2007a, p. 434). Shinar functions. goes on to say there are two general reasons for this. Compared 2. Concentrations of the drug and/or its metabolites in body with other drugs, alcohol is a very simple drug. It spreads fluids can be accurately and quantitatively measured quickly and evenly throughout different body tissues so that and related to the degree of impairment produced. blood alcohol levels correspond very well to concentrations of 3. Such impairment is confirmed by actual highway alcohol in the brain. There is a direct dose-response relation- experience. ship so that the amount of impairment is directly related to the 4. Simple behavioral tests such as can be done at the road- amount of alcohol that enters the blood and, consequently, the side by police officers with modest training can indicate relationship between alcohol intake, blood concentration, and the presence of such impairment to the satisfaction of impairment is quite reliable and straightforward (Moskowitz the courts. 2002, 2007; Shinar 2007a). Alcohol affects just about every 5. A range of concentrations of the drug can be incorpo- capacity we have including multiple perceptual, attentional, rated in laws relating to impaired driving as ipso facto cognitive, decision, memory, and motor functions--all critical evidence" (Blanke et al. 1985; McBay 1989, 1997). for safe driving (Ogden and Moskowitz 2004). The impairing effects can be demonstrated at very low alcohol levels and as McBay, Shinar, and others appear to be in agreement that the amount of alcohol in the blood rises, the number of func- these criteria have been met for just one drug, ethanol (alcohol). tions that are impaired and degree of impairment increases The adverse effects of alcohol on driving performance have (Moskowitz and Robinson 1988; Moskowitz and Fiorentino been well-established. Experts can testify to its effects based 2000; Ogden and Moskowitz 2004). both on blood and BACs. However, most of what is known about alcohol and driving performance is not available for The psychomotor performance research literature reports other drugs. It is not certain that these listed criteria can be a large number of alcohol and performance studies and a con- met for most other drugs that now are of concern to highway siderable number of those were done in driving simulators. safety (McBay 1997; Shinar 2007b). Moskowitz and Robinson (1988) analyzed 177 studies that examined effects of low levels of alcohol (BAC levels of 0.10% or less) on driving-related functions and behaviors. DRUG INFLUENCES ON PERFORMANCE COMPARED WITH ALCOHOL EFFECTS They summarized their results in terms of the likelihood of impairment as a function of the BAC for nine different The three issues outlined previously serve as a segue to the driving-related categories: reaction time, tracking, vigilance, fourth issue, which is that in the scientific literature, human divided attention, information processing, visual function, performance researchers often report lab study drug effects perception, psychomotor skills, and driving skill. Their sig- by comparing them with the better-identified effects of ethanol nificant findings were: (1) that alcohol in almost any amount (alcohol) on performance. Research on alcohol performance impairs driving or driving-related skills, for all functions effects provides a "baseline" for relating and understanding studied, and as the BAC level increases, impairment increases; how much impact other chemical substances have in affecting (2) all aspects of driving behaviors studied are impaired as driver performance. This is so largely because: the BAC equals 0.10% or higher; and (3) there are differences among the cognitive functions in their sensitivity to alcohol. Predictable processing of alcohol (ethanol) in the body The most sensitive function--producing impairment at the is well-understood; lowest levels of BAC--was divided attention. Approximately The effects of alcohol on so many forms of performance 50% of the studies demonstrated impairment in divided atten- have been thoroughly studied and described; and tion at BAC less than 0.05%. The next most sensitive function