I. INTRODUCTION
It has been almost 100 years since it became apparent that drivers' use of alcohol leads to an increased risk of crash (See Borkenstein, 1985). Traffic codes prohibiting alcohol-impaired driving had appeared in the United States by 1910, and the major approach to prevention then, as now, was deterrence by legal prohibition and law enforcement. By the 1940's, only three percent of traffic collisions were reported as being alcohol-related, due largely to officers' difficulties in assessing drivers. In the 1930s, epidemiological studies, which are studies examining the distribution of an event in a population, had begun to use breath and blood specimens to measure blood alcohol concentration (BAC) in crash-involved drivers. The measured BACs showed alcohol involvement in crashes to be much greater than three percent, and it was on the basis of those studies that the states began to establish BAC limits for drivers.
The first law in the United States establishing a BAC limit was enacted in 1939 in Indiana. Initially, the limits in Indiana and in other states were set at 0.15%(1), but they now have been lowered nationwide to either 0.10% or 0.08%. In other countries they are even lower. Limits defined by BAC assist with enforcement problems and also aid drivers in assessing their own impairment. There is worldwide agreement that alcohol-involved driving is curtailed when BAC laws are enacted and enforced.
The reduction of limits from the initial 0.15% BAC was prompted by evidence obtained from experimental and epidemiological alcohol research. As research continued over several decades, and as scientific investigators improved their techniques for examining relevant driving behaviors, evidence of significant driving impairment was reported at even lower BACs.
Studies have reported that the degree of impairment produced by alcohol may be modified by other variables. For example, the Grand Rapids study, which was the largest epidemiological study, suggested that the variables, age, gender, and drinking practices, produce differential impairment at similar alcohol levels (Borkenstein et al., 1964). Firm conclusions about those three variables on the basis of epidemiological data are difficult, however, because each is also associated with other variables which influence crash rates. For example, young people show a differentially high crash rate under alcohol, but they are also less experienced drivers. Also, when the Grand Rapids study was executed in 1962, women drove far less frequently and for shorter distances than men, possibly making them more susceptible to alcohol effects on driving. Analysis of the study's data relied primarily on uni-variate statistical methods, which could not isolate the effects of age, gender, and drinking practices from the effects of other variables.
The literature reporting data from laboratory research contains only equivocal evidence for an age interaction with alcohol (Jones and Neri, 1994; Morrow et al., 1990; Collins and Mertens, 1988). These studies, which included no subjects under age 21 and few subjects over age 55, do not resolve the issue, however, since it was drivers under age18 and over age 70 for whom the Grand Rapids study suggested an age and alcohol interaction. The question of whether young drivers are differentially sensitive to alcohol also remains unanswered by the current study. Because alcohol cannot be administered in the United States to anyone under age 21, the youngest subjects were ages 19 and 20. They were tested in Ontario, Canada where the alcohol age limit is 19 years.
In the Grand Rapids study, a gender and alcohol interaction did not occur until the BACs reached 0.08% and above. At those levels, women were more frequently accident-involved than men. Laboratory studies of the responses by men and women to alcohol, however, provide inconclusive results. As Sutker et al. (1983) noted, most experiments have given men and women the same alcohol dosage. Since the body fat and total body water of men and women differ greatly even when they are the same age, height and weight, women reach a higher BAC than men for the same alcohol amount. Many early studies failed to take this into account, but more recent studies have used comparable BACs rather than equivalent doses. These studies failed to find significant difference between male and female subjects (Burns and Moskowitz, 1978; Mills and Bisgrove, 1983; Oei and Kerschbaumer, 1990).
More reliable evidence exists for an interaction between alcohol and drinking practices. The Grand Rapids study reported that the likelihood of involvement in a collision for drivers at the same BAC was greatest for the drivers with the lowest daily alcohol consumption. A study by Moskowitz, Daily and Henderson (1974) supported this finding with a comparison of extremely heavy drinkers (recruited from bars) and moderate drinkers. They reported that heavy drinkers were less impaired than moderate drinkers at equal BACs on several psychomotor tasks. Also, a mean ethanol clearance rate of 0.020% per hour for the heavy drinkers, in comparison to a rate of 0.017% per hour for the moderate drinkers, demonstrated a physiological difference between the heavy and moderate drinkers.
This study examined skills performance of a representative sample of the driving population at BACs from 0.02% to 0.10%. It also examined whether variations in drivers' age, gender, or drinking practices interacted with BAC and resulted in variability in the impairment produced by alcohol. One hundred sixty- eight subjects were classified by four age groups, two genders, and three drinking practice categories. The three variables of age, gender, and drinking practice dictated the assignment of subjects to 24 groups of 7 each (Figure 1).
The youngest subjects in the study, who were ages 19 and 20, were tested at Human Factors North (HFN) in Ontario, Canada. Also, although evidence of an interaction of gender and alcohol is less substantial than the evidence of interactions of age and drinking practices and alcohol, the study included equal numbers of men and women in order to examine the issue.