Acnd. Anal. & Prev. Vol. Printed in Great Britain.
1R. No. 4. pp. 273-287.
YOUTH,
Oool-4575/86 0 1986 Pergamon
1986
ALCOHOL AND RELATIVE CRASH INVOLVEMENT
S3.00+ IHI Journals Ltd.
RISK OF
D. R. MAYHEW, A. C. DONELSON, D. J. BEIRNESS and H. M. SIMPSON Traffic Injury Research Foundation of Canada, 171 Nepean Street, Ottawa K2P OB4, Canada Abstract-Deaths and injuries due to road-crash involvement are a major health and safety problem, especially among youth. Numerous factors can account for the overrepresentation of young drivers in road crashes and one of these-alcohol-has received renewed attention. This paper examines evidence pertaining to drinking and driving among youth to determine the extent to which alcohol has special significance for crashes involving young drivers, Three sets of studies are reviewed: those providing data on the extent of drinking and drinking-driving among youth, those that examine alcohol use among youth involved in road crashes and those that estimate the relative risk of a road crash for young drinking drivers. Findings show that frequent and heavy alcohol consumption among teenagers and young adults is not unusual, although they are less likely than older age groups to drive after drinking. Nonetheless, those young people who drive after drinking have a greater risk of crash involvement than older drinking drivers at all blood alcohol concentrations. One explanation of this finding assumes that young people are “inexperienced” with drinking and with driving. Limited behavioural research done to date supports some but not all assumptions implicit in this reasoning. There is also evidence in favour of an alternative hypothesis, namely, that the comparatively higher crash-risk among young drinking drivers is attributable to a subset of this group, those that engage in risky driving behaviour and who also happen to use alcohol. Further investigation into the personal and social characteristics of young people who drive after drinking would provide a sounder empirical basis for policies and programmes to reduce crash involvement among youth.
Research has established that young adults are overrepresented in road crashes (e.g. Chipman, 1985; Jonah, 1984; Mayhew et al., 1981; OECD, 1975; Simpson er al., 1982; Stewart et al., 1984; Valentine et al., 1978; Williams, 1985). This finding holds even when the amount and nature of road travel are taken into account. For example, in the United States, Williams (1985) compared data from the 1977 National Personal Transportation Survey and from the Fatal Accident Reporting System to compute mileage-based rates of fatal-crash involvement for drivers of various age groups. The author found that rates were highest for drivers aged 16-19 years, declined through age groups up to 40-59, and then increased again, particularly among drivers 70 years of age or older. A similar Ushaped relationship was described by Stewart ef al. (1984), who calculated the relative risk of fatality and injury for Canadian drivers of various ages. Controlling for exposure, Stewart et al. (1984) found that the relative risk of fatal crash involvement is highest for drivers aged 16-19 years, decreases through age groups until 25-44, then rises, particularly for drivers 65 years of age and over. A similar pattern has been observed for the risk of injury accident, with youngest drivers (16-19) having the highest risk, drivers aged 2064 years the lowest, and an elevated risk of injury for those 65 and older (Jonah, 1984; Stewart et al., 1984). These findings consistently show that, compared to older age groups, youth have a higher relative risk of crash involvement. They do not, however, provide much insight into factors contributing to this phenomenon. Potentially important factors include amount and quality of driving experience, risk-taking, hazard awareness, sensation-seeking, aggressiveness and, of course, alcohol use. With the exception of alcohol use, direct empirical evidence concerning the extent to which these factors act to increase youth’s relative risk of crash involvement is scarce indeed. Because alcohol as a risk factor has received renewed attention (e.g. as a reason to establish higher minimum drinking age laws [Presidential Commission on Drunk Driving, 1983; Wagenaar, 1983; Vingilis and De Genova, 19841) this paper explores relationships among age, alcohol use and relative risk of crash involvement. Information for the present paper comes from three lines of research: studies of alcohol use and drinking-driving among youth, investigations of alcohol use among youth involved in road crashes and research on the relative risk of crash involvement. 273
274
D. R. MAYHEW el al. ALCOHOL
CONSUMPTION
AND
DRINKING-DRIVING
PRACTICES
Questionnaire-based surveys have described patterns of drinking among youth. With varying degrees of directness, results of these surveys indicate the potential overlap of alcohol use with driving, another activity usually initiated sometime during adolescence. More direct, objective and precise information on drinking-driving practices among youth comes from roadside surveys using breathtests to measure blood alcohol concentration (BAC). As might be expected, roadside surveys obtain less extensive and less detailed data on alcohol consumption per se. Taken together, however, studies of drinking and drinking-driving practices provide important information on the relationship between alcohol and crash risk for youth. Alcohol consumption among youth Despite minimum drinking age laws and other restrictions on the purchase and consumption of alcoholic beverages, alcohol use among adolescents is frequent and widespread. The majority of young people consume alcoholic beverages. In fact, studies have shown that 79.0 to 92.5% of 16-18-year-old students report having consumed alcohol at some time; that 25.4 and 45.0% drink at least once a week; and that 0.9 and 6.1% drink daily (Vingilis and De Genova, 1984). Among adolescents, the frequency of drinking generally increases with increasing age. For example, in a 1979 study of drinking practices among high-school students in Ontario (minimum drinking age of 19), 67.1% of youth aged 15 years or younger, 89.4% of 16-17 year olds, and 91.3% of 18-19 year olds reported having used alcohol in the past year (Smart, 1980). Similarly, data from a 1982 Canadian Gallup Poll show increases in reported consumption with increases in age (see Whitehead, 1984). In this survey 70.8% of 15-17 year olds and 88.9% of 18-19 year olds reported having used alcoholic beverages in the past 12 months. Among those who reported using alcohol, many were frequent, recent users; 29.6% of 15-17 year olds stated they had consumed alcohol about once or more a week in the past 30 days as did 50.4% of 18-19 year olds. These results are quite comparable to those of studies in the U.S. (e.g. Cameron 1982, 1985). In a recent household survey on drinking and driving in Canada, Jonah et al. (1985) found self-reported drinking highest among young adults aged 21-25 years (83.1%). Among 18-20 year olds, 75.5% reported drinking one or more alcoholic drinks in the last 30 days. With respect to the quantity of drinking this study showed that within the last seven days young adults aged 21-25 and 18-20 consumed a greater number of alcoholic drinks than older age groups (9.8 and 8.9 mean number of drinks for young adults compared with 7.5, 6.9 and 5.5 drinks for those aged 26-30, 31-40 and 41 and over, respectively). Moreover, on their last drinking occasion, 18-20 and 21-25 year olds drank more than older drinkers (4.7 and 4.6 drinks for 18-20 and 21-25 year olds, respectively, and 3.6, 3.8 and 2.9 drinks for those aged 26-30, 31-40 and 41 and over, respectively). In the U.S., studies have also found that alcohol use ranges from infrequent consumption (drinks less than once a month) among 12-15 year olds to frequent, heavy consumption (drinks five or more drinks in a sitting at least once a week) among 18-20 year olds (Cameron, 1982, 1985). As reviewed by Wagenaar (1983), since 1975 the proportion of older adolescents who report having “ever used” alcohol has remained at about 90%. The percentage of high-school seniors reporting frequent intoxication, however, increased from 37% in 1975 to 41% in 1979, 1980 and 1981. These self-report surveys indicate that frequent and heavy drinking increases through the adolescent years and becomes normative behaviour as young people approach the legal age for drinking. These findings have broader implications. For example, for many adolescents this developmental pattern begins well before they reach the minimum legal drinking age. During this period the consumption of alcohol seems to take on a greater, symbolic meaning, for example, as an indicator of maturity or adult standing. Jessor et al. (1975) suggested that drinking represents a departure from conventionality during adolescence. Beverage alcohol certainly offers a convenient means for such self-expression. Awareness of alcohol among adolescents and the possibility of drinking no doubt
Youth, alcohol and risk of crash involvement
275
increase throughout teenage years, perhaps fostered by such factors as parental example and media advertising. Age-specific prohibitions of the purchase and consumption of alcoholic beverages, which are intended to prevent underage drinking, probably accord to alcohol the status of “forbidden fruit”. These laws may thus act to enhance the attractiveness of drinking to persons who, under the legal age, have a need or desire to violate societally established norms of adolescent behaviour. Whatever the validity of the “forbidden-fruit” hypothesis, results of surveys summarized above clearly show that minimum drinking age laws do not completely deter the majority of teenagers from drinking. Research on the impact of changes in legal drinking age on youthful alcohol consumption do not provide definitive answers about the effects of this law-based measure. Nonetheless, there is some evidence (1) that reductions in the legal drinking age have been associated with increases in youthful consumption and (2) that reductions in self-reported youthful drinking follow an increase in the drinking age (Simpson et al. 1985; Vingilis and De Genova, 1984). Often, however, these reported changes in the use of alcohol among young people are small, or only temporary. Questions remain as to whether such changes are the result of changes in the drinking age or the consequence of changes in social norms and drinking practices among youth that might have occurred even in the absence of a law change. As noted by Simpson et al. (1985), any change in youthful drinking practices may result from several factors, acting independently, or in combination with changes in minimum drinking age. Moreover, it may be important to determine whether changes in youthful drinking practices as a result of changes in the legal drinking age are reflected in terms of either the quantity of consumption or its frequency. These two aspects of drinking behaviour might not be similarly affected by statutory changes and each may have different implications for driving after drinking. For example, if raising the drinking age served to reduce the frequency of consumption without altering the quantity of alcohol consumed, then only the opportunities for drinking-driving have been limited without affecting the relative risk of crash when drinking-driving does occur. This issue has yet to be examined in relation to changes in the legal drinking age. Given the prevalence of drinking among youth (especially frequent and heavy consumption among older adolescents) and given that licencing to operate a motor vehicle can occur at age 16 in most jurisdictions, it might reasonably be expected that driving after drinking among youth would occur at least occasionally. Several self-report and roadside surveys of the prevalence of alcohol use among young drivers confirm this expectation. These studies are reviewed below. Drinking-driving among youth A 1979 survey in the San Francisco Bay Area found that 25% of 16 and 17 year olds reported that they had driven after they had had a few drinks (Cameron, 1982). Similarly, data from a 1979 U.S. nationwide survey (Clark and Midanik, 1980) showed that 29% of 18-20-year-old current drinkers reported that they had at some previous time driven a car after drinking; in many of these cases (91%) the event had occurred during the past year. About 35% of current drinkers aged 21-24 years reported driving after drinking. When these data were examined according to gender of respondent, it was found that an almost identical proportion of 18-20-year-old and 21-24-year-old male drinkers indicated that they had driven a car after drinking (43% vs 44%). Among female drinkers, 21-24 year olds reported drinking and driving twice as often as did 18-20 year olds (25% and 12%, respectively). These surveys suggest that about 40% of male drivers and 10% of female drivers under the age of 20 have driven after drinking at some time. Other surveys provide insights into the frequency of this behaviour. For example, Jonah et al. (1985) reported that 53.8% of 18-20 year olds and 62.1% of 21-25 year olds surveyed in a recent Canadian household study had driven a vehicle within two hours of drinking any amount during the past 30 days. Moreover, 22.5% of 18-20- and 22.8% of 21-25-year-old respondents thought they had been legally impaired on these drinking-and-driving occasions. Hingson et al. (1982) found that 24% of 16-19 year olds reported driving after drinking between
276
D. R. MAYHEW el al.
1 and 5 times in the previous month; 7% reported drinking and driving as frequently as 6 to 14 times in the previous month. These results are consistent with those of earlier surveys by Pelz et al. (1975) and Wallack and Barrows (1981). In general, survey respondents report having been a passenger in an automobile operated by a drinking driver more often than they report having driven after drinking themselves. (It is unclear whether this finding indicates a tendency to give “socially desirable” answers or whether it actually reflects typical travel patterns of youth. Many youth may well travel more often as passenger than as driver.) In the 1979 nationwide survey (Clark and Midanik, 1980) 59% of 18-20 year olds reported having been transported by a driver who had had too much to drink. Wechsler et al. (1984) found similar results, with 50% of 10th grade students reporting having been a passenger in a car operated by a drinking driver. Of some interest, in the Clark and Midanik study (1980), among 18-20 year olds who did not report drinking in the past six months (about 17%), 35% indicated they had been a passenger in a car driven by someone who had had too much to drink. Cameron (1982) suggested that such findings support the idea that drinking drivers have friends who engage in the same behaviour, while abstainers are less frequently involved in drinking-driving situations. Nusbaumer and Zusman (1981) offered that riding with drinking drivers may be an intermediary step in a progression toward drinkingdriving itself. Findings from these questionnaire-based surveys suggest that driving after drinking is frequent, if not normative, among young people. Of course, such data are subject to the usual limitations and biases associated with self-reported information. They also provide little indication of the amount of alcohol consumed before drinking-driving occasions. For such data we rely on roadside surveys, which provide objective estimates of alcohol use among young drivers. In the Grand Rapids study, Borkenstein et al. (1964) found that 4% of 16-19-yearold drivers had consumed alcohol. No 16 or 17 year old had a BAC greater than 50 mg %; only 1% of 18-19 year olds had BACs exceeding 50 mg %. Perrine et al. (1971) found similar results among Vermont drivers. Of those under the age of 20, 5.5% had positive BACs below 50 mg %, and 4% had BACs greater than 50 mg %. A survey in Huntsville (Farris et al. 1977) also found that comparatively few teenagers combine drinking and driving; only about 8% of those under the age of 20 had been drinking (BAC equal to or greater than 30 mg %) compared to 12% of those over 20 years of age. These surveys covered most times of the day. Inasmuch as drinking occurs more frequently at night (OECD, 1975), it is instructive to examine the frequency of drinking among nighttime drivers. In reexamining the Grand Rapids data for the period between 9 p.m. and midnight, Zylman (1973) found that all 16-17 year olds were alcohol free and that 6% of 18- and 19-year-old drivers tested positive, all of whom were below 80 mg %. Similarly, other nighttime surveys such as those by Carlson et al. (1971) in Washtenaw County and by the Research Triangle Institute (1971) in North Carolina report low frequencies of drinking among youthful drivers. Only 1% of 16-20 year olds in Washtenaw County and 5% of 16 to 20 year olds in North Carolina had BACs of 50 mg % or higher. The U.S. roadside survey by Wolfe (1974) found 3.3% of 16-17-year-old nighttime drivers between 10 and 50 mg % and 8.2% above; the comparative figures for 18-20 year olds were 7.3% and 11.2%. One of the most extensive national surveys of nighttime drivers was carried out in Canada. Data on nearly 10,000 drivers were obtained compared to just over 3000 in the U.S. survey (Smith et al., 1975). In the Canadian survey, 11% of 16-19 year olds had BACs in the range of 20-50 mg % and 7.2% were above this BAC. More detailed breakdowns by age are given in Table 1. This table shows, for each age group, the proportion of persons who had been drinking (BAC over 15 mg %), the proportion with BACs over 95 mg %, and the mean BAC. Consistent with self-reported data, the most significant observation is that the lowest frequency of drinking and of high BACs occurred among 16-17 year olds. Indeed, the frequency of drinking-driving and of “impaired” driving increases systematically from age 16-17 to age 30-34. Thus, while driving after drinking is not infrequent among youth, the incidence appears lower than for most other age groups.
Youth, alcohol and risk of crash involvement
217
Table 1. Age distribution of drinking drivers (Canada, 1974). Percent of Nighttime Drivers with BLOOD ALCOHOL CONCENTRATION: over
42
over
15 mgX 14.7 19.3 21.7 20.0 26.0 22.3 17.6 20.8 15.2
16-17 18-19 20-24 25-29 30-34 35-39 40-44 45-49 50+
Mean BAC (mg%)
95 mgx 1.1 2.5 4.1 4.3 7.2 3.7 3.2 5.8 3.2
50 56 61 68 71 65 68 80 62
More recent roadside surveys in four Canadian Provinces-Ontario, British Columbia, Saskatchewan and Quebec-produced similar results (Interministerial Committee on Drinking-Driving, 1980; Lawson ef al., 1982; Mercer, 1982). The Ontario roadside survey in 1979 (data on about 10,000 drivers) found that, as a group, 16-24-year-old drivers had lower BACs than 25-49 year olds and that drivers aged 16-18 years were less likely to have BACs in excess of 50 mg % than drivers of other ages. In the Saskatchewan and Quebec roadside surveys, 16-24-year-old drivers were least likely of any age group to have BACs greater than 80 mg %, whereas in British Columbia 16-24 year olds were reported to have BACs over the statutory limit with about the same frequency as drivers aged 25-39 years. Nevertheless, among 16-24-year-old British Columbia nighttime drivers, the proportion of drinking and of impaired drivers was lowest for 16-19 year olds (Mercer, 1982). Comparison of the 1974 and 1979 roadside survey results in Ontario indicate an overall, but slight increase in impaired driving (81 mg % and over) across all age groups from 6.3% to 6.6% (Interministerial Committee on Drinking-Driving, 1980). As can be seen in Table 2, the trend toward more impaired driving was evident for most age groups, with the exception of those aged 30-39. Additionally, the proportion of drivers with a BAC of 50-80 mg % increased for those drivers 50 years old and younger. The greatest increase in drinking-driving (50 mg % and over) occurred among those aged 16-19 and 25-29. In 1974, 6.3% of 16-19 year olds and 12.3% of those aged 25-29 had a BAC of 50 mg % or greater. In 1979, 10.5% of 16-19 year olds and 16.6% of drivers aged 2529 had been drinking. While increases in drinking-driving occurred between 1974 and 1979, particularly for 16-19 and 25-29 year olds, those aged 16-19 still remained the least likely to have been drinking. Comparisons between the 1974 and 1981 roadside surveys in British Columbia, Saskatchewan and Quebec produce conflicting findings concerning trends in drivers’ alcohol use. Lawson et al. (1982) reported that, in British Columbia, the proportion of drinking drivers (15 mg % or over) remained unchanged but that the proportion of impaired drivers (75 mg % and over) declined from 7.8% to 6.1%. For Saskatchewan in 1981, the proportion of drinking drivers and impaired drivers were below those estimated for the Prairies region in 1974. In contrast, Quebec experienced increases in the proportion of drinking and impaired drivers between 1974 and 1981. These differences could be attributable to different study designs (although considerable effort was taken to ensure com-
Table 2. Age distribution of nighttime drivers by blood alcohol concentration (Ontario, 1974 and 1979). 1979
1974 BAC (rug%)
&E 16-19 20-24 25-29 30-39 40-49 50+
-
o-49
-50-80
-al+
-o-49
_50-80
_al+
93.7 87.8 87.7 84.4 88.0 92.2
3.3 6.4 6.4 6.2 6.0 4.3
3.0 5.9 5.9 9.3 6.0 3.6
89.6 85.1 83.5 85.3 85.6 90.6
5.6 a.4 8.5 6.7 6.8 4.2
4.9 6.5 8.1 a.1 7.6 5.2
278
D. R. MAYHEW et al.
parable techniques), to the differing BAC ranges selected for reporting the results, or to actual differences in alcohol-use patterns within these provinces. In addition, since no age-specific comparisons were made it is not possible to determine whether increases in drinking-driving occurred for some age groups but not for others. More detailed analyses would seem highly desirable. These roadside survey results describe the incidence of drink-driving and alcoholimpaired driving among teenage and older drivers. The results presented above do not indicate the extent to which young drivers contribute to the prevalence of drinking and driving, that is, what proportion of the total number of drinking and alcohol-impaired drivers are young. Relevant data, taken from the 1979 Ontario roadside survey, are shown in Table 3. As can be seen in the table, young drivers aged 16-19 are underrepresented among drinking and impaired drivers in comparison to the proportion of young nighttime drivers. Drivers aged 16-19 account for about 14% (1 in 7) of nighttime drivers, 13% (1 in 8) of drinking drivers and only 9.6% (1 in 10) of drivers with BACs over 80 mg o/c. In contrast, drivers aged 20-24 are overrepresented among drinking drivers, those aged 25-29 and 30-39 account for a disproportionate number of both drinking and impaired drivers, and those 40 years and over are underrepresented among drinking nighttime drivers. In the roadside surveys in Saskatchewan and Quebec, 52.1%, and 36.6% of drivers on the road were 16-24, but only 43% and 27% of the impaired drivers fell in this age range. In British Columbia, 16-24 year olds accounted for 35.3% of nighttime drivers, but 37.2% of impaired drivers. Thus, in general, young drivers, particularly those aged 16-19, contribute less to the total amount of drinking and driving than would be expected given their numbers among nighttime drivers. In summary, roadside surveys conducted over the past 20 years in Canada and the U.S. suggest that the lowest frequency of drinking and of high BACs occurs among young drivers. There is some evidence that there has been an increase in drinking and driving among 16-19 year olds in recent years, but increases have also occurred for other age groups. Roadside survey results show that teenage drivers are usually underrepresented among drinking drivers relative to their representation among nighttime drivers. The extent to which alcohol has been consumed by young drivers involved in road crashes is examined in the next section.
ALCOHOL
USE
AMONG
YOUNG
DRIVERS
INVOLVED
IN
ROAD
CRASHES
Several studies have determined the frequency of alcohol use among collision-involved drivers. Similar to results of roadside surveys, studies of collision-involved drivers have usually found that the youngest and the oldest drivers are least likely to have been drinking or to have BACs over the statutory limit. Moreover, given both crash involvement and alcohol use, young drivers also tend to have lower BACs than older drivers. For example, in the Grand Rapids study (Borkenstein et al., 1964), of drivers involved in injury-producing and property-damage accidents, those aged 16-17 and 18-24 years were least likely to have been drinking; only 5% of 16-17 year olds and 13% of 18-23 year olds had positive BACs, compared to approximately 20% for the older age groups. Reanalyzing the Grand Rapids data for the nighttime period between 9 p.m. and midnight.
Table 3. Age-specific
drinking
and alcohol-impaired
nighttime
driving
(Ontario.
Percent
Percent !t&c 16-19 20-24 25-29 30-39 40-49 50-59 60+
Nighttime 13.6 22.4 17.8 21.4 12.6 8.4 3.9
of Drivers
Positive BAC (1 mg% and over) 13.0 23.4 18.4 21.5 12.7 7.7 3.2
Percent Over 80 mg% 9.6 21.2 20.9 25.1 14.0 6.1 3.0
1979).
219
Youth, alcohol and risk of crash involvement
Zylman (1973) found that young collision-involved drivers and those over 65 were least likely to have been drinking, with 3% of 16-17 year olds and 14% of 18-19 year olds testing positive. Moreover, young collision-involved drivers who had been drinking had lower BACs than older drivers. As described above, this study involved both injuryproducing and property-damage collisions. If injury accidents are examined separately, the frequency with which alcohol is detected increases. For example, in the Huntsville study (Farris et al., 1977), of drivers involved in injury-producing accidents, those between the ages of 14 and 17 and those 55 years and older had the lowest percent of positive BACs (BAC 30 mg % and over); however, the incidence of alcohol use was greater than in the Grand Rapids study. Among 18-19 year olds, 30% had been drinking, compared to 34% of drivers age 20-24 and 28% of drivers 25 years or older. That the frequency of alcohol use among drivers increases as a function of the severity of the crash is also shown in the study by Pelz et al. (1975). They found alcohol among 4%) 7%, 10% and 22% of 18-year-old drivers involved in no-injury, moderate injury, severe injury and fatal accidents, respectively. Although the frequency with which alcohol is detected is greatest in serious crashes, even studies of fatal collisions have shown that the teen-aged, crash-involved driver is less likely to have consumed alcohol than young adult and middle-aged drivers and that, on average, the BAC of teen-aged drinking drivers is lower. For example, Perrine et al. (1971) reported that, among fatally injured Vermont drivers, the youngest and the oldest groups were least likely to have consumed alcohol; 44% of those under 20 and 23% of drivers 60 years of age or older tested positive (20 mg % and over) compared to 69% and 57% of those aged 20-24 and 25-59 years, respectively. Persons under age 20 were also less likely to have a BAC of 100 mg % or higher than were individuals in the two middle-age categories. More recent data from the Fatal Accident Reporting System (FARS) for 1982 show that 62% of tested drivers [BAC data in 1982 were available for only 33% of all drivers in fatal motor vehicle accidents] aged 16 to 19 years had positive BACs (BAC greater than or equal to 10 mg %) compared to 71% of 20-34 year olds (Center for Disease Control, 1983). Moreover, young drinking drivers involved in those fatal crashes had lower BACs than older drinking drivers. Similar results have been reported in Canada by the Traffic Injury Research Foundation of Canada, which maintains a Fatality Database for seven Canadian provinces that demonstrate a high rate of testing for alcohol (Simpson et al., 1979; Beirness et al., 1985). For example, Beirness et al. (1985) examined alcohol use among driver fatalities over a ten-year period, from 1973 to 1982. They found that the distribution of BAC by age of driver has remained relatively consistent across the study period. The proportion of drivers who were found to have been drinking was lowest among young drivers (19 and under), increased among drivers aged 20-45, and then decreased again in older drivers (46 and over). Young drivers and older drivers were most likely to have low BACs (80 mg % and below), compared to drivers between the ages of 20 and 45, who were most likely to have BACs in excess of 150 mg %. The results for 1982 are shown in Table 4. As can be seen, 16-17- and 18-19-year-old fatally injured drivers tested for alcohol, 61% and 71%, respectively, were known to have been drinking and half of both age groups had BACs over the statutory limit of 80 mg %. However, the incidence of high
Table 4. Frequency of alcohol detection among fatally injured drivers by age, Canada (Seven Provinces), 1982.
Age
Percent tested for alcohol
range
16-17 18-19 20-25 26-35 36-45 46-55 55+
79 143 392 332 202 133 254
79 73 79 a0 76 78 65
Percent BAC positive 61 71 70 67 60 49 30
Percent with BAC 81 rag% and over 50 52 57 58 52 42 21
280
D. R. MAYHEW et al.
BACs is even greater among 20-25 and 26-35 year olds; about 58% of these fatally injured drivers had BACs over 80 mg %. In summary, in both nonfatal and fatal crashes, the youngest and the oldest drivers are least likely to have been drinking or to have BACs over the statutory limit. This pattern has been consistent over time and across jurisdictions. The review thus far has examined alcohol use among drivers on the road and among drivers who crash. Although these descriptive data are useful in their own right, their value is enhanced when the two sets of data are combined to estimate relative risk of crash involvement as a function of BAC. Several studies have produced relative risk estimates and they are reviewed below. RELATIVE
RISK
OF
COLLISION
FOR
YOUNG
DRINKING
DRIVERS
Early field studies (Holcomb, 1938; Lucas et al., 1955; McCarroll et al., 1964), the classic Grand Rapids study (Borkenstein et al., 1964), and a number of similar, more recent investigations (Perrine et al., 1971; Farris et al., 1977; McLean et al., 1980; Warren and Simpson, 1980) have established a general relationship between BAC and relative risk of crash. Typically, these studies compare the incidence of alcohol among crashinvolved drivers to that among drivers on the road to determine the likelihood (or risk) of crash involvement at a given BAC relative to the likelihood of a crash at zero (or very low) BAC. Differences are found among the relative risk values calculated in these studies, due to a variety of factors, including differences in study method, the type of collision population studied and the time and location of the study. The overall pattern of results, however, is strikingly similar. In all cases, risk of collision increases marginally with increases in BAC up to 80 to 100 mg %, after which relative risk increases rapidly with further increases in BAC.
Driver Fig.
Age
1. Relative likelihood
of fatal crash for impaired
drivers
as a function
of age.
Youth, alcohol and risk of crash involvement
281
The relative risk estimates produced in these studies are based on aggregate data, describing the results for the “average” drinking driver compared to the “average” nondrinking driver. It is important to note that other factors such as the type of accident, temporal factors (time of day and day of week), annual mileage, drinking frequency, age, sex and marital status (Borkenstein et al., 1964; Hurst, 1974) clearly influence this relationship. This paper is concerned specifically with the interactive effects of age. Case-control studies have established that the relative crash risk of young drinking drivers is higher than that of drinking drivers of other ages. For example, data from the Huntsville study (Farris et al., 1977) showed that the relative risk of involvement in an injury-producing accident is higher for the young and elderly drinking driver (BAC 30 mg % and over) than for middle-aged drivers. Allsop (1966), using Grand Rapids data, also found that the risk of collision is higher for drivers aged 18-24 years than for older drivers, especially in BAC ranges greater than 80 mg %. Although data for 16-17-yearold drivers were available for only one positive BAC range (lo-49 mg %), the relative risk was about 5 times that of drivers aged 18-24 years in that range. In Canada, Warren (1976) computed relative risk ratios for fatally injured drivers of various ages, all of whom had BACs over the statutory limit. His study was intended to determine whether the relative risk of collision for all “legally impaired” drivers is similar. The results are shown in Fig. 1. As can be seen, there is considerable variability in the relative risk of fatal collision for drivers with BACs over the legal limit and, of relevance to the present paper, the highest relative risk occurs for young adults.
(415)
80.0
80.0
0
20
40
80
80
100
120
Blood Alcohol Concentration
140
180
(mg%)
Fig. 2. Relative risk of fatal crash as a function of BAC and age
D. R.
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More recently, Mayhew and Simpson (1985) extended that work to include a greater range of BAC categories. This study used two Canadian data sets containing information on (1) nighttime drivers on the road (population at risk) and (2) fatally injured, nighttime drivers. These groups were compared using a risk-factors method (Hurst, 1970, 1974; Warren and Simpson, 1980) to estimate relative risk of fatal crash. Relative-risk curves were generated for six age groups. The results are shown in Fig. 2. As can be seen, the relative risk of fatal crash involvement increases with increasing BAC ranges for each age group. There are, however, important differences. For example, the relative risk of fatal crash among 16-19-year-old drivers is higher than for other age groups in all BAC ranges. In addition, the slope of the curves indicate that relative risk rises faster for 1619 year olds than it does for those aged 20 years and older. Of interest here is the finding that drivers aged 16-19 years have a higher relative risk of fatal crash than older drivers in all BAC ranges. In lower BAC ranges, when data for all drivers are aggregated, relative risk estimates are very close to those for the average nondrinking driver (1.0). To determine whether or not observed differences are reliable, 95% confidence intervals (Gart, 1962) were calculated for relative risk values for young drinking drivers and for drinking drivers aged 20 years and older. Figure 3 shows the age-specific relative risk curves with confidence intervals placed around each data point. In the BAC range 15-49 mg %, 16-19 year olds have a relative risk that differs significantly from that of older drivers as well as from non-drinking drivers. This is not the case for drinking drivers aged 20 years and older. As can be seen for this latter age group, the risk of collision at 15-49 mg % does not differ significantly from 1.0. In the BAC range 50-79 mg %, the relative risk of fatal crash for 16-19-year-old drinking drivers is about nine times greater than that for nondrinking drivers; for drinking drivers aged 20 years and older the risk is about 2 times greater. Despite considerable variance in these estimates, especially among young drinking drivers, the confidence intervals do not overlap. Accordingly, the relative risk esti-
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mates for young drivers and older drivers are significantly different. Moreover, the relative risk estimates for both age groups in this BAC range appear significantly different from the relative risk value (1.0) for the average non-drinking driver. For BACs above 80 mg %, the relative risk estimates for both groups differ significantly from the reference value 1.0, and differences between the two age-specific curves still hold. Indeed, even at BACs above 150 mg %, the relative risk estimate is greater for drivers aged 16-19 years than for older drivers. In summary, case-control studies have consistently found that young people who drive after drinking have a higher relative risk of crash involvement than older drinking drivers. Recent findings suggest that this pattern holds in all BAC ranges, including ranges not always associated with “impaired driving” (e.g. lo-49 mg %). Age-specific risk curves seem to show two main effects and an interaction. The first main effect involves alcohol. As is well documented, relative risk of fatal crash increases with increasing BAC. The second main effect involves driver age. For example, 16-19 year olds have a higher relative risk of fatal crash than older drivers, regardless of BAC. In addition, there is an interaction between driver age and alcohol. While relative risk of crash involvement increases with increases in BAC for both groups, the rate of increase is greater for 16-19-year-old drivers.
DISCUSSION
Given the prevalence of drinking, especially frequent and heavy consumption among young adults, it is not surprising that drinking-driving occurs. Many 16-19 year olds report doing so and some 50% of teen-agers report riding in a car with a drinking driver. Data from roadside surveys confirm that young people drive after drinking. They also show that the percentage of drinking drivers is less among people under the age of 20 than older age groups under the age of 50. Moreover, on average, young drinking drivers consume less alcohol than older drinking drivers. Nevertheless, those young people who drive after drinking have a higher relative risk of crash involvement than older drinking drivers in all BAC ranges. These data provide solid evidence for the greater relative risk of crash involvement faced by young drivers. At the same time, they provide few insights into why this is so. Unfortunately, almost all risk-factors research done to date has focused mainly on alcohol-crash involvement and basic demographic characteristics such as age (Borkenstein et al., 1964; Perrine et al., 1971; Farris et al., 1977). This research does not permit close examination of interrelationships among age, alcohol and other age-related factors, such as skill development and personal and social characteristics of youth who drive after drinking. Thus basic questions remain. What accounts for the higher relative risk of young drinking drivers compared to older drinking drivers? Further, why is the rare at which relative risk increases with increasing BAC greater for youth than for older age groups? Aside from scientific interest, these questions seem highly relevant to the development and implementation of plans and programmes to reduce crash involvement among youth. Given limited resources for tactical intervention, it would appear critical to know (1) which factors or which combinations of factors (e.g. age-specific, person-specific, alcoholspecific, driving-related) contribute most to observed differences and, once the relative importance of factors has been determined, (2) how (or even if) key factors can be addressed effectively. In this context, perhaps because during adolescence most youth begin to consume alcoholic beverages and to operate motor vehicles, many explanations for their higher relative risk of crash involvement focus on inexperience-with drinking, with driving or with driving after drinking (e.g. Cameron, 1982; Jones and Joscelyn, 1979; Klein and Waller, 1970; and Zylman, 1973). Basically, it is thought that, during adolescence, both learning to drink and learning to drive take place. The assumption is that both learning processes are incomplete for adolescents. Thus, when teen-agers combine driving and drinking, their inexperience manifests as a greater inability to drive after drinking, resulting in the observed higher risks of crash involvement, even (in contrast to older age groups)
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at low to moderate BACs. Unfortunately, complementary lines of investigation, especially behavioural research, have not tested this hypothesis and related assumptions in a direct, systematic fashion. Some studies, however, have produced results relevant to aspects of the general issue. Despite the fact that these studies involve experimental designs and are, therefore, subject to the criticism that they do not reproduce real world driving tasks, they do provide insights into understanding the complex relationships among age, alcohol and experience. For example, “inexperience with alcohol” implies that young drinkers are more affected by alcohol than older persons who have used alcohol over a longer period of time. Vogel-Sprott and Barrett (1984), however, found that, even controlling for drinking patterns and individual differences in BAC, alcohol-induced impairment of task performance increased, rather than decreased, with increasing age. Thus, younger subjects exhibited less impairment than older subjects at the same BAC. Although young people may not have a “special sensitivity” to the effects of alcohol (Smart, 1976), the higher crash risk of young drinking drivers might relate to different degrees of acquired tolerance. Tolerance is defined as lesser impairing effects of alcohol upon repeated doses over time. Tolerance is further characterized by the necessity to increase the dose in order to produce the same intensity and/or duration of effect originally observed. Studies that have examined different degrees of alcohol-induced impairment displayed by individuals with various histories of alcohol consumption (Goldberg, 1943; Goodwin, Powell and Stern, 1971; Moskowitz, 1971) have consistently found heavy drinkers less impaired, and presumably more tolerant, than light or moderate drinkers given the same amount of alcohol. Young drinkers, lacking the extensive exposure often believed necessary to acquire such tolerance, are assumed to be similar to a group of light drinkers in terms of the amount of impairment induced by alcohol. Regrettably, age was not treated as an independent variable in the studies cited. Moreover, results demonstrating tolerance have been found in both older (Goldberg, 1943) and younger (Goodwin et al., 1971) subjects. More recent work on tolerance has shown that factors other than pharmacologic resistance to the effects of alcohol may play an important role. For example, when rewards for achieving levels of performance equal to those under alcohol-free conditions are given to subjects performing under the influence of alcohol, individuals can quickly learn to compensate for the effects of alcohol (Beirness and Vogel-Sprott, 1984; Mann and VogelSprott, 1981). This finding leads to speculation that young drinkers have not had sufficient opportunity to develop an ability to compensate adequately for alcohol’s effects when driving. There may also be disincentives (e.g. peer pressure) or no desire (e.g. risk-taking) to exhibit tolerant behaviour when driving after drinking. The fact that young drivers have a higher relative risk of crash involvement at low to moderate BACs suggests that alcohol affects the performance of inexperienced drivers more so than that of more highly skilled drivers. However, one recent study, which manipulated the level of acquired skill on a perceptual motor task, found that the degree of alcohol-induced impairment was independent of prior task proficiency (Beirness and Vogel-Sprott, 1982). Alcohol did not produce a greater degree of impairment in subjects less skilled at the task, although absolute differences in performance between groups were maintained under drug-free and alcohol conditions. Assuming that these findings can be generalized to the driving task, then we would expect that performance of inexperienced drivers would not be any more or less impaired by moderate amounts of alcohol than the performance of more skilled drivers. Nonetheless, if inexperienced (or less skilled) drivers must devote all their attentional capacity to driving (Shinar, 1978), even small amounts of alcohol might decrease performance to the point where it falls below driving demands and, thereby, increases the risk of crash. Experienced drivers who require less attentional capacity allocated to the driving task (i.e. a greater “spare” capacity) might require more alcohol to decrease performance to the point where it is exceeded by demands. If this were so, even though alcohol produces the same degree of impairment in driving ability, both in experienced and inexperienced drivers, inexperienced drivers
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would more likely exceed the limits of their spare capacity at lower BACs than experienced drivers. This would result not from the differential influence of alcohol on the skill level of the driver, but rather because the less experienced driver’s performance was closer to the “critical point” (the point at which demand exceeds capacity) to begin with. Given this situation, almost any condition-be it alcohol, noise, distraction, a novel situationthat increases the attentional demands placed on the driver would be more likely to decrease the performance of the inexperienced driver below the “critical point” required for safety. Thus, some experimental evidence does not support the view that the higher crash risk faced by young drinking drivers is due to their relative inexperience with driving or drinking. Nevertheless, the limited state of present knowledge precludes definitive answers to questions surrounding relationships among age, alcohol, driving experience and relative risk of crash involvement. For example, an alternative hypothesis seems viable: that the higher crash risk of young drivers is attributable to personal and social characteristics (Goldstein, 1972; Pelz and Schuman, 1971, 1973; Preusser et al., 1975) of a subset of this group-those who engage in risky driving behaviour and who also happen to consume alcohol. Jonah (1984), for example, in reviewing research on risk taking by youth, concluded that risk-taking was the major factor underlying the higher accident risk of young drivers. If so, drinking and driving may not only be an expression of risktaking behaviour, but alcohol may also enhance behaviour that is already risky. Moreover, if drinking-driving involves a special group of young risk takers, it may be that the majority of young drinking drivers are in many respects indistinguishable from older cohorts. The importance of alcohol as a risk factor cannot be denied. Epidemiologic findings reviewed here provide some empirical support for alcohol-specific measures aimed at young drivers, for example, (1) lower statutory BAC limits for drivers aged 16-19 years, and (2) licence restrictions prohibiting their driving after consuming any amount of alcohol. Questions remain, however, about the potential of such tactics to effect substantial reductions in the frequency and severity of road crashes involving young drivers. Young drinking drivers most likely to become crash involved may also be least likely to obey laws and regulations in the first place. More target-specific programmes may ultimately prove more effective in terms of resource allocation and crash-loss reduction. Present knowledge does indicate that the problem of young driver accidents is a multi-dimensional phenomenon. It is probable that no one factor such as alcohol consumption will account for all or even a major part of the problem. Similarly, no one solution will likely substantially reduce the problem, short of complete prohibition of driving among youth. To understand the problem, we may have to take into account the broader social context and avoid the pitfall of “premature closure”-fixing on “solutions” before a more complete definition of the problem is achieved (Knapper, 1985). Even measures that would appear to produce almost certain benefits in terms of accident reduction must be carefully considered in light of possible individual and social costs that may be incurred. Research has shown, for example, that higher minimum drinking ages are generally associated with lower collision rates among young persons below that age (Vingilis and De Genova, 1984; Wagenaar, 1983). However, in a recent review of studies on the effects of raising the drinking age, Simpson et al. (1985) found that the magnitude of the effect shows considerable variability (from no effect to significant reductions in alcohol-related crashes involving youth) and the duration of the impact is virtually unknown. Moreover, while a higher drinking age may produce some benefits, it will likely produce a number of costs as well. For example, it may result in alienation among the affected groups, many of whom may view such a law as arbitrary and perhaps discriminatory. Such a law encompasses all persons within an affected age group, including young women drivers, who are involved in considerably fewer alcohol-related accidents than young males, young people who do not drive, and the majority of young people who do drive but refrain from drinking and driving. Thus, the introduction of any new programme can have broader implications than traffic safety per se and involve matters of economy, morality, justice and individual rights and freedoms.
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