This report is a comprehensive review of the state of knowledge of alcohol-impaired driving. The review covers the entire spectrum of related research, from the nature of the societal problem created by alcohol-impaired driving on through the description and effects of programs that have addressed that problem. The review covers scientific literature published since 1990.
This report deals with two major areas, the alcohol-crash problem and possible solutions to that problem. In the first area, the alcohol-crash problem, we briefly discuss the epidemiologic literature addressing crashes in which drivers, pedestrians or bicyclists have measurable amounts of alcohol in their blood. Both the number and risk of such crashes are examined in order to obtain an estimate of the magnitude of the alcohol-crash problem nationwide. Next, we examine in more detail how alcohol affects people and degrades their performance in driving or walking in ways that may lead to crashes. Finally, we describe the characteristics of individuals who drink and drive.
The second major area of this review is concerned primarily with alcohol-crash countermeasures that have been tried and evaluated. We define several types of countermeasures and discuss the impact of specific countermeasure programs of each type. The possible impact of some other countermeasures that have been proposed but not implemented or evaluated is also discussed briefly.
We have exercised considerable care in selecting the materials used in this report, concentrating on the most scientifically reliable studies that are available to the general reader. The main focus is on studies relevant to the alcohol-crash problem in the U.S., but some studies from other countries are included as appropriate. Sources include both collections and individual documents that have not been placed in traditional collections.
While the emphasis was on documents published since 1990, some earlier studies are included where needed to provide perspective and a basis for comparison with more recent studies. Also, some studies have been reported in more than one document. We have cited only one of the documents for such studies.
Not all of the studies identified in our literature search are discussed in detail in the review. We have sought those studies that best illustrate current thinking and have looked for background material from earlier research that led to current thinking. For the most part, the treatment is from the perspective of the traffic safety generalist, with departures into more specialized technical matters occurring only when these matters are central to the subject under discussion. The reader is asked to refer to the studies cited for a more detailed treatment.
Finally, we have emphasized literature that defines alcohol-crash involvement and risk objectively in terms of the blood alcohol concentration (BAC) of the individuals that were studied. Exceptions to this rule include studies based on well-designed surveys and some studies that measure impairment or alcohol-crash involvement by the opinion of a police officer who investigated the crash. Studies have used a variety of units for measuring BAC. In this review, we use percent alcohol, weight per unit volume of blood, as the unit, and have converted BACs measured in other units to this unit.
At the millennium, a driver’s blood alcohol concentration (BAC) remains as the single most important independent variable for measuring the extent of the alcohol-crash problem. Prior research has clearly established that human performance related to driving is substantially impaired in virtually everyone at BACs of .10 and higher, and recent research provides evidence that many such behaviors are impaired significantly at BACs as low as .05. Epidemiologic studies provide evidence that impaired performance at such low BACs is manifested in increased crash risk
The BAC level at which crashes involving alcohol becomes a societal problem deserving of widespread societal action is dependent upon the level of alcohol-crash risk an informed public is willing to tolerate, given available alternatives to reducing that risk. A level of .10 has been used in prior state of knowledge reviews in defining crashes of sufficiently high alcohol-crash risk to use the number of crashes involving a driver at .10+ as a measure of the alcohol-crash problem. By this measure, the alcohol-crash problem currently includes about 12,500 fatalities per year in the U.S. This figure would be even higher if a lower BAC level (such as .05) were used in defining the alcohol-crash problem. The National Highway Traffic Safety Administration (NHTSA) has described the problem in terms of fatalities with any alcohol present, that is, a BAC of .01 and higher. By that metric, the number of alcohol-related fatalities would be about 16,000 per year.
This measure (and related measures) of the alcohol-crash problem have declined markedly since objective data on the problem became available. The fatality rate, an especially important measure since it accounts for population growth, has declined nearly 50% since 1982, but has flattened out in recent years, suggesting that the problem needs increased emphasis to maintain the overall downward trend. Clearly, alcohol-related fatal crashes are a much smaller societal problem at the millennium than they were 20, or even 10, years ago.
The situation with respect to non-fatal crashes involving alcohol is less clear -- BACs are not routinely measured in non-fatal crashes. However, data from NHTSA indicate that 9% of the injury crashes and 5% of property-damage-only crashes involved alcohol in the judgement of police officers investigating the crashes.
Research continues to show that young drivers are more often involved in alcohol-related crashes than any other comparable age group. Alcohol-crash involvement rates, share of the alcohol-crash problem, and alcohol-crash risk all reach their peaks with young drivers, with the peaks for fatal crashes occurring at age 21.
A large proportion of the alcohol-crash problem involves young White males. In 1998, 84% of fatal-crash involved drivers with BACs of .10+ were male, and more than 70% were White. However, certain racial / ethnic subgroups have higher involvement rates than other subgroups. Of these, American Indians have the highest rate, and Asian / Pacific Islanders the lowest.
The impact of other biographical variables on drinking and driving is less understood. Crash data suggest that:
Many studies have found that beer is the preferred beverage of drinking drivers. There is evidence that heavier drinkers prefer to drink at bars and other persons’ homes, and at multiple locations requiring longer driving distances. Younger drivers have been found to prefer drinking at private parties, while older, more educated drivers prefer bars and taverns.
Studies continue to show that drinking-driving is primarily a nighttime, weekend phenomenon. Household surveys indicate that male drivers make three times as many trips within two hours after drinking any amount of alcohol than do females. Using this measure, such drinking drivers as a whole made 1.7 drinking-driving trips in the past 30 days, with the oldest drivers making the most trips and the youngest drivers making the fewest. Motorcycles have the highest rate of alcohol-related fatal crashes, followed by light trucks, passenger cars, and large trucks in that order.
Data from the Fatality Analysis Reporting System (FARS) suggest that only a few percent of fatal crashes involve drivers who have recent convictions of DWI, and studies in California have found that only 8% of drivers in fatal crashes had one or more DWI offenses on their driver record. However, studies in Minnesota suggest a much higher percentage. In California, crashes of all types actually decreased with number of priors, and in terms of sheer number of alcohol-related crashes, persons with no priors had the highest rate of involvement. The characteristics of repeat offender DWIs and first offender DWIs have in general been found to be quite similar in many respects, but DWIs with large numbers of priors have been found more often to have long-standing problems of alcohol dependency, and to differ on the severity of their alcohol problems, rather than on their demographics. A history of participation in multiple treatment programs is common for these individuals, as well as diagnoses of psychiatric pathology.
Research suggests that experiencing a prior negative event (such as an arrest or a crash) has a positive effect on one’s decision to drive after drinking, tending to make a driver less inclined to drive after drinking or to drive more cautiously after drinking. Factors that have a negative effect include a lack of knowledge of the impairing effects of alcohol or a misinterpretation of the cues of impairment, a reduction of inhibitions at higher BACs, a lowered perception of alcohol-crash risk, and a neglect of social norms after drinking. Research suggests that it is not just the impairing effects of alcohol that favors a decision to drive after drinking; some drivers plan to drink knowing they will drive afterward.
The scientific literature on the characteristics of alcohol-impaired pedestrians and bicyclists indicates that the alcohol-crash problem for pedestrians is, as it is for drivers, predominately a male problem. Very high BACs are common for pedestrians in alcohol-related fatal crashes, especially for those in the 35-44 age group (which is estimated to have 41% at .10+ and 18% at .20+). Alcohol-impaired bicyclists in fatal crashes are also more likely to be male, with the highest percentage of bicyclists at .10+ occurring for those the 45-54 age group, an older peak age group than that for either drivers or pedestrians.
Locations of pedestrian alcohol-related crashes as a whole are most likely to be near the victim’s home or a short distance from the starting point of the trip. Recent research on race and ethnicity indicates that Native Americans have the highest prevalence of alcohol-related pedestrian crashes, roughly three time that of Caucasians at .20+. Blacks and non-Black Hispanics fall somewhere between these two extremes.
We find that the literature reporting scientific evaluations of alcohol-crash countermeasures deals overwhelmingly with countermeasures using strategies of deterrence and incapacitation carried out by elements of the criminal justice system, that is, the Traffic Law System (TLS). These evaluations clearly indicate that many TLS-based countermeasures have been effective in reducing alcohol-related crashes. These countermeasures include those seeking general deterrence as well as specific deterrence.
Among those that were primarily legislative in nature, laws establishing administrative license revocation (ALR) have been found to reduce alcohol-related fatal crashes by up to 40%. This adds support for prior research showing the effectiveness of driver license sanctions in general. Also, there is considerable evidence that laws that lower the legal BAC limit from .10 to .08 can reduce alcohol-related fatalities by some 6-8%.
Zero tolerance laws for persons under 21, which in effect prohibit driving with any alcohol in one’s system, have been shown to result in reductions in alcohol-related fatal crashes ranging from zero to 40%. The key elements for this law are also effective enforcement and increased awareness of the law. States where DUI or DWI cases are essentially diverted into zero tolerance cases and there is no net increase in youth alcohol-related driving citations, are less likely to receive the potential beneficial effects of such laws.
Recent evaluations of comprehensive changes in State laws that have been accompanied by activities to implement those laws have shown positive results in reducing surrogates of alcohol-related crashes from 8% to 20%.
Enforcement-oriented programs that use multiple strategies (including sobriety checkpoints and saturation patrols) and are supported by strong public information and education efforts can be effective in reducing various surrogates of alcohol-related crashes by some 10% - 30%. Sobriety checkpoint programs alone can achieve positive impacts on alcohol-related crashes of the order of 20%.
Countermeasures emphasizing sanctions for drivers convicted of DWI have received considerable attention in the recent evaluative literature. There is now evidence that treatment and rehabilitation combined with driver’s license suspension can be more effective than suspension alone, obtaining recidivism reductions in the 30% range.
Three forms of sanctions used as an alternative to jail can be effective in reducing recidivism for DWI. These sanctions are: intensive supervision probation, electronic monitoring, and sanctions tailored expressively for individual offenders. For these sanctions, 33% - 50% recidivism reductions over traditional sanctions have been found. Other alternative sanctions that show promise but need further evaluation are day reporting centers, and possibly, victim impact panels.
A number of evaluations of vehicle-oriented sanctions provide evidence that sanctions that require the vehicle to be impounded or seized can achieve recidivism reductions of 50% or more. A similar effect has been noted for just seizing and destroying the offender’s license plates. Ignition interlocks that prevent an offender from starting his or her car can also reduce recidivism (by up to 69%) during the period in which interlocks are attached, but the effect disappears after the interlocks are removed. The use of specially marked license plates for DWIs also reduced recidivism in one state.
Large-scale experimentations with countermeasures not involving the Traffic Law System (TLS) have begun to appear in recent years. Two recent community trials programs had TLS components, and one of these evaluated the TLS components which were composed primarily of enforcement system support. The evaluation indicated a positive impact on alcohol-related crash surrogates in the 10% - 20% range. A large-scale, long-range prevention program consisting of school-based education in avoidance of alcohol-related problems suggested reductions in alcohol use and misuse, and a recent study suggests a positive effect on alcohol-related crashes as well. Another education-oriented program has just been developed, this time to enable medical practitioners to identify and help patients with alcohol problems, but its impact on alcohol-related crashes is not yet known. A year-around ride-service program providing a transportation alternative to drinkers has been evaluated recently and appears promising.
Another class of countermeasures has been aimed at limiting the availability of alcohol. Of these, legislation raising the legal minimum drinking age has been the most extensively evaluated, with results that clearly indicate a reduction of 9% to 14% in alcohol-related fatal crashes for the affected age group. Early attempts to limit alcohol availability have not shown any positive impact on alcohol-related crashes, but preliminary evaluations of alcohol-server programs indicate positive results. Server training now appears to offer alcohol-crash reduction potential, especially if used as a component of a broad community-based program. Evaluations of behavioral tests that help servers and social hosts identify alcohol-impaired guests also suggest potential impact on alcohol-related crashes. Finally, there is some hard evidence that more vigorous enforcement of alcohol sales to minors through such programs as “Cops in Shops”can reduce alcohol-related crashes for under-21 years old drivers.
Several countermeasures that attempt to reduce the probability of an impaired driver becoming involved in a crash have been suggested and even tested in field experiments, but their possible impact on crashes has not been determined. Included among such countermeasures are wider and better-marked road edges, rumble strips to warn drivers they have left the roadway, and drunk driver warning systems to alert roadway users of the presence of an alcohol-impaired driver.
While a significant number of alcohol-related crashes involve pedestrians and bicyclists, there have been very few evaluations of countermeasures for this component of the alcohol-crash problem. A multi-faceted pedestrian countermeasure program in Baltimore, Maryland, involving an extensive public information and education (PI&E) campaign, reflective caps for persons in high-risk zones, and improvements to the roadway environment in the high-risk zones was evaluated recently. The program achieved positive results, reducing surrogate measures of pedestrian alcohol crashes by 16 - 22%.
The recent scientific literature on programs to reduce excessive drinking among college students (and resultant alcohol-related crashes) does not reveal many program evaluations, although many colleges have such programs in place. We found four types of programs that had been the subject of an evaluation:
Three evaluations of normative programs were identified, all indicating a positive effect on excessive drinking. The evaluations of the alcohol education programs (at six colleges) could be more accurately described as assessments and also indicated a positive effect on excessive drinking. The one evaluation of a peer education program found a positive effect, but the brief intervention evaluations obtained conflicting results, one indicating a positive effect and the other indicating no effect. While none of these evaluations measured drinking-driving or alcohol-crash impact, the normative programs seemed to offer the most potential. Also, other non-evaluative studies suggest that the programs that have a component that promotes the use of designated drivers could reduce the prevalence of drinking-driving.
In short, the literature on anti-drinking programs for college students provides considerable information on the nature and location of such programs, but very little information on the effects of such programs, especially their effects on the alcohol-crash problem.
Some of the limitations to alcohol-crash countermeasure evaluations should be kept in mind when interpreting the above findings on effectiveness. It is especially important to note that many evaluations have been conducted against a background of other anti-DWI activity, making it difficult to ascribe an observed effect to any single countermeasure. This is especially true of multi-state evaluations of legislative countermeasures where the evaluations often have not analyzed the level of activity supporting the implementation of the countermeasure in the various States. Clearly, when considering the simultaneous application of more than one countermeasure, one cannot assume that their total impact would be the sum of their individual impacts. For programs involving many countermeasures, this could lead to the absurd conclusion that the program would eliminate more than 100% of the alcohol-crash problem.
Another limitation is that most of the evaluations were of countermeasures that were implemented in just one or a few jurisdictions. Thus, it cannot be assumed that such a countermeasure would be effective in every jurisdiction, regardless of local conditions. Similarly, a finding of no effect in one or two jurisdictions does not necessarily mean that the countermeasure would be ineffective in every jurisdiction. And of course, the lack of any evaluation at all also does not necessarily indicate that a countermeasure is ineffective.
We conclude that currently available hard data on the nature of the alcohol-crash problem are adequate for defining the gross prevalence of alcohol-impaired drivers in fatal crashes. For example, it is known that some 12,500 persons are killed each year in crashes in which one or more drivers had a BAC of .10+. (This figure increases to about 16,000 for crashes in which a driver had a BAC of .01+.) There is also evidence that drivers at BACs much higher than .10 account for a disproportionate share of the alcohol-crash problem. Since virtually all drivers are impaired at .10+ (and recent research indicates impairment and high risk at even lower BACs), using a BAC of .10+ as a measure is reasonable for determining a lower bound to the current magnitude of the problem. Less is known about the role of alcohol in non-fatal crashes, since comprehensive data based on objective measures of impairment (such as driver BAC) do not exist at the national level.
Research also clearly indicates that the size of the alcohol-crash problem in general has declined significantly in recent years, to the point that it can be said that alcohol-related fatal crashes are a smaller societal problem at the millennium than they were 10 or 20 years ago.
The characteristics of persons who drink and drive are also generally better known than they were at the times of prior state-of-knowledge updates. Basic demographic data for such variables as age and sex exist in abundance, and data are starting to appear on ethnic and racial characteristics. From this knowledge, it is more clear than ever that, overall, young drivers, and especially, young White males account for a large share of the alcohol-crash problem. Other demographics are available for certain groups of drinking-drivers (e.g., DWIs), but, except in small studies, generally not for drivers in crashes. Also, the drinking patterns and drinking-driving patterns of drinking drivers are becoming better defined. The role of prior DWI convictions in drinking-driving, is now better understood, indicating that while multiple DWI offenders have higher recidivism rates than first offenders, persons with no priors at all may have the highest involvement in total crashes and in alcohol-related crashes of all degrees of severity. Further, research suggests that repeat DWI offenders and first offenders share many of the same characteristics.
Our review found that pedestrians and bicyclists account for a much smaller, but still highly significant, portion of the alcohol-crash (approximately 1,500 fatally injured pedestrians at .10+ BAC). Data from FARS indicate that fully 34% of fatal pedestrian crashes involved either a pedestrian or a driver whose BAC was .10 or higher, and that very high BACs were common among alcohol-positive pedestrians. The contribution of alcohol-impaired bicyclists to the problem is much lower than that of pedestrians, probably of the order of a few hundred fatalities a year at the .10+ level.
In general, the literature suggests that data from existing research are sufficient for defining broad groups of alcohol-crash targets, but are still inadequate for identifying more narrowly defined target groups. For example, there are sufficient data to say that young male drivers should be a target group, but not enough data to say that young, unemployed males without a college diploma who drive light trucks are an important subgroup to be singled out for special countermeasure action. In a word, more research is needed on the characteristics of alcohol-crash involved drivers and their relative risk. Specific areas where significant knowledge gaps exist and where significant research efforts are recommended are:
Our review reveals that nearly all alcohol-crash countermeasure programs that have been evaluated have focused on the pre-crash phase. Their objective has most often been to reduce driving after drinking, although there has been increasing attention given to reducing excessive drinking before driving. The great majority of programs have used strategies of deterrence and incapacitation carried out by elements of the criminal justice system.
Countermeasures with strong evidence favoring their effectiveness are:
Countermeasures that have shown promise but for which evaluations of alcohol-crash impact are as yet inconclusive are:
While the state of knowledge about ways of dealing with the alcohol-crash problem has grown enormously since the first comprehensive report on alcohol and traffic safety, significant knowledge gaps remain. The most glaring of these is knowledge about the effect of countermeasures that do not rely on the Criminal Justice System. These other countermeasures include approaches focusing on technology, the vehicle, the highway environment, and the more effective control of alcohol consumption. To date, such approaches have either been insufficiently developed, insufficiently evaluated, or both. Two additional areas where significant new knowledge is needed are: countermeasures targeted at specific groups of drinking drivers, (e.g., groups defined by such variables as race, ethnicity, and type of vehicle), and pedestrian countermeasures.
We recommend a coordinated program of countermeasure research and development to fill these gaps. For the short term, the major thrust of operational programs should be on maintaining the 20-year downward trend in alcohol-related crashes. This will require refining current deterrent / incapacitation programs and generating and evaluating new such programs. But concurrently, new approaches will have to be developed, evaluated, and refined for later widespread adoption as the marginal utility of deterrence-based programs becomes exhausted.