Q&As: Alcohol — general

January 2010

1 What is drunk driving?

This term is an inaccurate characterization of the problems caused by motorists who are impaired by alcohol. The first criminal laws targeting this problem prohibited "drunk driving," encouraging the notion that the problem was restricted to drivers who were visibly drunk. In fact, many alcohol-impaired drivers do not appear drunk in the traditional way. Research has shown that even small amounts of alcohol can impair the skills involved in driving, but the persistent notion that the problem is predominantly one of drunk drivers has allowed many drinking drivers to decide they are not part of the problem. For these reasons, the term "alcohol-impaired driving" is a more accurate and precise description of what is commonly referred to as drunk driving.

2 What does blood alcohol concentration (BAC) measure?

A BAC describes the amount of alcohol in a person's blood, expressed as weight of alcohol per unit of volume of blood. For example, 0.08 percent BAC indicates 80 mg of alcohol per 100 ml of blood. For most legal purposes, however, a blood sample is not necessary to determine a person's BAC. It can be measured more simply by analyzing exhaled breath.

3 What BAC is considered illegal?

All 50 states and the District of Columbia have per se laws defining it as a crime to drive with a BAC at or above a proscribed level, 0.08 percent. In all 50 states, drivers younger than 21 are prohibited from operating a vehicle with any detectable blood alcohol. Most states define this as a BAC at or above 0.02 percent but less than 0.08 percent.

4 What is the effect of alcohol on driving skills and crash risk?

The probability of a fatal crash increases significantly after 0.05 percent BAC and climbs more rapidly after 0.08 percent.1 Drivers with very high BACs (at or above 0.15 percent) have a very high risk of being involved in a police-reported crash or dying in a crash.1,2

5 How many drinks does it take to reach significantly impairing BACs?

The effects of alcoholic drinks vary greatly because the rate of absorption and BACs attained vary from person to person due to factors such as weight, amount of fat tissue, and stomach contents. Nevertheless, various organizations have developed charts intended to help people estimate their BACs based on the number of drinks consumed. These tables can be used to estimate BACs, but they are subject to error. Some people can be substantially impaired after two drinks. In addition, women can attain higher BACs and become more impaired than men who weigh the same and consume the same amount of alcohol because alcohol is processed differently by women and men.

6 Are beer and wine less impairing than hard liquor?

Impairment is not determined by the type of drink but rather by the amount of alcohol ingested over a specific period of time. There is a similar amount of alcohol in such standard drinks as a 12-ounce glass of beer, a 4-ounce glass of wine, and 1.25 ounces of 80-proof liquor. Beer is the most common drink consumed by people stopped for alcohol-impaired driving or involved in alcohol-related crashes.3

7 What proportion of motor vehicle crashes involves alcohol?

The most reliable information about alcohol involvement comes from fatal crashes. In 2008, 35 percent of fatally injured passenger vehicle drivers had BACs at or above 0.08 percent. Pedestrians had the highest proportion of BACs at or above 0.08 percent (38 percent). Such statistics do not mean that a third or more of all fatal crashes are caused solely by alcohol, because alcohol may be only one of several factors that contribute to a crash involving drinking drivers. An Institute study estimated that 13,452 deaths in 2005 were directly attributable to alcohol. These lives could have been saved if all drivers had been restricted to no blood alcohol. An estimated 8,916 deaths would have been prevented if all drivers on the road had BACs below 0.08 percent.4 Applying the same methods yields an estimate of 8,104 preventable deaths if BACs were below 0.08 percent in 2008.

Alcohol involvement is much lower in crashes involving nonfatal injuries and lower still in crashes that do not involve injuries at all. A study conducted during the 1960s estimated that 9 percent of drivers in injury crashes and 5 percent of drivers in noninjury crashes in Grand Rapids, Michigan, had BACs at or above 0.10 percent.5 A 1977 study found that 12 percent of drivers in injury crashes in Huntsville, Alabama, and San Diego, California, had BACs at or above 0.10 percent, compared with 1 percent of a sample of drivers not involved in crashes.6 According to a research review conducted during this same period, studies of fatally injured drivers found that 40-55 percent had BACs at or above 0.10 percent.7

8 How has the prevalence of alcohol-impaired driving changed over time?

The prevalence of alcohol-impaired driving has been reduced but remains a major problem. In 2007, the National Highway Traffic Safety Administration (NHTSA) undertook a national roadside breath survey in which data were collected during weekend nights. Patterned after 1996, 1986, and 1973 surveys, the 2007 survey found that 2.2 percent of drivers had BACs at or above 0.08 percent. This compares with 4.3 percent in 1996, 5.4 percent in 1986, and 7.5 percent in 1973.8 According to NHTSA, the percentage of drivers with any detectable alcohol in their systems declined almost as much from 1973 to 2007 as the percentage of drivers with BACs at or above 0.08 percent.

Although the roadside surveys suggest that the prevalence of alcohol-impaired driving has gone down over time, the percentage of fatally injured drivers with BACs at or above 0.08 percent has remained about a third since 1994. The percentage of fatally injured drivers with BACs at or above 0.08 percent declined steadily from 49 percent in 1982 to 33 percent in 1994 (a 32 percent decline).

9 Does alcohol-impaired driving differ by gender?

Crashes among male drivers are much more likely to involve alcohol than those among female drivers. Among fatally injured male drivers of passenger vehicles in 2008, 40 percent had BACs at or above 0.08 percent. The corresponding proportion among female drivers was 21 percent. Alcohol involvement in fatal crashes was highest for males ages 21-40. Between 1982 and 2008, the proportion of fatally injured passenger vehicle drivers with BACs at or above 0.08 percent declined from 56 to 40 percent among males and from 33 to 21 percent among females. Male drivers were significantly more likely to have illegal BACs than female drivers (2.6 percent versus 1.5 percent) according to the 2007 national roadside alcohol survey.8 Compared with 1996, a lower percentage of male drivers had illegal BACs than in 2007 (3.5 percent in 1996). The percentage of female drivers with illegal BACs did not change between 1996 and 2007.

10 When do alcohol-impaired driving crashes occur?

They happen at all hours, but alcohol involvement in crashes peaks at night and is higher on weekends than on weekdays. Among passenger vehicle drivers who were fatally injured between 9 pm and 6 am in 2008, 60 percent had BACs at or above 0.08 percent compared with 20 percent during other hours. Forty-nine percent of all fatally injured drivers on weekends (6 pm Friday to 6 am Monday) in 2008 had BACs at or above 0.08 percent. At other times the proportion was 25 percent.

11 Who qualifies as a "hard-core drinking driver"?

The term was coined to refer to people who repeatedly drive while impaired and are resistant to changing their behavior despite previous sanctions, treatments, or education. The underlying premise is that many, if not most, of these people are problem drinkers.9 The term is not precisely defined, although two criteria commonly used to identify hard-core drinking drivers are prior alcohol-impaired driving convictions and very high BACs (0.15 percent or higher) at the time of arrest for alcohol-impaired driving. Hard-core drinking drivers have drawn extra attention in recent years as policymakers have tried to renew progress against alcohol-impaired driving. The concept of hard-to-change chronic heavy drinking drivers ignores many who account for a large portion of alcohol-impaired driving crashes. These include drivers who drink heavily on occasion and drivers who drink at more moderate levels that elevate crash risk. Some countermeasures aimed at the hard-core group have been effective in reducing recidivism, but attention and resources also need to be given to general deterrent initiatives (e.g. sobriety checkpoints, administrative license suspension).10 As described below, general deterrence has been shown to be effective in reducing alcohol-impaired driving for all BAC categories.

12 Are most alcohol-impaired driving crashes caused by repeat offenders?

No. It is true that people with prior convictions for driving while impaired by alcohol are overrepresented among drivers in fatal crashes. According to a federal study, drivers convicted of alcohol-impaired driving during the past 3 years are at least 1.8 times as likely to be in fatal crashes as drivers with no prior convictions during the same time period and are at least 4 times as likely to be in fatal crashes in which drivers have high BACs (0.10 percent or higher).11

However, in 2008 only 8 percent of drivers in fatal crashes with high BACs (0.08 percent or higher) had previous alcohol-impaired driving convictions on their records. The actual incidence of previous convictions could be higher, because information on convictions was available for only the prior 3 years. In addition, some alcohol offenses are not included on driver records because of court programs that allow drivers to remove or avoid a conviction if they attend educational programs. Still, most alcohol-impaired driving fatal crashes do not involve drivers with a long history of multiple alcohol convictions.

13 Is the problem of alcohol-impaired driving limited to people with very high BACs?

No. Among passenger vehicle drivers with illegal BACs (0.08 percent or higher) who died in crashes in 2008, 28 percent had BACs lower than 0.15 percent.

14 Have hard-core drinking drivers been unaffected by countermeasures directed at all drivers?

No. It often is suggested that alcohol-impaired driving crashes are increasingly caused by people with very high BACs and repeat offenders. However, between 1982 and 2008, all categories of illegal BACs declined among fatally injured passenger vehicle drivers: 0.08-0.14 percent, 0.15-0.19 percent, 0.20-0.24 percent, and 0.25 percent and higher. In 1982, 14 percent of fatally injured passenger vehicle drivers had BACs of 0.08-0.14 percent compared with 8 percent in 2008. The percentage of fatally injured passenger vehicle drivers with BACs of 0.15 percent or higher declined from 37 percent in 1982 to 27 percent in 2008. Most of the declines occurred between 1982 through the mid-1990s. Since then, the percentages have stayed about the same or risen slightly for all BAC categories.

Previous alcohol convictions reflect both driving behavior and law enforcement patterns. During 1991-1995, about 12 percent of fatally injured passenger vehicle drivers with BACs at or above 0.08 percent had alcohol convictions during the previous 3 years. This percentage declined to 8 percent during 2006-2008. These statistics do not support the claims that hard-core drinking drivers have become a larger part of the problem or that they have been unaffected by countermeasures directed at all drivers.

References

¹Zador, P.L.; Krawchuk, S.A.; and Voas, R.B. 2000. Alcohol-related relative risk of driver fatalities and driver involvement in fatal crashes in relation to driver age and gender: an update using 1996 data. Journal of Studies on Alcohol 61:387-95.

²Peck, R.C.; Gebers, M.A.; Voas, R.B.; and Romano, E. 2008. The relationship between blood alcohol concentration (BAC), age, and crash risk. Journal of Safety Research 39:311-19.

³Smart, R.G. 1996. Behavioral and social consequences related to the consumption of different beverage types. Journal of Studies on Alcohol 57:77-84.

⁴Lund, A.K.; McCartt, A.T.; and Farmer, C.M. 2007. Contribution of alcohol-impaired driving to motor vehicle crash deaths in 2005. Proceedings of the 18th International Conference on Alcohol, Drugs and Traffic Safety (CD-ROM). Oslo, Norway: International Council on Alcohol, Drugs, and Traffic Safety.

⁵Borkenstein, R.F.; Crowther, R.F.; Shumate, R.P.; Ziel, W.B.; and Zylman, R. 1964. The role of the drinking driver in traffic accidents. Bloomington, IN: Department of Police Administration, Indiana University.

⁶Farris, R.; Malone, T.B.; and Kirkpatrick, M. 1977. A comparison of alcohol involvement in exposed and injured drivers. Report no. DOT HS-400-954. Washington, DC: National Highway Traffic Safety Administration.

⁷Jones, R.K. and Lacey, J.H. 2001. Alcohol and highway safety 2001: a review of the state of knowledge. Report no. DOT HS-809-383. Washington, DC: National Highway Traffic Safety Administration.

⁸Compton, R. and Berning, A. 2009. Results of the 2007 National Roadside Survey of Alcohol and Drug Use by Drivers. Report no. DOT HS-811-175. Washington, DC: National Highway Traffic Safety Administration.

⁹Simpson, H.M. and Mayhew, D.R. 1991. The hard core drinking driver. Ottawa, Ontario: Traffic Injury Research Foundation.

¹⁰Williams, A.F.; McCartt, A.T.; and Ferguson, S.A. 2006. Hardcore drinking drivers and other contributors to the alcohol-impaired driving problem: need for a comprehensive approach. Traffic Injury Prevention 8:1-10.

¹¹Fell, J.C. 1991. Repeat DWI offenders: their involvement in fatal crashes. Washington, DC: National Highway Traffic Safety Administration.