Q&A: Alcohol — general

March 2013

1 What is alcohol-impaired driving?

What many people refer to as "drunk driving" is better described as "alcohol-impaired driving." In fact, many alcohol-impaired drivers do not appear drunk in the stereotypical way. 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. 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 for drivers?

All 50 states and the District of Columbia have per se laws making it a crime to drive with a BAC at or above 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.

4 What is the effect of alcohol on crash risk?

In general, the probability of a fatal crash increases steadily with increasing driver BAC. Fatal crash risk increases substantially after 0.05 percent BAC and climbs more rapidly after 0.08 percent. Voas, R.B.; Torres, P.; Romano, E.; and Lacey, J.H. 2012. Alcohol-related risk of driver fatalities: an update using 2007 data. Journal of Studies on Alcohol and Drugs 73(3):341-50. At all BACs, the fatal crash risk is much higher among 16-20 year-old drivers than among drivers 21 and older. At a BAC of 0.08 percent compared to a zero BAC, the likelihood of involvement in a fatal crash is 10 times as high among 16-20 year-old drivers, 7 times as high among drivers ages 21-34, and 6 times as high among drivers 35 and older. At the same BAC, fatal crash risk is the same for male and female drivers in a given age group.

Relative risk of fatal crash involvement at various BACs compared to zero BAC, passenger vehicle drivers by age

The likelihood of involvement in a crash of any severity also increases steadily with increasing driver BAC. Peck, 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(3):311-9.  The risks for drivers younger than 21 are much higher than the risks for drivers 21 and older across the range of BACs. At a BAC of 0.08 percent compared to a zero BAC, 16-20 year-old drivers are more than 7 times as likely to crash, and drivers 21 and older are about 1.6 times as likely to crash.

5 How many drinks does it take to become significantly impaired?

The effects of alcoholic drinks vary greatly because the rate of absorption and the BAC attained varies from person to person due to factors such as weight, amount of fat tissue and stomach contents. 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.

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.

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.

7 What proportion of motor vehicle crashes involves alcohol?

The most reliable information about alcohol involvement comes from fatal crashes. The proportion of fatally injured drivers with BACs at or above 0.08 percent has remained about a third since 1994 after declining from nearly half in 1982. 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 10,600 deaths in 2010 were directly attributable to alcohol. Lund, A.K.; McCartt, A.T.; and Farmer, C.M. 2012. Contribution of alcohol-impaired driving to motor vehicle crash deaths in 2010. Arlington, VA: Insurance Institute for Highway Safety.   These lives could have been saved if all drivers had BACs of zero. An estimated 7,082 deaths would have been prevented if all drivers with BACs of 0.08 percent or higher were kept off the roads. Applying the same methods yields an estimate of 6,794 preventable deaths if all drivers with BACs of 0.08 or higher were kept off the roads in 2011.

Alcohol involvement is much lower in nonfatal crashes, but it is still quite high. 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. Borkenstein, 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. 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. Farris, 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. More recently, a study conducted during 1996-98 gathered BAC measurements from drivers involved in evening and nighttime crashes of all severities in two cities and from a comparable group of non-crash-involved drivers. Blomberg, R.D.; Peck, R.C.; Moskowitz, H.; Burns, M.; Fiorentino, D. 2005. Crash risk of alcohol involved driving: a case-control study. Stamford, CT: Dunlap and Associates, Inc.  The proportion of crash-involved drivers with BACs of 0.09 percent or higher was 11 percent, compared to 2 percent for non-crash-involved drivers.

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

Alcohol-impaired driving has become less prevalent 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. Lacey, J.H.; Kelley-Baker, T.; Furr-Holden, D.; Voas, R.B.; Romano, E.; Torres, P. Tippetts, A.S.; Ramirez, A.; Brainard, K.; and Berning, A. 2009. 2007 National Roadside Survey of Alcohol and Drug Use by Drivers. Report No. DOT HS-811-248. Washington, DC: National Highway Traffic Safety Administration.  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 proportion of fatally injured drivers with BACs at or above 0.08 percent has remained about a third since 1994 after declining from nearly half in 1982.

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 2011, 38 percent had BACs at or above 0.08 percent. The corresponding proportion among female drivers was 20 percent. Alcohol impairment in fatal crashes was highest for males ages 21-40. In 1982, the proportion of fatally injured passenger vehicle drivers with BACs at or above 0.08 percent was 56 percent among males and 33 percent among females. Male drivers are 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. Lacey, J.H.; Kelley-Baker, T.; Furr-Holden, D.; Voas, R.B.; Romano, E.; Torres, P. Tippetts, A.S.; Ramirez, A.; Brainard, K.; and Berning, A. 2009. 2007 National Roadside Survey of Alcohol and Drug Use by Drivers. Report No. DOT HS-811-248. Washington, DC: National Highway Traffic Safety Administration.  Both percentages were lower than in 1996, when 5 percent of males and 3 percent of females had illegal BACs. Insurance Institute for Highway Safety. 2012. [Unpublished analysis of data from the U.S. Department of Transportation's and Insurance Institute for Highway Safety’s1996 National Roadside Survey of Alcohol Use by Drivers and  the U.S. Department of Transportation’s 2007 National Roadside Survey of Alcohol and Drug Use by Drivers]. Arlington, VA.

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 p.m. and 6 a.m. in 2011, 60 percent had BACs at or above 0.08 percent compared with 18 percent during other hours. Forty-six percent of all fatally injured passenger vehicle drivers on weekends (from 6 p.m. Friday to 6 a.m. Monday) in 2011 had BACs at or above 0.08 percent. At other times the proportion was 24 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, treatment or education. The underlying premise is that many, if not most, of these people are problem drinkers. Simpson, H.M. and Mayhew, D.R. 1991. The hard core drinking driver. Ottawa, Ontario: Traffic Injury Research Foundation. The term is not precisely defined, but hard-core drinking drivers are often identified as those with prior alcohol-impaired driving convictions or very high BACs (0.15 percent or higher) at the time of arrest for 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. Among passenger vehicle drivers with illegal BACs (0.08 percent or higher) who died in crashes in 2011, 25 percent had BACs lower than 0.15 percent.

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

No, although people with prior convictions for alcohol-impaired driving are overrepresented among drivers in fatal crashes. According to a federal study, drivers convicted of alcohol-impaired driving during the past three 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). Fell, JC. 1993. Repeat DWI offenders: their involvement in fatal crashes. In: Utzelmann H-D, Berghaus G, Kroj G, eds. Alcohol, Drugs and Traffic Safety – T92: Proceedings of the 12th International Conference on Alcohol, Drugs and Traffic Safety, Cologne, Germany, September 28 – October 2, 1992. Cologne, Germany: Verlage TÜV Rheinland GmbH; 1044-9.

In 2011 8 percent of drivers in fatal crashes with BACs of 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 three 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 Do we need a separate set of policies to address the problem of hard-core drinking drivers?

Some have argued that hard-core drinking drivers are resistant to countermeasures such as sobriety checkpoints and administrative license suspension that have reduced alcohol-impaired driving overall. In response, some states have passed laws mandating stiffer penalties for repeat offenders or drivers with very high BACs (e.g., 0.15 percent or higher).

However, hard-core drinking drivers have not been immune to the general deterrence efforts of the past three decades. Between 1982 and 2011, there were large declines in all categories of illegal BACs 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. 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 three years. This percentage declined to 8 percent during 2007-2011. Previous alcohol convictions reflect both driving behavior and law enforcement patterns.

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. Some countermeasures aimed at the hard-core group have been effective in reducing recidivism Williams, A.F.; McCartt, A.T.; and Ferguson, S.A. 2007. Hardcore drinking drivers and other contributors to the alcohol-impaired driving problem: need for a comprehensive approach. Traffic Injury Prevention 8(1):1-10. , and these countermeasures also may be effective in reducing recidivism among other offenders. Attention and resources also need to be given to general deterrent initiatives.