ONLINE FACTS
Driving While Impaired
by John Brick,
Ph.D., M.A., F.A.P.A.
(John Brick is a Biological Psychologist and Executive Director of Intoxikon International, an education and consulting firm located at 1006 Floral Vale, Yardley, Pennsylvania 19067.)
Many factors contribute to motor vehicle related injuries, including vehicle design and function, roadway characteristics (e.g., traffic volume, speed, lighting, weather conditions) and driver characteristics such as experience, gender, personality and health, including state of brain function. These variables should be considered in reaching conclusions about the causes of a crash. For example, unintended acceleration accidents, once thought to be due to faulty throttle or braking systems, are now believed to have been due largely to driver error.
Highways and rural areas are the scene of more fatal crashes than urban roadways, in part, because of higher driving speeds and resulting crash severity on these roadways. Young males have more driving accidents and are at greater risk than young females because of a combination of factors, and older drivers are also at increased risk in comparison to younger drivers because of nervous system changes in sensory perception, reaction time and other age-related changes. However, one variable stands out--up to 40% of all fatal motor vehicle crashes involve alcohol.
The most important driver characteristic necessary for safe motor vehicle operation, and the human variable most quickly altered by a variety of factors, is mental state. Alcohol, other drugs, sleep deprivation, competing tasks (e.g., cell phone use) significantly impair the psychomotor and cognitive skills needed to drive safely.
There are many identified risk factors in DWI crashes. In 1997, one in seven drivers aged 16-20 years of age, and one in four drivers aged 21-24 years old who were involved in fatal crashes had blood alcohol concentrations (BACs) of .10% or more. Men who die in car crashes are twice as likely to be intoxicated at .10% or more, than women. When compared to drivers who have never been arrested for drunk driving, 21-34 year olds who are arrested for drunk driving are four times as likely to die in a future crash due to intoxication. About one in four Americans are likely to be involved in an alcohol-related crash during their lifetime. More drunk driving crashes occur late at night and on weekends.
Alcohol and other central nervous system depressants impair the ability to divide attention among the many factors involved in safe driving. Current studies have demonstrated that impairment at a BAC of .02% is measurable not only in laboratory studies of divided attention, as indicated by prior research, but in field investigations of fatal crashes. Attending to lane position, curves, intersections, traffic control devices, presence of other vehicles, etc., while driving is particularly difficult, even at relatively low levels of intoxication. A BAC of .02% can be obtained in most drinkers after approximately 2 standard drinks in an hour, depending upon various biological factors (e.g., body size, gender, rates of absorption and metabolism). At BACs well below .08-.10%, variability in lane position, increased brake use, decreased steering ability, gear changing and steering errors are commonly detected during closed course driving tests.
Although some studies suggest that there is a relationship between level of intoxication and crash responsibility, some drivers, intoxicated or sober, are involved in crashes due to the mistakes of others, through no fault of their own. Therefore, studies of relative risk based upon single vehicle crash data are particularly useful in demonstrating the multiplicative increase in relative risk for a fatal crash as a function of BAC, younger age and male gender. Table 1 shows the relative risk for a crash for drivers 21-35 years old. Younger drivers (16-20) are at significantly greater risk than illustrated (e.g., at a BAC of .10-.149%, 16-20 year old males have a relative risk for a fatal crash that is about 50-1200 times greater than non-intoxicated controls).
TABLE 1. RELATIVE RISK FOR FATAL CRASH*
AND BIOBEHAVIORAL EFFECTS OF ALCOHOL**
%BAC |
~Relative Risk Males/Females
Ages 21-34 |
Biobehavioral Effects |
.020-.049% |
2 ½ - 3x |
Impaired on some lab tests. Start of increased risk for fatal crash |
.050-.079% |
6-8x |
0.04% and higher defines intoxication in many European countries and for commercial vehicle operators in the US |
.080-.099% |
11-17x |
0.08% defines intoxicated driving in the majority of the states in the US |
.100-.149% |
28-49x |
At 0.10%, most drinkers show impairment in SFSTs and would be by law, intoxicated in about 15 states |
.150+% |
> 343x |
Most people appear visibly intoxicated at 0.15% without special tests |
.30% |
|
Most people lose consciousness above this level |
.35% |
|
Realm of surgical anesthesia |
.40% |
|
Lethal concentration for about half the population |
* Modified from Zador, et al., (2000).
**Modified from Brick and Erickson (1999)
At this time, 34 states (plus the District of Columbia and Puerto Rico) have established that a blood alcohol level of .08% is a per se violation of the drinking-driving laws of that state. A per se statute defines the BAC at which it is presumed that all drivers are intoxicated and cannot drive safely, regardless of actual motor vehicle operation. Most other states use .10% to define intoxicated driving with a clear trend towards .08%. Some states, such as Massachusetts, define intoxication at .08%, but not as a per se violation. Federal law uses a BAC of .04% to define intoxication for commercial vehicle operators, and all states have lower or zero tolerance legal definitions (e.g., zero to .02%) for underage drinkers. Most other countries use BACs of .02-.05% to define intoxicated driving. For drugs other than alcohol, many legislators use the mere presence of the drug, not its concentration in the human body to define impaired or intoxicated driving.
Although beer is the most commonly reported alcohol beverage consumed prior to a crash, the type of beverage bears no relationship to driving impairment. It is the drug ethanol, not the beverage in which it is delivered, that causes impairment. Even though driving ability is significantly impaired at low BACs, the ability to detect impairment due to intoxication without special tests or knowledge does not reliably occur until BACs are very high (e.g., .15% or more).
Numerous behavioral tests are employed by law enforcement officers to determine if someone is impaired, but only one test battery of Standardized Field Sobriety Tests (SFSTs) is specifically designed to detect intoxication at BACs greater than or equal to .10%. The original scientifically validated study for alcohol used three tests: Walk and Turn (68-80% accuracy in determining BAC of.10% or more), One Leg Balance (65-78%), and a test of eye movement, Horizontal Gaze Nystagmus (77-82%). Higher accuracy scores are achieved when tests are combined. When properly administered and scored, and particularly when combined with chemical tests (blood or breath), these tests provide convincing evidence of impairment due to alcohol use. Drug Recognition Evaluations (DREs) which include SFSTs and other measures, may provide evidence that a drug was ingested and psychoactive at the time of testing.
Alcohol and other drugs are a particularly fatal combination, especially among men aged 25-54. Although drugs other than alcohol are involved in about one in four fatal crashes, other factors, including sleep deprivation, have detrimental effects on drivers. For example, over 1300 fatal drowsy-driver crashes occur annually. Preliminary studies suggest that 36 hours of wakefulness, like alcohol intoxication, results in increased driving speed, lane excursions, and crashes.
Impaired driving takes many forms. Future prevention efforts would benefit from increased research and public awareness of this problem.
References:
Brick, J., (2001). Accident reconstruction: biobehavioral factors in unintended acceleration. Forensic Examiner,10 (5/6), 26-30.
Brick, J., Adler, J., Cocco, K., and Westrick, E. (1992). Alcohol intoxication: pharmacokinetic and behavioral analysis. Current Topics in Pharmacology, 1, 57-67.
Brick, J. and Carpenter, J.A., (2001). Identification of alcohol intoxication by police. Alcoholism: Clinical and Experimental Research, 25 (6), 850-855.
Brick, J. and Erickson, C. (1999). Drugs, the brain and behavior: the pharmacology of abuse and dependence. New York: Haworth Medical Press.
Moskowitz, H., Burns, M. and Williams, A.. (1985). Skills performance at low blood alcohol level. Journal of Studies on Alcohol, 46(6), 483-485.
National Highway Traffic Safety Administration. (1993). The incidence and role of drugs in fatally injured drivers. Traffic Tech, No. 57.
National Highway Traffic Safety Administration. (1996). Traffic safety facts, 1995: alcohol. Washington, DC: National Center for Statistics and Analysis.
National Highway Traffic Safety Administration. (1998). Traffic safety facts, 1997:alcohol. Washington, DC: National Center for Statistics and Analysis.
National Highway Traffic Safety Administration. (2000). Relative risk calculated for driver fatalities in alcohol-related crashes. Traffic Tech, No.222.
Peters, R.D., Wagner, E., Alicandri, E., Fox, J.E., Thomas, M.L., Thorne, D.R., Sing, H.C. and Balwinski, S.M. (1999). Effects of partial and total sleep deprivation on driving performance. Public Roads, 62(4). Retrieved December 17, 2002 from http://www.tfhrc.gov/pubrds/janfeb99/effects.htm.
Zador, 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(3), 387-395.
Center of Alcohol Studies
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