Study on DUI and Accidents

FACTS ON: DRIVING WHILE INTOXICATED

By John Brick, Ph.D., F.A.C.A.

Traffic accidents account for more deaths of individuals between age 6 and 33 than any other type of accident. Although many factors contribute to highway accidents including highway and vehicle design, traffic volume, driver characteristics (e.g., experience, personality), and the mechanical function of the motor vehicle, the single most significant predictor of an accident is the driver's state of sobriety. The results from experimental and epidemiological studies indicate that alcohol intoxication greatly increases the risk of accidents. Many sources state that about half of all fatal traffic accidents involve alcohol; however, such reporting should not be interpreted to mean that alcohol intoxication caused the accident. The fact that a driver is intoxicated does not mean s/he caused the accident. However, when the intoxicated driver is assumed responsible, the relative risk of "crash probability" clearly increases as a function of blood alcohol concentration or %BAC (see figure). For example, in 1992, alcohol-related accidents resulted in approximately 20,000 fatalities.

Driving under the influence of drugs other than alcohol may increase the relative risk of a motor vehicle accident as well but epidemiological studies similar to those conducted with alcohol are not yet available. In those field studies in which marijuana was detected, for example, alcohol was often present in high enough amounts so that alcohol intoxication alone could have accounted for the accident. Even so, many of the effects of marijuana (e.g., memory lapses, distortion of time) are likely to interfere with the ability to drive safely.

The effects of cocaine on driving ability and risk for accidents are equivocal at best. Although cocaine may alter vision (e.g. hallucinatory "snow lights", sensitivity to light) and mood (euphoria, depression, paranoia, etc.) at the present time it is speculative as to whether such effects are present to the extent that they will affect driving. In the future, drug screens may routinely be given following all accidents and more will be learned regarding the relationship between cocaine use and other drugs on driving ability. Other related but less well-studied public health risks include pedestrian, bicycling, and watercraft accidents that occur during intoxication. Because alcohol use is many times more prevalent than other drugs, much more is known about the relationship between alcohol intoxication and driving. Generally, alcohol is a central nervous system depressant that causes a dose-dependent decrease in cognitive and motor functioning. As the blood alcohol level rises, the signs and symptoms of alcohol intoxication increase in number and intensity so that laws restricting drinking and driving are necessary.

In response to the many problems that befall society as a result of drinking and driving, most states have what is called a –per se†statute. A per se statute defines the BAC at which it is presumed that all drivers are intoxicated and cannot drive safely. Blood alcohol levels are usually measured and defined by most legislatures as the number of grams (g) or milligrams (mg) of alcohol in 100 milliliters of blood (100 milliliters is usually called a deciliter or dl). Most law enforcement agencies use instruments that measure alcohol in breath and in some states the drinking/driving statute is written in terms of blood or breath alcohol concentration.

Persons can be convicted of intoxicated driving under a per se conviction regardless of their actual driving performance at the time of arrest. The majority of states use .10 grams % (the same as 100 mg/dl) as prima facie evidence for drunk driving (i.e., .10% alone is enough evidence to justify a conviction). At the present time California, Maine, Oregon, Utah, and Vermont use .08% as a per se violation, and most European countries have adopted a blood alcohol level of .05%-.08% to define legal intoxication. It has been recommended that all states make it a per se criminal offense to drive with a blood alcohol level of .08% or more. For drivers under the age of 21, some states have a "zero tolerance law" in which the standard for legal intoxication is .02%.

More is known about the relationship between alcohol intoxication and driving than about driving and any other drug. While people of all types and backgrounds drink and drive, the most consistent profile for drivers arrested for driving under the influence of alcohol is that they tend to be men, 20-40 years of age with some history of problem drinking. About twice as many men than women are involved in fatal motor vehicle accidents when the blood alcohol level is greater than .10%. Although beer is reported as the most commonly consumed beverage prior to the crash, the type of beverage alcohol bears no relationship to driving impairment. Other statistical correlations with driving while intoxicated include being divorced or separated and having a low income, and previous DWI arrests and moving violations involving the use of alcohol. Consistent with drinking patterns and driving after a night partying, most drinking/driving accidents occur late at night and on weekends.

Laboratory studies designed to evaluate the effects of alcohol on psychomotor performance reveal that alcohol affects vision, eye-hand coordination and reaction time only at moderate to high doses (generally at blood alcohol levels well above .10%). However, the most relevant driving-related behaviors affected by alcohol are probably divided attention tasks. Alcohol impairs the ability to attend and respond to complex stimuli at very low BACs ( e.g., .02%†.03%). It is believed that such laboratory findings translate well to real-world driving situations where it is necessary to pay attention to many different events, such as road and traffic conditions, speed, traffic control devices, lane position, pedestrians, other vehicles, roadway signs, etc., often in a nearly simultaneous fashion.

On closed-course driving tests, at BACs of about .06%-.09%, variability in lane position and brake use increased and steering ability decreased. Using a driving simulator approach, it has been found that at BACs approaching .10%, there are increases in braking (up to 45%), and gear changing and steering errors. In some laboratory tests, alcohol produces impairment on complex tests when blood levels are as low as about .02%. Epidemiological data indicate that the risk for an accident begins to increase at blood alcohol levels above .05% so that at .08% the risk of an accident is about 5 times greater than when sober. However, the probability of being innocently involved in an accident remains level and does not increase with increasing BACs. In other words, alcohol intoxication, even at high levels, does not significantly change the relative risk of an accident when other drivers or factors are found to be responsible for the accident.

Numerous behavioral tests are used to determine if an individual is intoxicated. Often these tests are not quantitative. Three tests recommended by the U.S. Department of Transportation (DOT) and commonly used to screen suspected intoxicated drivers at the roadside were designed to discriminate drivers above .10%. The recommended tests are: the Horizontal Gaze Nystagmus, Walk and Turn Test and One Leg Balance Test. The nystagmus test measures eye movements during a simple tracking task. Alcohol intoxication (as well as some neurological conditions) may result in jerky eye movements. The Walk and Turn Test measures the ability to walk a straight line in a heel-to-toe manner, turn on one foot and walk back. Alcohol intoxication may result in walking off the line, inability to turn and failure to follow instructions (e.g., keeping arms at sides, walking a fixed number of steps). The One Leg Balance Test is a divided attention-type task that measures the ability to balance with one leg raised six inches off the ground while counting aloud rapidly from 1001-1030. Alcohol intoxication may make it difficult to stand without putting your foot down and/or making counting errors. According to the DOT, when properly administered and scored, these psychophysical tests have the following accuracy in determining intoxication above .10% BAC: Nystagmus Gaze: 77-82%, Walk and Turn: 68-80%; One Leg Balance: 65-78%. For law enforcement purposes, behavioral tests coupled with an objective blood or breath alcohol test, provide convincing evidence of impairment due to alcohol use.

When a laboratory test is administered to an intoxicated person it is fairly easy to detect intoxication. However, in the absence of specific tests, it is difficult to reliably detect intoxication until the person is well above most current legal limits for intoxication. Reliable signs and symptoms of alcohol intoxication will not be present until the blood alcohol level reaches about .15%. Above this level, signs and symptoms of intoxication may include one or more of the following: stumbling, inability to walk or stand normally, and changes in speech, affect, or various psychomotor skills, depending upon individual differences and actual blood alcohol level. At a blood alcohol level of .15%, the relative risk for an accident is nearly 20 times greater than when sober. In other words, when you see someone that clearly –looks†drunk, that person will not be able to safely drive a car regardless of how they feel or their previous drinking experiences.

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Tags: Fatalities & Accidents