Study on DUI and Accidents


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

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.


Borkenstein, R.B., Crother, R.F., Shumate, R.P., Zeil, W.B. &
Zylman, R. (1974). The role of the drinking driver in traffic accidents
(The Grand Rapids Study). Bluthkohol, 11, 1-131.

Brick, J. (1990). Marijuana. New Brunswick, NJ : Rutgers Center of
Alcohol Studies.

Brick, J., Adler, J., Cocco, K. and Westrick, E. (1992). Alcohol
intoxication: Pharmacokinetic prediction and behavioral analysis. Current
Topics in Pharmacology, 1, 57-67.

MacAvoy, M.G. & Marks, D.F. (1975). Divided attention performance
on cannabis users and non-users following cannabis and alcohol.
Psychopharmacology, 44, 147-152.

National Highway and Traffic Safety Administration. (1985). Alcohol
and highway safety 1984: A review of the state of knowledge (Technical
Report No. DOT-HS-806-569). Washington, D.C. : U.S. Govt. Printing

Perrine, M.W., Peck, R.C. & Fell, J.C. (1989). Epidemiologic
perspectives on drunk driving. Surgeon General’s workshop on drunk
driving (pp. 35-76). Washington, D.C. : U.S. Dept. of Health and Human

Siegel, R.K. (1987). Cocaine use and driving behavior. Alcohol, Drugs
and Driving, 3, 1-9.

Simpson, H.M. (1986). Epidemiology of road accidents involving
marijuana. Alcohol, Drugs and Driving, 2, 15-30.

Smiley, A., Ziedman, K. & Moskowitz, H. (1981). Pharmacokinetics
of drug effects on driving performance: Driving simulator tests of
marijuana alone and in combination with alcohol (Contract 271-76-3316).
Washington, DC: National Institute on Drug Abuse and National Highway
Traffic Safety Administration .

United States Department of Transportation. (1984). Improved sobriety
testing (DOT-HS-806-512). Washington, DC: National Highway Traffic Safety

The White House: President’s Commission on Model State Drug Laws.
(1993). Vol. III, Crimes Code. Washington, DC:Office of National Drug
Control Policy.

Zador, P.L. (1991). Alcohol-related relative risk of fatal driver
injuries in relation to driver age and sex. Journal of Studies on
Alcohol, 52(4), 302-310.

Zylman, R. (1974). A critical review of the literature on “alcohol
involvement” in highway deaths. Accid. Anal. & Prev. , 6,


John Brick, Ph.D., F.A.P.A is the Executive Director of Intoxikon
International, an education and consulting firm located in

Center of Alcohol Studies
607 Allison Road,Piscataway, NJ 08854-8001
Telephone: (732)445-2190
Fax: (732)445-350
CAS Library (732)445-4442
Fact Sheet No. 5 (2)

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