Drivers
under the influence of alcohol are thought to be responsible for half the fatal
traffic accidents in the U.S. To
curtail this loss of life, it is imperative to have a means of detecting
whether drivers are impaired. Several models of driving under the influence are
used in Texas.
For people your age (18-24) in the U.S., 51% of the 8242 traffic deaths in 2001 involved alcohol. About 32% of that age group was in college, so an estimated 1349 traffic deaths of college students involved alcohol. The rate was 15 per 100,000, or about 7 per UT population. (This value is about twice that of the national average for suicides.) From another perspective, there is about a 1% chance that at least one of your classmates in your Bio301D section will die in an alcohol-related traffic accident this semester.
To put these
numbers into perspective, the traffic deaths in your age group is about the
same level as for U.S. personnel who died each year during the main 8 years of
the Vietnam War. The turmoil
caused by U.S. protests against the Vietnam War caused probably the most
extreme social disruption of the post WWII generation. There has never been much of a protest
against the same magnitude of traffic fatalities.
The
solution: deterrence
Alcohol is such
an integral part of our society, especially in social gatherings, that people
have not voluntarily avoided driving after drinking. In 2001, nearly 1/3 of college students in the U.S. reported
driving while under the influence of alcohol. To increasingly discourage “driving under the influence,” we
have increased the chance of being caught and increased the penalties. Nor are these tactics limited to the
U.S. – Canada and many European countries are very aggressive about catching
impaired drivers.
The ideal
model of impairment
Once it is
decided that driving under the influence (DUI) is unacceptable (i.e.,
criminal), we face the problem of establishing criteria for being impaired
while driving. From our
perspective in this class, we need models of DUI. The main issue is a person’s
ability to drive safely, so if we were to consider the most accurate model of DUI,
it would include the driver’s performance in
·
coordination
·
judgement
·
reaction
time
It would be
great to have a model of DUI that included each of these criteria, but we
don’t, although as we will see below, one model adopts some of these criteria.
(Note that there is a legal distinction in Texas between DUI and DWI – the
latter means driving while intoxicated – but for our purposes here, we are not
concerned with the distinction.
DWI is the more serious offense; DUI is reserved for drivers under 21
and does not require the same level of proof as DWI.)
The reason that
we don’t have the perfect model of DUI is the usual problem with all of our
models – all models have limitations.
In particular, it is not practical to administer a test that covers all
of these criteria, and it would probably be difficult to measure these
behaviors objectively. But you
might live to see a test of this sort in the future, administered as a video
game in a police car to test your ability to drive in a simulation. While such a scenario might seem
far-fetched, the concept of a breathalyzer was equally unimaginable forty years
ago.
Texas law
The law in our
state is both vague and specific about what constitutes impaired driving. The law (penal code 49.04) considers a
driver to be legally impaired when
i)
not
having normal use of physical faculties or mental faculties, or
ii)
having
a blood alcohol concentration (BAC) of 0.08% or greater.
Older laws in
some states used a BAC threshold of 0.15, later down to 0.10%, and most now use
0.08%. In Texas, a BAC may be
measured in blood (gm EtOH in 100 mL of blood), breath (gm EtOH in 210L
breath), or urine (gm EtOH in 67mL urine).
The vague model
in this law is (i), lacking “normal use of physical or mental faculties.” It is vague, because there is no
criterion for “normal use.” The
test that is used to assess these behaviors is the Standardized Field Sobriety
Test (SFST), although it is not part of the penal code. It typically consists of 3 parts
administered where the driver is apprehended:
(A) The Walk and Turn test (WAT)
(B) The One Leg Stand (OLS)
(C) Horizontal Gaze
Nystagmus (HGN)
We will return
to these tests in the section on Data, but for now the WAT test consists of
walking along a straight line for 9 steps, turning around in a specific way,
and returning along the line for 9 steps.
The OLS test consists of standing on one leg, arms at sides, for about
30 seconds, while counting. These
two tests are tests of coordination and ability to follow directions. The HGN test is a measure of the involuntary
behavior of your eyes as they track an object to the side of your field of
vision. Each of these tests is
scored according to a strict set of criteria that includes following
directions.
Appraising
models of DUI
Many of you can
relate to the limitations of the models of DUI, but it is also important to
acknowledge the benefits:
Model |
Strengths |
Limitations |
BAC of 0.08%
using blood |
easy to
obtain accurate reading |
one threshold
does not produce the same level of impairment in all people |
is an
objective criterion |
||
BAC of 0.08%
using breath |
easy to
obtain accurate reading of breath alcohol |
one threshold
does not produce the same level of impairment in all people |
is an
objective criterion |
breath
concentration may differ from blood concentration |
|
SFST |
performance
is relevant to driving impairment |
scoring is
subjective |
easy to
administer; no equipment required |
performance is
affected by many factors unrelated to driving (road surface, physical
properties of the person, age, shoes) |
|
no baseline
data exist for each person |
Perhaps the
major limitation of any legal definition of DUI is that there are no
gradations, because the legal system either finds you guilty or not. Impairment is instead a graded property
of a person’s behavior. A person
at 0.070% may be legal to drive, but they obviously will not be as safe as at
0.04%, and even that will not be as safe as 0.01%. The law needs to set a threshold, but that threshold is a
compromise which allows some impairment, at least in some people. So the legal definition of DUI is not
an accurate model of impairment.
Suppose you are stopped while driving soon after leaving a
restaurant, where you had a meal and 2 beers (or 1-2 glasses of wine). The SFST
ordeal takes two hours. You pass
(maybe they don’t tell you this.)
But then, in a moment of overconfidence, you blow for the
breathalyzer. It comes out at
0.04, and you breathe a sigh of relief.
Off the hook, you think.
Not necessarily ....
A new tactic in some parts of Texas and perhaps throughout
the U.S. is to back-calculate your BAC at the time you were stopped. If you were 0.04 some 2-3 hours after
you last consumed alcohol, you may have been over 0.08 when you were stopped
even if you were under 0.08 when you blew.
When people quickly consume alcohol on an empty stomach, a
common pattern is that the BAC spikes soon after the alcohol is consumed and
then the BAC beings a linear decay toward zero (this pattern is called a
Widmark plot):
You can
easily see how simple it would be
to do the back calculation if you had one measure out near the right end (and
knew the slope of the line). And
this is what the courts are starting to do.
In this case, model accuracy is critical. Use of the Widmark plot is certainly
‘convenient,’ but if it is not accurate an accurate backwards measure of BAC,
then most of us would think it should
not be used to decide someone’s guilt or innocence. It may come as no surprise to you that the model is not an
accurate representation of BAC,
largely because people and the circumstances under which it is applied, are not
uniform. Measurements of breath
alcohol content from people given known amounts of alcohol (of various types)
have revealed that
i)
the
Widmark curve does not always apply, and even when it does,
ii)
there is
considerable variation in the time of the peak and slope of the decay.
From this
work, it seems almost impossible to make reasonably accurate back
calculations. This limitation of
the model has not stopped its use in court; in Texas, higher courts have
even overturned lower courts’ rejections of the method.
What this
means is that a person could maintain their BAC well below 0.08 and still be
convicted of DWI. Of course, this
same outcome could happen from poor performance on the SFST.
Copyright 2006
Craig M. Pease & James J. Bull