DWI Science

DWI Science

DWI cases usually rely heavily on scientific evidence. The DWI charge itself is a science-based offense. There are two ways to be “intoxicated” under the law: a blood alcohol content (BAC) above a 0.08% or loss of the “normal use of mental or physical faculties” (learn more about the definition of intoxication).

To collect evidence that your BAC is above 0.08%, police officers use scientific testing in the form of field sobriety tests, breath tests and blood tests. All of these types of evidence have a fundamental basis in science.

The loss of “normal use of mental or physical faculties” type of intoxication is a blurry line, but “normality” itself is a scientific concept. What is objectively “normal” is an empirical question, and one that is certainly subject to debate (and probably different for different people). Usually the state uses the results of the same police-administered standardized field sobriety tests to prove or help prove the “normality” type of intoxication.

In order to fight DWI charges, it is crucial to understand the science behind DWI laws and the science related to all of the evidence and the evidence collection methods. If you took a breath test or a blood test and the state alleges the results indicated a BAC over a 0.08, then in order to best fight the charge, your DWI attorney will need to understand the mechanics of the breath test, proper administration procedures, the science behind the test and the physiology of alcohol consumption.

If you did not take either the breath test or blood test, then your DWI attorney should be familiar with the field sobriety tests that were administered by the police in order to determine whether or not they were properly administered. DWI investigators almost always say the same thing in their reports. Clues like slurred speech, the scent of alcohol and bloodshot eyes are noted in almost every report. So in order to poke holes in the state’s case, you have to know where to look.

There are certain field sobriety tests that have been standardized and validated through rigorous scientific testing. These are known as the standardized field sobriety tests or “SFSTs.” DWI investigators almost always administer these tests, but sometimes they also administer other tests. Learn more about SFSTs

Sometimes it is impractical or even impossible for DWI investigators to administer the standardized tests, so they use other tests that have little or no scientific validation. One circumstance in which this is common is an investigation into a “Boating While Intoxicated’ offense.

Field Sobriety Test Validation

DWI field sobriety tests were initially developed and used by individual police units across the country. Extensive scientific research studies only bega in 1975 when the NHTSA contracted the Southern California Research Institute (SCRI) to assess the reliability and accuracy of these field sobriety tests. SCRI published reports in 1977, 1981 and 1983.

Six tests were used in the initial study, but only three of them, and only when administered in a standardized manner, were highly accurate and reliable for distinguishing a blood alcohol content (BAC) of above a 0.10. These three tests were the Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn (WAT) and the One-Leg Stand (OLS).

NHTSA determined, through an analysis of the lab data provided by SCRI, that the HGN was 77% accurate, the WAT was 68% accurate and the OLS was 65% accurate. When used together, the HGN and WAT achieved 80% accuracy. Of course this means that 20% (one-fifth!) of the time, even the two tests together are inaccurate. And this is, of course, data obtained in a lab by scientists, not police officers.

The final phase of the study was a field validation where standardized procedures were developed. The NHTSA concluded that the three standardized tests were reliable in identifying people with BACs above a 0.10.

Three standardized field sobriety test (SFSTs) validation studies were commenced between 1995 and 1998, including studies in Colorado (1995), Florida (1997) and San Diego (1998). The results of the three studies supported the institutionalization of SFSTs.

The Science of Nystagmus

Nystagmus is a condition where a person’s eyes involuntarily “jerk.” Alcohol and certain other drugs cause Horizontal Gaze Nystagmus. Of course, Nystagmus in and of itself does not increase the likelihood that someone is unable to drive; instead it only indicates the presence of alcohol and certain other drugs. And also, of course, people are able to drive safely with a multitude of drugs in their system. The question, therefore, is whether Nystagmus can reliably be determined to be caused by prohibited drugs and whether that may not rise to the level of impairment.

There are three types of Nystagmus:

  1. Vestibular Nystagmus. Vestibular Nystagmus is caused by movement or action to the vestibular system. Rotational Vestibular Nystagmus is caused when fluid in the inner ear is disturbed when a person is spun around. Post Rotational Vestibular Nystagmus occurs after a person stops spinning around. Caloric Vestibular Nystagmus occurs by changes in temperature. Positional Alcohol Vestibular Nystagmus (PAN) occurs when some fluid enters the body and changes the specific gravity of the blood in unequal concentrations between the blood and the vestibular system.
  2. Nystagmus from Neural Activity. Optokinetic Nystagmus results from fixating on an object that suddenly moves out of sight or from watching sharply contrasting moving images (for instance, watching strobe lights, rotating lights or rapidly moving traffic nearby). Physiological Nystagmus is natural and occurs in everyone, and very frequently. It is the most common type of Nystagmus. It produces very minor tremors and are generally impossible to be seen by the naked eye. Gaze Nystagmus occurs when the eye moves from center position. There are three types of Gaze Nystagmus, including Horizontal Gaze Nystagmus that occurs as the eyes move to the side, Vertical Gaze Nystagmus that occurs when eyes look upward at maximum elevation and Resting Nystagmus that occurs when the eyes look straight ahead.
  3. Nystagmus may also be caused by pathological disorders, including brain tumors, brain damage or particular diseases of the inner ear.

Why did I get arrested if I did well on the roadside tests?

As discussed on our DWI page, the police don’t tell you what clues they are looking for. You may think you did well on the tests, but unless you know what the police were looking for, there’s no way to tell whether you really passed the roadside field sobriety tests. In addition, if they tested your eyes with the HGN test, you can’t see what they saw, so you don’t know whether your eyes displayed any nystagmus.