DWI Street Science: Introduction

By Allen, Texas Criminal Defense Lawyer Kyle T. Therrian
Office Number: (972) 562-7549
24 Hr Jail Release: (214) 403-6522


Part 1: Horizontal Gaze Nystagmus
Part 2: Walk and Turn
Part 3: One Leg Stand


Rarely does a misdemeanor offense call for the use of scientific evidence. This is probably why you will never see a CSI: Allen or Law and Order Misdemeanor Unit. Putting on scientific evidence can be costly and time consuming for the State, and a cost-benefit decision is usually made to not do things such as take fingerprint or DNA evidence at the scene of a misdemeanor. In addition to it being not worth collecting, there are normally significant issues in presenting this kind of evidence. In most cases, before scientific evidence is presented to a jury, the judge will require supporting testimony from an expert witness. Unfortunately, these general rules of law enforcement and procedure don’t apply to a misdemeanor offense of Driving While Intoxicated.

When a person is stopped for a DWI, the officer will request that the person submit to a series of tests which are designed to determine whether or not a person is likely intoxicated. These tests require the officer to use his training and experience to make conclusions regarding number of scientific or medical phenomena including the physiology of the human eye, short-term memory, and small muscle control. These conclusions will inevitably be part of the police report and the State’s DWI prosecution.

Normally, when a person seeks an expert opinion regarding his or her eyes, memory, or muscles he or she would go to a doctor’s office–not the police station. Among the many reasons that we make this decision is the fact that we recognize that an education in medicine is a minimum requirement in advising and treating medical conditions: 4 years of undergrad, 4 years in medical school, 3-5 years in a residency program.

So what experience does an officer have (or need to have)? Most courts will only require that the officer be certified in administering the Standardized Field Sobriety Tests before allowing the officer to give conclusions about a person’s performance on those tests. The grand total hours required for this certificate is 24–more than 45,000 fewer hours in school than is required to become a doctor.

As a consolation, most courts recognize that officers are merely practitioners of these methods developed by doctors and scientists and require that the officer adhere to standardized techniques. However, if an officer performs the test in any manner that somewhat resembles the proper way to perform the test, their conclusions will most likely be presented to the jury. It then becomes the job of the defendant to demonstrate to the jury why the tests were performed incorrectly.

Understanding what the field sobriety tests are and how officers commonly mess them up is important in almost all DWI cases. The following links are to a series of articles which discuss the three Standardized Field Sobriety Tests:

  1. Horizontal Gaze Nystagmus
  2. Walk and Turn
  3. One Leg Stand

*Kyle Therrian is an attorney licensed to practice in the State of Texas. Nothing in this article is intended to be legal advice. For legal advice on any case you should contact an attorney directly.

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