In Texas, police officers use the so called Standardized Field Sobriety Tests at the roadside following a traffic stop. The standardized tests involve a three part battery of physical tests.

I. The Horizontal Gaze Nystagmus (HGN) test

In this test, an officer moves a stylus, typically a pen, in front of your eyes for the purpose of determining whether nystagmus is present. Nystagmus is, generally speaking, an involuntary jerking of the eyes. The HGN is a three part test, and the police are looking for the presence of certain "clues" that nystagmus is present.

The police claim that if nystagmus is present, it indicates that you are intoxicated due to the introduction of alcohol into your system. There are several problems with this claim. First, in order for the HGN test to have any validity at all, it must be administered in accordance with the training the police receive. The police are trained to conduct all the standardized field sobriety test according to the protocol developed by the National Highway Traffic Safety Administration (NHTSA). Secondly, nystagmus may be due to sources other than alcohol. Third, even if properly performed, there is no scientifically valid, reliable evidence that a particular result on HGN demonstrates a particular alcohol concentration. In other words, the HGN proves, at most, that a person may have consumed alcohol. It cannot prove the person's alcohol concentration.

A skilled DWI attorney is required to communicate to the jury the limitations of the Horizontal Gaze Nystagmus test.

II. The Walk and Turn

In this test, the officer will instruct you to take nine heel to toe steps on an imaginary line, then pivot as instructed and take nine heel to toe steps back.

In the Walk and Turn test, like the HGN, the officer is looking for supposed clues of intoxication. In this test, there are a possible total of 8 clues. Would it surprise you to learn that if, in the officer's subjective opinion, you scored 2 clues, you have failed the test? That is correct. You may execute six of the eight portions correctly and still be considered to have failed.

The Walk and Turn test is a highly subjective test that must be administered in accordance with the officers training to yield a valid result. In addition, like the HGN, it cannot be reliably correlated to a specific alcohol concentration. In other words, even if you "fail" the Walk and Turn test, it does not prove that you have a specific alcohol concentration.

III. The One Leg Stand

The final test in the battery of standardized field sobriety tests is the One Leg Stand. In this test, the officer asks a person to stand on one leg while counting. The officer is looking to see if the person can maintain that position for thirty seconds without placing their leg back on the ground. Frequently, the police do not inform the subject of the length of time they are expected to maintain their leg in the air.

On this test, the office is judging a person's performance on four "clues." A score of two or more clues is a failure.

The police may also ask you to engage in one or more non standardized tests such as reading from a document, reciting the alphabet, or performing the Rhomberg test.

Challenging the results of the field sobriety tests

I know the weaknesses inherent in the standardized field sobriety tests as well as the methods police must use to administer them properly. In addition, there may be numerous reasons unrelated to intoxication that one might "fail" the sobriety tests. The tests can be effectively challenged. It is important to hire an experienced DWI attorney who knows how to mount an effective challenge.