Police who suspect a motorist of drunk driving typically conduct field sobriety tests to justify an arrest and gather evidence for a court case. The horizontal gaze nystagmus test is one of three standardized field sobriety tests recognized by the National Highway Transportation Safety Administration (NHTSA). Texas DUI / DWI lawyer David Hunter is a certified field sobriety test instructor who has the knowledge needed to aggressively challenge the horizontal gaze nystagmus test or any other field sobriety test.

The term horizontal gaze nystagmus refers to the involuntary jerking of the eyes as they move from side to side. The test was designed on the principle that an individuals eyes jerk more quickly with higher blood alcohol concentrations.

The horizontal gaze nystagmus test begins when the officer directs the driver to follow a small object such as the tip of a pen using only the eyes. As the driver performs the test, the officer will watch for sustained jerking and uneven eye movement when the eye reaches the furthest point. The officer also will note if the eye begins jerking before it reaches a 45-degree angle.

Each of these three occurrences is considered a “clue” if it is observed, and police will count the number of clues in each eye. If the officer spots four of the six clues, he or she is trained to believe that there is a 77 percent chance that the driver’s BAC exceeds .10 percent, and a driving while intoxicated arrest will follow.

Even though the horizontal gaze nystagmus test is standardized by the NHTSA, it’s definitely not a foolproof gauge of alcohol intoxication. Nystagmus, or involuntary jerking of the eye, occurs in everyone, regardless of alcohol consumption. Alcohol and drug use can intensify nystagmus, but so can many other factors, including injury or illness.

Field sobriety tests like the horizontal gaze nystagmus test are purportedly designed to detect the symptoms of mental and physical impairment to support a DUI / DWI charge, but they are inherently flawed. Here’s why – experts agree that mental impairment always occurs before physical impairment when an individual drinks alcohol. Physical impairment can be masked by drivers with a high alcohol tolerance, but mental impairment cannot be hidden. Therefore, physical impairment such as involuntary eye jerking without any corresponding mental impairment cannot be considered a symptom of intoxication.

Many police officers don’t even administer the horizontal gaze nystagmus test properly, so their conclusions may be inadmissible in court. The driver’s head and body should be facing the test object for the exercise to be valid. If the officer had the accused drunk driver perform the test through the car window while the motorist remained in the vehicle, the test was improperly conducted, because the driver’s head was turned at a 45-degree angle.

Fortunately, field sobriety test evidence doesn’t always hold up in court. The officer’s conclusions are extremely subjective and can be often be successfully challenged during an aggressive cross-examination. An experienced Texas DUI / DWI attorney will challenge the results of the horizontal gaze nystagmus test as part of an effective defense strategy.