When Harris County law enforcement officers pull over a driver on suspicion of drunk driving, they typically administer one or more field sobriety tests. The results can be used by the officer to establish probable cause for a DWI arrest, and as evidence in support of a conviction at trial.
There is a standard battery of three field sobriety tests which can be given. The first is known as the walk-and-turn. In this test, the officer directs the suspect to walk a specified number of steps in a straight line, heel-to-toe, and then turn on one foot and walk back along the same straight line. The officer will note whether the suspect steps out of line, fails to keep heels to toes, stops to maintain balance, or uses their arms to stay balanced. Any of these is considered a sign of impairment.
The second test is known as the one-leg-stand, which is exactly what it sounds like. The suspect is told to lift one foot off the ground while counting, for a period of about 30 seconds. Hopping to keep balanced, swaying or using arms for balance, or putting the foot down are considered indicators of impairment.
The third test is the horizontal gaze nystagmus. In this test the officer asks the suspect to follow a flashlight or pen with their eyes as the officer moves it horizontally. An impaired person will often experience an involuntary jerking of the eyes while doing this, and may have trouble tracking the moving object evenly.
There are two major problems with the standard field sobriety tests. First, they depend heavily on the subjective observations of the arresting officer. Second, they fail to take into account other factors that can affect a suspect's ability to perform the tests, such as age, weight and medical condition. An experienced DWI defense attorney can often call into question the results of field sobriety tests, casting doubt on the prosecution's case.
Source: NHTSA.gov, "Development of a Standardized Field Sobriety Test," accessed Oct. 25, 2014