There are methods law enforcement uses to indicate probable cause that a Texas driver is impaired and should not be behind the wheel.

The series of three standardized field sobriety tests were developed in the 1970s, and according to the AAA DUI Justice Link, when officers conduct them correctly, they are often accurate.

Walk-and-turn

The officer instructs the driver to take a specific number of steps on a straight line touching heels to toes, then turn on one foot and repeat the process going the opposite direction. Being able to remember and follow the instructions precisely without losing balance are the keys to successful completion of this test.

Only 79% of the people who fail this test are likely to be legally impaired, according to research. Health conditions, medication, age or injuries could affect a driver’s ability to complete this test to the officer’s satisfaction.

One-leg stand

A driver who has a blood alcohol content of 0.10% or higher is likely to be unable to stand on one foot with the other in the air while counting to 30 beginning at 1,000.

Research indicates that 17% of other individuals who fail the test will not be impaired, but may instead have other factors that affect their ability to perform the tasks.

Horizontal gaze nystagmus

This test involves following the officer’s pen, finger or flashlight with the eyes. When the eyes get to a certain point, if the driver is impaired, there will be an involuntary jerking movement of the eyeballs that he or she cannot feel, but that the officer can see. Accuracy is about 88%.

According to Healthline, medications, stroke, head injuries, brain tumors, eye diseases, inner ear diseases, vitamin deficiencies, genetic conditions and other health issues also cause nystagmus.