Field sobriety tests are not without their problems. To start, the officer doesn't have a baseline to know how a driver would perform the walk-and-turn, one-leg stand and horizontal gaze nystagmus tests in the most ideal of circumstances—he or she certainly doesn't know other extenuating factors (besides intoxication) that could have an impact on the driver's performance. In fact, there are a number of physical and mental matters that could cause someone to fail the field sobriety tests.
There are certain people who should not be asked to perform the walk-and-turn and one-leg stand tests: those over the age of 65, more than 50 pounds overweight or who have a physical disability that would preclude them from doing so.
There are other physical impairments that can make performing these tests difficult. For example, someone with an inner ear condition would have difficulty maintaining the balance required for some field sobriety tests. Back, leg and foot injuries could also make it impossible to obtain a passing score.
The horizontal gaze nystagmus test measures the involuntary jerking of the eyes that can occur when looking from side to side. While nystagmus can become more pronounced due to alcohol consumption, there are a number of neurological conditions that could also cause someone to fail the test.
Anxiety could also make performing the field sobriety tests difficult. For many people, being stopped by the police and ordered to perform a number of unfamiliar physical tasks is intimidating and could result in a failing score.
In addition, the police officer is supposed to ensure that the driver comprehends all of the directions before administering the test. Those with certain mental disabilities who cannot understand or follow the directions could not be expected to successfully pass these tests.