One aspect of DWI law that most people do not understand are the Field Sobriety Tests.  They think that people who are drunk will fail them, and people who are sober will fail them.  If only that were the case.  While properly administered field sobriety tests may be a tool to help an officer determine if a person is sober or not, they are far from perfect.  There are several ways that tests can lead to false positives. In the same way, the test can lead to false negatives. Lets take a quick look at each test:

sobriety testsField Sobriety Tests

Horizontal Gaze Nystagmus

This test involves testing a person’s eyes to see if nystagmus is present. Nystagmus is the involuntary jerking of the eyes.  When a person is intoxicated, their eyes will display nystagmus when gazing horizontally. While this is true, it is also true that other factors can cause nystagmus, which the officer can mistake for an indication of impairment.  Some of these include naturally visible nystagmus, which would cause a person to seem impaired regardless of whether they have had any alcohol, positional alcohol nystagmus, which would cause nystagmus when some alcohol has been consumed and the head is tilted or moved, or optokinetic nystagmus which is caused by strobe lights, rotating lights, or fast-moving lights (like headlights in traffic).  Each of these can get a person arrested even though they only have no alcohol or a small amount of alcohol in their system.

Walk and Turn

This test requires a person to follow multiple instructions at once. These include 9 heel to toe steps while counting out loud, keeping hands down by their side.  While doing this, are supposed to look at their feet, not step off the line, and not stop before completing the test.  You are also supposed to complete a certain type of turn, just as the officer demonstrates it.  Failing to follow any of these instructions will result in a clue of impairment.

Obviously, there can be reasons other than drinking for poor performance on the test.  Physical problems can inhibit a person’s ability to perform on the tests.  Improper testing conditions, such as an un-level, wet, or debris filled testing area, can contribute to poor performance.  Finally, sometimes a person who has a high BAC level is able to complete the test satisfactorily because they have extremely high tolerance for alcohol.

One Leg Stand

The final test is the One Leg Stand.  During this test, an officer will instruct that the  person lift one leg off of the ground and count out loud, while keeping their arms down by their side.  The person is to keep counting until the officer instructs them to stop.  One obvious reason that a sober person could fail this test is because they naturally have bad balance.  In addition, the factors mentioned above about the testing surface come into play as well.  As mentioned above, sometimes drunk people are able to pass this test, especially those who are used to having a BAC level.

Because each of these tests is subject to error, even a sober person can falsely appear impaired. On the other hand, an impaired person’s performance on the test can indicate sobriety. However, if the officer fails to administer the tests correctly, then the results of the tests are invalid.