There are three main field sobriety tests that police may use during a drunk driving stop. These are the one-leg stand, the walk-and-turn and the horizontal gaze nystagmus test.
They all have a different focus. The HGN test looks at eye movement and a person’s ability to track objects within their vision. The walk-and-turn test helps to gauge their ability to perform simple tasks and follow instructions. The one-leg stand, of course, just measures a person’s ability to maintain their balance.
In many cases, police officers may ask a person to perform all three tests.
These tests are not 100% reliable
One important thing to note about these tests is that they are not 100% reliable. False arrests can and do happen.
For instance, one study looked at traffic stops involving 297 different drivers. Researchers determined that the field sobriety tests produced 24 false positives. Even though most of the arrests were correct, this means that 24 people did face a wrongful arrest.
When looking at the HGN test in particular, 81 of the drivers showed four HGN cues, even though it later turned out that they were actually below the BAC threshold of 0.08%. In other words, 37% of the drivers who triggered the HGN test did not actually violate the legal limit.
This does not mean that these tests are worthless, but it does mean that they should be taken with a grain of salt. They do not guarantee a conviction, and they are not always accurate. Those who are facing DUI charges need to know exactly what defense options they have.