When a driver is suspected of drunk driving in New York, he or she may be to perform a field sobriety test. While there are numerous tests that can be employed, three standard tests are Horizontal Gaze Nystagmus (HGN) testing, walking and turning and standing on a single leg.
HGN is the involuntary movement of a person's eye that takes place when a person looks to their left or right. If a driver is under the influence of alcohol, this movement is heightened and the angles of the movement are different. In addition, a drunk driver may find it difficult to smoothly follow an object visually in a horizontal manner. Flashlights or pens are common objects to use in these tests.
Walking and turning is another commonly used field sobriety test for those accused of drunk driving. This requires a person to have divided attention - the person must listen to the officer while at the same time engage in a motion. For example, in the walk-and-turn test, the officer may ask the driver to walk several steps in a row with their heel following their toe, and then turn around and do the same thing. If a person is under the influence of alcohol, they may have trouble keeping their balance. They may not make the required number of steps or turn properly. These could be indicators of intoxication.
Another divided attention field sobriety test is standing on a single leg. In administering this test, the officer will ask the driver to stand on one foot for half a minute while counting out loud at the same time. Signs of intoxication indicated by this test include hopping, swaying, steadying oneself with one's arms or being unable to stand on one foot.
One may think that one can beat such tests, even under the influence, but this is difficult to do. On the other hand, even a sober person facing drunk driving charges could have trouble performing such tests. For this reason, it is important to understand all the circumstances surrounding a person's physical abilities before declaring that, based on these field sobriety tests, a person is under the influence when in fact they may not be.
Source: nhtsa.gov, "Appendix A Standardized Field Sobriety Testing," accessed April 7, 2015