If you have had a few alcoholic drinks and then drive a vehicle, chances are you will do something that, if seen by a police officer, will give probable cause for pulling you over. This might be something like crossing the pavement lines, weaving within your lane, or a number of things that might give a clue that you’ve been drinking. What likely will happen next is that the officer may request you to perform a “field sobriety test”.

The Standardized Field Sobriety Test (SFST) is a battery of three tests performed during a traffic stop in order to determine if a driver is impaired. The tests that make up the SFST are the horizontal gaze nystagmus (HGN), the walk-and-turn, and the one-leg stand tests. Developed in the 1970s, these tests are scientifically validated, and are admissible as evidence in court in a majority of states, including New York.

Horizontal gaze nystagmus is an involuntary jerking of the eyeball which occurs as the eyes gaze to the side. An alcohol impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct and sustained nystagmus when the eye is at maximum deviation, or if the angle of onset of jerking is prior to 45 degrees of center.

In the walk-and-turn test, the subject is directed to take nine steps, touching heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The examiner looks for eight indicators of impairment: if the suspect cannot keep balance while listening to the instructions, begins before the instructions are finished, stops while walking to regain balance, does not touch heel-to-toe, uses arms to balance, steps off the line, takes an incorrect number of steps, or makes an improper turn.

In the one-leg stand test, the subject is instructed to stand with one foot approximately six inches off the ground and count aloud by ones beginning with one thousand (one thousand-one, one thousand-two, etc.) until told to put the foot down. The officer times the subject for 30 seconds. The officer looks for four indicators of impairment including: swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down.

There are many factors that might render a person unable to successfully complete one or more of the SFSTs. For instance, regarding the HGN test, the person asked to consent to such a test might be suffering from an eye disease or condition that affects his/her ability to see and consequently confound the test and results. Age, injury or disease could also affect the ability of a person to perform the one-leg stand test or the walk and turn test. Other disabilities, such as deafness, should be taken into consideration and noted as well.

Next week, we’ll discuss measuring the amount of alcohol in a person’s blood by chemical analysis of a person’s blood, breath, urine or saliva and what may happen if you refuse the test.