Officers working the night shift in a busy or college town are bound to cross paths with a drunk driver from time to time. After conducting a traffic stop, if there is suspicion that the driver is under the influence an officer will put the driver through a field sobriety test. We’ve all seen the three main tests on a television show or movie, but what’s the science behind them? I’ve set out to discover and report just that. The Standardized Field Sobriety Test (SFST; you know how the criminal justice profession loves their acronyms) is a set of three physical tests that can give an officer probable cause for an arrest. The three tests are the horizontal gaze nystagmus, walk-and-turn, and one-leg stand. These tests are administered and evaluated by the officer conducting them.
The horizontal gaze nystagmus is the first test administered. Nystagmus is essentially the rapid, uncontrollable movements of one’s eyes. This is usually found in infants and can lead to vision loss; however it also occurs to those under the influence of alcohol. An officer will hold an object such as a pen or pencil or flashlight and tells the driver to follow it with their eyes while keeping their head completely still as the officer waves it slowly from side to side. The three criterion the officer is looking for is inability to follow the moving object with the eyes, if there is distinct jerking of the eye at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. The officer does this test for each eye and if four or more of the criterion are present between both eyes, then the suspect is likely to have a BAC over the legal driving limit. This test can also be used to determine someone who is under the influence of other drugs such as some inhalants, barbiturates and other depressants.
The next test is the walk-and-turn. This is the first of the two tests that are “divided attention” tests. These tests are designed to be relatively easy for persons who are not impaired by drugs and/or alcohol. A suspected drunk driver has to listen to oral instruction from the officer while performing simple physical tasks. In this test a person is told by the officer to take nine steps, heel to toe, along a straight line. After nine steps the driver is instructed to turn around one hundred and eighty degrees on one foot and take nine more steps in a straight line. There are several indicators of impairment in this test including loss of balance, not touching heel to toe, using arms for balance and making an improper turn. If two or more of the indicators are demonstrated, research states that 79% of people will have a BAC of .08 or higher.
Last but not least is the one leg stand test. A driver is told to stand with one foot six inches off the ground and count out loud to the officer by thousands (one one thousand, two one thousand etc.) until the officer says stop. The test runs 30 seconds. In this test there are four indications of impairment which include swaying while balancing, using arms to balance, putting the raised foot down before being instructed to and hopping to maintain balance. If two or more of these indicators are present during the test, research shows that 83% of the time the subject will have a BAC over .08.
When all three of these tests are combined (and they usually are), officers are accurate in 91% of cases. A good question to ask yourself is if you think a 91% success rate is sufficient for these tests to be used by police or not. There might be times when it is difficult as an officer to tell if someone is under the influence or not, which may bring up issues such as police ethics and discretion. Other times, it is readily apparent that someone is under the influence such as our friend “Mr. Turner” in the video below.
**Disclaimer: This video contains adult language. Viewer discretion is advised.