How Field Sobriety Tests Work

Written by Joe Suhre on March 13, 2015 in Defenses, Field Sobriety Test

How Field Sobriety Tests Work

Law enforcement officers have a variety of different tools at their disposal to detect drunk drivers. However, many times the initial screening is performed in a much less technologically advanced way through the use of field sobriety tests. These tests are a battery of different examinations done by the officer during a traffic stop in order to see whether there is probable cause to arrest the person and test them with a chemical breath test back at the police station. The three tests performed during a standard field sobriety test are the horizontal gaze nystagmus test, the walk and turn, and the one legged stand. Officers may also administer a preliminary breath test at this stage, using a portable breathalyzer.

What Are Field Sobriety Tests?

These three tests are supposed to be standardized ways created by the National Higway Traffic and Safety Administration for officers to judge whether there is probable cause to believe that a person is driving under the influence. The horizontal gaze nystagmus test is a test of a person’s involuntary eye reflexes. The officer will move a pen of finger back and forth in front of a person’s gaze, and have the person follow it with their eyes. If the person’s eyes exhibit automatic jerking reflexes, it may be a sign of intoxication.

The walk and turn involves a motorist taking a set number of steps, walking in a heel to toe manner. Then, after those steps, the driver has to turn around on one foot, and walk back the same way. This is a test of both balance and a person’s ability to follow simple directions.

The final test is the one legged stand. This test, as the name suggests, is performed by the motorist having to stand in place on one leg for some amount of time. As with the prior test, this is a test of the person’s balance.

The field sobriety tests are designed to have standard success rates that can act as markers for possible intoxication. However, the tests do not take into account the fact that different people may have different levels of skill apart from any intoxication. For instance, the one legged stand test usually requires people to hold their balance for 30 seconds. Some people cannot do that sober, while others have good enough balance to do it even when they are intoxicated.

The Purpose of Field Sobriety Tests

The purpose of field sobriety tests is not actually to prove that a person has been driving under the influence. In fact, the prosecutor in a DUI case cannot even use a failure of the test as evidence of intoxication. Instead, they are designed to give the officer probable cause. Probable cause is important because officers cannot simply arrest people without any reason, they have to suspect that the person has been committing some sort of crime, in this case driving under the influence. Then, the officer can arrest the person, bring them back to the station, and perform a chemical breath test on them that is actually designed to prove intoxication.

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