Jan

7

The Science Behind Sobriety Tests

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Law enforcement officers are on the lookout for drunk drivers. Part of their training involves learning the Standardized Field Sobriety Test (SFST) through a program administered and accredited by the International Association of Chiefs of Police (IACP).

Developed by the National Highway Traffic Safety Association (NHTSA), the SFST is a collection of three tests designed to help officers determine whether or not someone is driving under the influence of alcohol.

The three tests are: Horizontal Gaze Nystagmus (HGN), Walk-and-Turn (WAT), and the One-Leg Stand (OLS).

HGN

HGN is an involuntary eye jerking movement that occurs at the extreme periphery of the eye. When a person is intoxicated, HGN is present at lesser angles. Officers use a small object, such as a pen or flashlight, to test for intoxication.

The suspects are required to follow the object as they slowly move it in front of the suspect’s eyes. While conducting this test, officers look for three key signs:

  • Is the subject able to follow the object smoothly?
  • Does the subject show HGN at maximum deviation?
  • Does the onset of HGN occur within 45 degrees of the center?

If the officer sees for or more signs between the two eyes, it is likely the suspect has a blood alcohol concentration (BAC) of .08 or higher.

The NHTSA has found this test has an 88% accuracy rate, primarily because HGN can be an indication of factors other than alcohol consumption.

HGN also occurs with the consumption of seizure medications, various inhalants, phencyclidine, barbiturates, and other depressants.

WAT

The WAT test is a divided attention test because it aims to see how well a suspect can do two things at once. When conducting this test, the officer asks the suspect to walk a straight line. Suspects are asked to take nine heel-to-toe steps, while keeping their arms down to their side. Suspects are then asked to turn on one foot, and take another nine heel-to-toe steps in the opposite direction.

The officer looks for eight signs of intoxication:

  • Does the suspect start walking before instructions are finished?
  • Does the suspect maintain balance during the instruction?
  • Does the suspect stop to regain balance?
  • Does the suspect touch heel-to-toe?
  • Does the suspect step off the line?
  • Does the suspect use their arms to balance?
  • Does the suspect make an improper turn?
  • Does the suspect take the correct number of steps?

If the suspect displays two or more of these signs, it is likely they have a BAC of .08 or higher.

The NHTSA has found this test has a 79% accuracy rate.

OLS

The OLS test is another divided attention test, aiming to see how well a suspect can do two things at once. When conducting this test, the officer asks the suspect to stand with one foot about six inches off the ground, and count out loud by thousands, until asked to put their foot down.

Suspects should count like this: “One thousand-one, one thousand-two, one thousand-three,” etc. until the officer tells them to stop. The officer will observe the suspect for about 30 seconds and watch for the following signs of intoxication:

  • Does the suspect sway back and forth?
  • Does the suspect use their arms to balance?
  • Does the suspect hop to maintain balance?
  • Does the suspect put their foot down?

If the suspect displays two or more of these signs, it is likely they have a BAC of .08 or higher.

The NHTSA has found this test has an 83% accuracy rate.

Disclaimer: This information is not intended to promote drunk driving. Do not attempt to beat these tests.

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