Traffic Law

The Mechanics Behind Field Sobriety Tests

When an individual is pulled over by a law enforcement officer for suspicion of driving under the influence (DUI), they are usually put through a series of tests called Standardized Field Sobriety Testing (SFST). These tests are designed to determine if the driver is impaired because of alcohol consumption or the use of controlled substance. If an individual does not complete these tests to the officer’s satisfaction, they can be arrested for a DUI.

The SFST series of tests were developed as the result of research sponsored by the National Highway Traffic Safety Administration (NHTSA) and conducted by the Southern California Research Institute. The three tests of the SFST are the Horizontal Gaze Nystagmus (HGN), Walk-and-Turn (WAT) and the One-Leg Stand (OLS).

According to the NHTSA, Horizontal Gaze Nystagmus is the involuntary jerking of the eye that occurs naturally as the eyes gaze to the side. Normally, nystagmus occurs when the eyes are rotated at high peripheral angles when they move horizontally left to right or right to left. During an HGN test an officer will observe the eyes of a suspect as they follow a slow moving object such as a pen or small flashlight, left to right with their eyes. The officer will look for three indicators of impairment:

  • The eye cannot follow the moving object smoothly,
  • Distinct jerking of the eye, and
  • If the angle of jerking is within 45 degrees off center

The NHTSA research found that the HGN test allows proper classification of 88 percent of individuals who are driving under the influence and that they likely have a blood alcohol concentration (BAC) of 0.08 or greater.

The two additional tests of the SFST, the Walk and Turn and the One Leg Stand are “divided attention” tests that are considered easily performed by most unimpaired people. The tests require an individual to follow instructions while performing physical movements.

During the Walk and Turn test, an individual is told to take nine steps, heel-to-toe, along a straight line and at the end of that line, they must turn on one foot. From here, they must take the same nine steps, heel-to-toe in the opposite direction. Throughout this test, test the law enforcement officer looks for eight signs of impairment. These signs of impairment within the individual can include one or more of the following:

  • Being unable to keep their balance while listening to the instructions
  • Beginning the test before the instructions are finished
  • Ceasing to walk to regain their balance
  • Unable to touch heel-to toe
  • Stepping off the line
  • Using their arms to balance
  • Making an improper turn
  • Taking the improper number of steps

The research concerning this test indicates that 79 percent of individuals who display two or more signs of impairment during the test will have a BAC of 0.08 or greater.

The One Leg Stand requires a person to stand with one foot approximately six inches off the ground while counting aloud by thousands until they are told to put their foot down. The officer administering the test times the test for 30 seconds and looks for the four following indicators to determine impairment:

  • Swaying while balancing,
  • Use of arms to balance,
  • Hopping to maintain balance, and
  • Putting the foot down.

NHTSA research indicates that 83 percent of individuals who exhibit two or more indicators during the test will have a BAC of 0.08 or greater.

Though these tests are considered the standard, there are additional non-standardized tests that officers administer such as reciting the alphabet or a portion of it, counting backwards, standing with feet together and tipping the head backwards and counting the number of fingers that the officer raises.

These tests are designed to measure normal faculties of the body such as balance, the ability to walk, talk, see or hear. However, these tests do not take into consideration prescription medications or illnesses that may interfere with a person’s normal faculties. Should you find yourself in a situation where you have been wrongfully accused of DUI because of factors such as these, you should speak with a criminal defense lawyer as soon as possible. Circumstances such as an illness or a legally obtained prescription drug that could possibly affect a field sobriety test can be used by a knowledgeable criminal defense attorney as a defense in a DUI case.