Standardized Field Sobriety Tests

The National Highway Traffic Safety Administration (NHTSA) sponsored research that led to the development of a Driving While Impaired (DWI) detection guide that listed 20 driving cues and the probabilities that a driver exhibiting a cue would have a BAC of at least 0.10 percent. A similar study was conducted more recently that identified 24 driving cues that are predictive of DWI at the 0.08 level; the latter study also identified ten post-stop cues with probabilities of DWI of at least 90 percent.  Based on this research, NHTSA developed a standardized battery of tests for officers to administer to assess driver impairment after an enforcement stop has been made.

Beginning in 1981, law enforcement officers used NHTSA's Standardized Field Sobriety Test (SFST) battery at roadside to help determine whether motorists who are suspected of DWI have blood alcohol concentrations (BACs) greater than 0.10 percent. Since 1981, however, many states have implemented laws that define DWI at BACs below 0.10. For this reason, NHTSA sponsored additional research to systematically evaluate the accuracy of the SFST battery to discriminate above or below 0.08 percent and above or below 0.04 percent BAC.

The SFST battery is composed of three tests: Horizontal Gaze Nystagmus (HGN), Walk-and-Turn (WAT), and One-Leg Stand (OLS);

Horizontal Gaze Nystagmus (HGN)

Horizontal gaze nystagmus is an involuntary jerking movement of the eyeball that occurs naturally as an individual's eye gazes to the side. Normally, nystagmus (jerking movement) occurs when the eyes are rotated at high peripheral angles. When an individual is impaired by the effects of alcohol, this jerking is exaggerated and can occur at lesser angles. Additionally, an individual Driving While Intoxicated (DWI) or Driving Under the Influence (DUI) will have greater difficulty tracking a moving object. As the test is administered, law enforcement personnel look for three indicators of impairment. The indicators are:

1. If the eye cannot follow a moving object smoothly.
2. If jerking is distinctly noticeable when the eye is looking as far to the side as it can (maximum deviation).
3. If the jerking begins when the eye is within 45 degrees of center.

The officer tests each eye. If, between both eyes, four or more of the indicators are observed, research indicates that approximately 77 percent of suspects will likely have a Blood Alcohol Content (BAC) of .10 or higher.

Walk and Turn

In the Walk-and-Turn test, the individual is instructed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the individual is instructed to turn on one foot and return in the same manner along the original path. There are seven indicators of impairment for this test. The indicators are:

1. If the individual cannot maintain balance while listening to the instructions.
2. If the individual begins before instructions are finished.
3. If the individual stops while walking to regain balance.
4. If the individual does not touch heel-to-toe.
5. If the individual uses arms to balance.
6. If the individual loses balance while turning.
7. If the individual takes an incorrect number of steps.

Research indicates that 68 percent of individuals that demonstrate two or more of these indicators will have a Blood Alcohol Content (BAC) of .10 or higher.

One-Leg Stand

In the One-Leg Stand Test, an individual is instructed to stand with one foot approximately six inches off the ground and count aloud in thousands until instructed by law enforcement personnel to put the foot down. The individual is timed for thirty seconds. There are four indicators looked for during this test. The indicators are:

1. If the individual sways while balancing.
2. If the individual uses their arms to balance.
3. If the individual hops to maintain balance.
4. If the individual puts the foot down.

NHTSA research indicates that if two or more indicators are observed there is a 65 percent chance that the individual has a Blood Alcohol Content (BAC) of .10 or greater.