Field Sobriety Tests and Horizontal Gaze Nystagmus (HGN) Information for Drinking and Driving

Information for persons who have been stopped for suspicion of DWI in Texas. Field Sobriety Tests are generally used to determine if a person has been driving while intoxicated, and whether or not there is probable cause to arrest the driver for DWI.

Most drivers are not aware that there are only four recognized field sobriety tests. The Standardized Field Sobriety Test (SFST) is a battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest. These tests were developed as a result of research sponsored by the National Highway Traffic Safety Administration (NHTSA). A formal program of training was developed and is available through NHTSA to help police officers become more skillful at detecting DWI suspects, describing the behavior of these suspects, and presenting effective testimony in court. Formal administration and accreditation of the program is provided through IACP. Unfortunately, many officers either did not attend the formal training, or relax the strict standards of the four tests, sometimes adding tests that are not recognized. The four tests of the SFST are:

  • the horizontal gaze nystagmus (HGN)
  • divided attention testing
  • the walk-and-turn
  • the one-leg stand.

These tests are administered systematically and are evaluated according to measured responses of the suspect.

HGN Testing

Horizontal gaze nystagmus is an involuntary jerking of the eyeball which occurs naturally as the eyes gaze to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, nystagmus is exaggerated and may occur at lesser angles. An alcohol-impaired person will also often have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a suspect as the suspect follows a slowly moving object such as a pen or small flashlight, horizontally with his eyes. The examiner looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center. If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.10 or greater. NHTSA research indicates that this test allows proper classification of approximately 77 percent of suspects. HGN may also indicate consumption of seizure medications, phencyclidine, a variety of inhalants, barbiturates, and other depressants.

Divided Attention Testing

The walk-and-turn test and one-leg stand test are “divided attention” tests that are supposed to be easily performed by most sober people. They require a suspect to listen to and follow instructions while performing simple physical movements. Impaired persons have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises.

In the walk-and-turn test, the subject is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The examiner looks for seven indicators of impairment: if the suspect cannot keep balance while listening to the instructions, begins before the instructions are finished, stops while walking to regain balance, does not touch heel-to-toe, uses arms to balance, loses balance while turning, or takes an incorrect number of steps. NHTSA research indicates that 68 percent of individuals who exhibit two or more indicators in the performance of the test will have a BAC of 0.10 or greater. In the one-leg stand test, the suspect is instructed to stand with one foot approximately six inches off the ground and count aloud by thousands (One thousand-one, one thousand-two, etc.) until told to put the foot down. The officer times the subject for a 30 seconds. The officer looks for four indicators of impairment, including swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down. NHTSA research indicates that 65 percent of individuals who exhibit two or more such indicators in the performance of the test will have a BAC of 0.10 of greater. The effectiveness of SFST in court testimony and evidence depends upon the cumulative total of impairment indicators provided by the four-test battery. The greater the number of indicators, the more convincing the testimony. Because SFST is administered according to national standards and is supported by significant research, it has greater credibility than mere subjective testimony.

Testing Accuracy

The NHTSA manuals (plural here, because six separate versions have now been released) say that if the FST tests are not performed properly, or if conducted without adhering to the training protocols, such actions “compromise” the validity of these evaluations.

The HGN evaluation, when performed correctly on proper subjects, had a 77% “claimed” reliability rating. The Walk-and-Turn exercise, when conducted properly on a qualified subject on a dry, level surface, was found to be 68% reliable. The One-Leg-Stand exercise, when conducted properly, on a qualified subject on a level, dry surface and under proper instructions and where correctly demonstrated and scored, reportedly yields about 65% reliability. Cumulatively, if all are done correctly, up to 83% correlation to a BAC of 0.10% or more may be expected.

Knowledgeable DWI lawyers know that 98% or more of the officers administering these evaluations do them wrong, or conduct them in a manner (or on a test subject) not approved by the SFST manual, or grade the evaluations improperly, as per the manual, or ALL OF THE ABOVE. When done incorrectly, these evaluations have ZERO predicted reliability.

Also, recent research and scientific review of the testing protocols and scoring methodology have brought the NHTSA “Standardized Field Sobriety Tests (“SFSTs”) into serious question. More and more courts are now saying “no” to these questions. In a recent New Mexico case, a high-level court has declared that the person who “developed” the tests (Dr. Marcelline Burns) was not qualified to testify as an expert witness about the scientific principles behind the HGN test. (Lasworth v. State, 42 P.2d 844 (N.M. App. 2001).