Traffic Safety Facts Banner
Number 207
Septeber 1999 

U.S. Department of Transportation
National Highway Traffic Safety Administration
400 Seventh Street, S.W., Washington, DC 20590


The National Highway Traffic Safety Administration (NHTSA) and the American Prosecutors Research Institute's National Traffic Law Center have developed a Resource Guide for judges, prosecutors, and law enforcement that succinctly explains the science and the law about horizontal gaze nystagmus (HGN). This 41 page report (plus detailed appendices) describes the science behind HGN: what it is, how alcohol affects this involuntary eye reflex, how it is detected and measured, and how valid and accurate the HGN test is.

The Guide is designed to assist judges, prosecutors, and law enforcement in gaining a basic understanding of HGN and its correlation to the consumption of alcohol. It distinguishes between HGN and other types of nystagmus and discusses the HGN test's scientific reliability.

What is Nystagmus?

Nystagmus is a term that describes a "bouncing" eye motion that is seen in two ways -- pendular nystagmus, where the eye wavers equally in two directions (like a pendulum), and jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a fast movement. Horizontal gaze nystagmus is a type of jerk nystagmus and is an involuntary motion, meaning that the person showing it cannot control it, and in fact, is unaware that it is happening. When impaired, a person's nervous system displays a breakdown in the smooth and accurate control of eye movements, resulting in a number of observable changes.

It is these observable changes that prompted NHTSA to research and standardize HGN for field sobriety tests in the 1970s. The Standardized Field Sobriety Tests (SFSTs) are used across the country whenever a law enforcement officer has reason to believe a motorist is driving impaired.

HGN in the Courtroom

Despite the proven correlation between alcohol consumption and HGN, many trial courts across the country still do not admit the results of the HGN test into evidence. Prosecutors and law enforcement have lacked either the knowledge or the resources, or both, to lay an adequate foundation for the admissibility of HGN.

Legal Admissibility of HGN Test Results

The Guide discusses several issues that affect the legal admissibility of HGN test results. These are:

  • whether the HGN test is characterized as a scientific test or as simply an observation of a physical trait of a suspect
  • the reliability of the HGN as a scientific test
  • whether the law enforcement officer is properly trained to administer the test
  • whether the officer properly administrated the test in a particular case
  • the purpose for which the HGN test result will be used -- as evidence of impairment or as probable cause

The Guide describes the requirements for the admissibility of HGN results under Frye, the Federal Rules of Evidence (Daubert) or other state evidentiary standards.

State Case Law Summaries

Prosecutors and judges will find the state by state summaries of horizontal gaze nystagmus a useful resource. Shown to the right is a sample for the state of California. There is a separate chart for each of the states in an appendix.

Reference Materials

Appendices contain state standards for admitting evidence, a comprehensive bibliography of references of HGN, predicate questions for law enforcement officers, SFST instructors, optometrists, emergency room physicians, and researchers in the area of alcohol impairment.


For a copy of Horizontal Gaze Nystagmus: The Science and the Law, A Resource Guide for Judges, Prosecutors, and Law Enforcement, write to Media and Marketing, NHTSA, NTS-21, 400 Seventh Street, S.W., Washington, DC 20590, or send a fax to (202) 493-2062.

Excerpt for California

I. Evidentiary Admissibility

HGN is a scientific test and the Kelly/Frye "general acceptance" standard must be applied. People v. Leahy, 8 Cal 4th 587, 607, 34 Cal Rptr. 2d 663 ,___(1994). People v. Joehnk, 35 Cal, App.4th 1488, 1493, 42 Cal. Rptr, 2d 6,8 (1995) "...[A] consensus drawn from a typical cross section of the relevant, qualified scientific community accepts the HGN testing procedures..." Joehnk, 35 Cal. App. 4th at 1507, 42 Cal. Rptr. 2d at 17.

II. Police Officer Testimony Needed to Admit HGN Test Result

Police officer testimony is insufficient to establish "general acceptance in the relevant scientific community." Leahy, Cal, 4th at 609, 34 Cal Rptr, 2d at 676. Police officer can give opinion, based on HGN and other test results, that defendant was intoxicated. Furthermore, police officer must testify as to the administration and result of the test. Joehnk, 35 Cal, App. 4th at 1508, 42 Cal. Rptr. 2d at 18.

III. Purpose and Limits of HGN

HGN may be used, along with other scientific tests, as some evidence that defendant was impaired. Joehnk, 35 Cal App. 4th at 1508, 42 Cal. Rptr. 2d at 17. HGN test results may not be used to quantify the BAC level of the defendant. People v. Loomis, 156 Cal. App. 3d Supp. 1,5-6, 203 Cal. Rptr. 767, 769-70 (1984).

U.S. Department
of Transportation
National Highway
Traffic Safety

400 Seventh Street, S.W. NTS-31
Washington, DC 20590

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