Understanding the Horizontal Gaze Nystagmus Test in DUI Investigations.

As a Georgia DUI Attorney, I have encountered numerous cases where law enforcement officers have utilized the Horizontal Gaze Nystagmus (HGN) test to determine whether a driver is under the influence of alcohol or drugs. In this blog post, I will provide an in-depth explanation of the HGN test, its purpose, and its potential limitations. 

The Horizontal Gaze Nystagmus test is one of the three standardized field sobriety tests developed by the National Highway Traffic Safety Administration (NHTSA) to assist law enforcement officers in identifying impaired drivers. Nystagmus refers to an involuntary jerking or bouncing of the eyeball, which can be exacerbated by the consumption of alcohol or certain drugs. The HGN test is designed to measure the smoothness of a person's eye movement as they track an object, such as a pen or a finger, horizontally from side to side. 

During the HGN test, the officer will typically follow these steps: 

  1. The officer will ask the driver to stand with their feet together, arms at their sides, and head still. 
  2. The officer will then position an object (usually a pen or a finger) approximately 12 to 15 inches from the driver's nose, just above eye level. 
  3. The officer will instruct the driver to keep their head still and follow the object with their eyes only, as the officer moves the object slowly from side to side. 

The officer will look for the following indicators of impairment during the test:

  1. Lack of smooth pursuit: The officer will observe whether the driver's eyes can smoothly follow the object from side to side. An inability to smoothly track the object may indicate the presence of alcohol. 
  2. Distinct nystagmus at maximum deviation: The officer will move the object to the edge of the driver's peripheral vision and hold it there for a few seconds. If the driver's eyes exhibit nystagmus (jerking or bouncing) at this point, it may suggest the presence of alcohol.
  3. Angle of onset of nystagmus prior to 45 degrees: The officer will move the object at a speed that would take about four seconds for the driver's eyes to reach the edge of their peripheral vision. If the driver's eyes exhibit nystagmus before the object reaches a 45-degree angle from the center, it may indicate the presence of alcohol.

These clues build on each other and must be seen in both eyes. As BAC increases, many people first show inability of smooth pursuit, then show distinct jerkiness at maximum deviation, and finally show an onset within 45 degrees.” (2023 SFST Instructor Manual / Instructor Note: session VIII, page 31) While the HGN test can be a useful tool for law enforcement officers in identifying impaired drivers by estimating potential blood alcohol levels, it is not infallible.

First, DUI field sobriety tests like the HGN do not measure impairment despite police testimony and training manuals that say otherwise. Georgia Police Officers are trained from the National Highway and Traffic Safety Administration manual with regards to administering and interpreting field sobriety evaluations. Dr. Marceline Burns is the author of the validation studies that form the basis of NHTSA's field sobriety tests. She authored various validation studies including one in 1998 called Validation of the Standardized Field Sobriety Test Battery at BACs Below .10 Percent (hereinafter “San Diego Study.”). In the 1998 San Diego study, Dr. Burns warned that, “Measures of impairment are irrelevant because performance of the SFSTs must be correlated with a BAC level, rather than driving performance.” Validation of the Standardized Field Sobriety Test Battery at BACs Below .10 Percent, Dr. Marcelline Burns, August 1998, p. 10. The study concluded, “Driving a motor vehicle is a very complex activity that involves a wide variety of tasks and operator capabilities. It is unlikely that complex human performance, such as that required to safely drive an automobile, can be measured at the roadside. The constraints imposed by roadside testing conditions were recognized by the developers of NHTSA' s SFST battery. As a consequence, they pursued the development of tests that would provide statistically valid and reliable indications of a driver's BAC, rather than indications of driving impairment...Thus, SFST results help officers to make accurate DWI arrest decisions even though SFSTs do not directly measure driving impairment.”

NHTSA Final Report, Validation of the SFST Battery at BACs Below 0.10 Percent, Stuster and Burns, August 1998. (San Diego Study), p. 28.

Second, factors such as medical conditions, medications, and environmental conditions can also cause nystagmus or affect the results of the test.

Third, the test requires proper administration by the officer, and any deviation from the standardized procedure can compromise the accuracy of the results. As a Georgia DUI Attorney, I have successfully challenged the validity of HGN test results in court, helping my clients avoid the severe consequences of a DUI conviction. 

If you or a loved one has been charged with a DUI in Georgia and the HGN test was a factor in your arrest, do not hesitate to contact me for a free consultation. Together, we can review the details of your case and develop a strong defense strategy to protect your rights and your future.

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For a free consultation, contact the Law Offices of George Creal today on the web at www.georgecreal.com or on the phone at (404) 333-0706. We are here to help you navigate the legal system and fight for your rights.


The information in this blog post is for general informational purposes only and should not be construed as legal advice. Please consult with an attorney to discuss your specific legal situation.