In 2024, drunk driving killed 11,904 people in the US. This figure accounts for 30 percent of all the traffic fatalities in America. The NHTSA statistics for 2025 add that the number of traffic fatalities reached 36,640, which is a 6.7 percent decrease when compared to 2024 figures.
The Horizontal Gaze Nystagmus (HGN) test is but one of the standardized tests that the police use to conduct investigations of driving under the influence (DUI) cases.
The HGN test measures a person’s ability to visually track an image in a controlled movement. Officers watch for an involuntary jerking motion, which signals impairment. The test is only one part of the DUI investigation and may not conclusively show that the driver was under the influence of both alcohol and drugs.
Let’s discuss how the HGN test is carried out, what factors police officers look for, and what they could interpret from the outcomes of such investigations.
What the Officer Is Actually Looking At
During the test, the subject holds their head still and tracks an object, such as a pen or penlight, as the officer moves it laterally across the subject’s visual field. The officer is checking for three separate signs in each eye.
The first is a lack of smooth pursuit. If the eye jerks instead of moving fluidly as it follows the object, then it may suggest impairment. The second is distinct and sustained nystagmus at maximum deviation. This condition is clearly exhibited when the eye jerks noticeably when held at the furthest point it can turn. The third is the onset of nystagmus prior to 45 degrees, which happens when the jerking begins before the eye reaches a 45-degree angle from the center.
The officer scores each of these three signs separately for the left eye and the right eye, for a maximum of six. A total of four or more is generally treated as an indicator of impairment.
Four Clues Is the Number That Matters
The field validation research behind the test set a specific threshold. Testing positive for four or more clues out of the six possible, across both eyes, is treated as an indicator that a driver’s blood alcohol concentration is at or above 0.08. That threshold comes from the 1998 San Diego study by Stuster and Burns, which NHTSA cites for putting the HGN test alone at 88 percent accuracy against that mark. When HGN is combined with the other two tests in the standard battery, the walk-and-turn and the one-leg stand, the combined accuracy rises to 91 percent. The walk-and-turn test on its own reaches 79 percent accuracy, using a lower bar of two or more clues. The one-leg stand reaches 83 percent, using that same two-or-more standard.
Those figures do a great deal of work in courtrooms. They are also narrower than they sound.
The Same Test Shows Up in Two Different Cases
A stop that ends in a collision can lead to two separate proceedings. The state may prosecute the driver, while anyone injured can pursue a civil claim of their own. The parties, evidential tasks, and terms and conditions all differ. This variation means that a resolution in one case will in any way affect the argument in the other. If your case is on drunk driving claims, then a drunk driving accident lawyer will explain to you your steps and help you institute the appropriate civil proceedings.
The evidence in these two proceedings overlaps, but the cases themselves do not.
The Accuracy Figure Comes With Conditions
NHTSA attaches an explicit condition to its numbers. The correlation holds when the test is administered in the prescribed standardized manner, when the standardized clues are used and when standardized criteria interpret the result. A change in any of these elements may compromise its validity.
The officer conducting the test can affect the test’s accuracy by holding the maximum deviation for less than the full four seconds. Moving the stimulus too quickly for the eye to track, or estimating a 45-degree angle by eye at the roadside in the dark, are other factors that can affect the test’s accuracy. Since the whole thing is an observation rather than a recording, the record of what happened is usually the officer’s notes plus whatever the dashcam caught.
What the Validation Studies Actually Measured
A San Diego study looked at 297 motorists. Roughly 12 percent of them were women. The mean blood alcohol concentration in the sample was 0.122, comfortably above the threshold the test is used to detect. It produced 24 false positives.
The Oklahoma Bar Journal published an analysis in 2021. It looked at drivers pulled over for apparent alcohol influence who tested below the legal limit. About sixty-seven percent of them showed all six clues on the HGN test.
Nystagmus Has Causes Other Than Alcohol
This symptom has a very broad differential diagnosis. It occurs in many inner ear conditions as well as in some neurological conditions, head trauma, various medications, and even congenital nystagmus. The flashing lights on the patrol car will cause the type of optokinetic movements seen in this condition.
An officer running a roadside test is not positioned to sort those apart, and the training does not ask them to. That limitation is built into the test rather than a knock on any particular officer.
The test estimates whether a number sits above a line. It does not produce the number. In many states it comes in as evidence of impairment, while the actual blood alcohol figure has to arrive through a chemical test, and the space between those two things is where most of the argument happens.
Anyone reading a police report about one of these stops should focus on how the test was run, not just on how many clues the officer marked.
