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The Eyes Have It

What in the world are the police looking for when they do the "eye test" on subjects who are believed to be under the influence? Many years ago the concept was that "drunk" driving was a punishable offense. Police officers would watch motorists driving in an erratic or unlawful manner and would stop them to see if they were intoxicated. We tend to know what intoxication looks like but at the years passed, states realized that intoxication was too high a standard for prosecutors to prove. The revenue stream provided by drunk driving convictions was drying up and it was useful to look for something short of intoxication, something that could only be recognized by individuals who had received specialized training. That "something" was an amorphous concept called "IMPAIRMENT."

How to show impairment? As a DUI defense lawyer, I hear many of my clients still clinging to the belief that because they weren't intoxicated or "drunk" that this will protect them from a DUI conviction. Nothing could be further from the truth. Impairment is presumed when one or more physiological manifestations are present, manifestations that have nothing to do with intoxication or, for that matter, with the act of driving.

Hence, the famous "eye test" that is conducted by police at roadside when they suspect that a person may have had more than their fair share to drink. This test is known officially as the Horizontal Gaze Nystagmus test and it has been validated to predict with some degree of accuracy a blood alcohol level, or BAC. Indeed, the National Highway Traffic and Safety Administration (NHTSA) has stated that if the HGN is properly administered and scored, it is 77% reliable in predicting when a motorist's BAC will exceed 0.10g/100mL of blood. In a Validation Study conducted in San Diego, California in 1997, this test was revitalized and is now believed to have a 77% reliability factor in discriminating out to the lower BAC of .080g/100mL of blood. Regardless whether the San Diego Validation Study was properly conducted to declare the HGN test even more accurate than before, the fact remains that even with the new study, the test is still a shot in the dark.

A 77% reliability factor means that almost 1 in 4 motorists will be incorrectly catalogued as impaired drivers and possibly arrested, based primarily upon this test. It is said that the HGN is impossible to game and therefore is a much more reliable indicator of impairment than the other two tests included in the Standardized battery of Field Tests (SFSTs). In fact, however, most law enforcement officers do not consistently administer the HGN in a correct manner and it is not uncommon for officers to mistake fleeting or transitory movements of the eyes for nystagmus, which is best described as a "jerking" of the eyeball.


This jerking is subtle, and officers who have been trained and certified by NHTSA often are unable to actually detect true nystagmus from what they "think" is nystagmus, and sometimes their incorrect administration of the HGN test actually causes nystagmus to appear where it would not have appeared had the test been properly undertaken. Angles are critical, distances from the stimulus (a finger, pen, or small flashlight) even more so. The speed with which the stimulus is moved can ruin any predictive value of the test. Officers overly confident in their ability to accurately guage offset angles by using a "one size fits all" approach consistently overstate the presence of nystagmus and its significance. This "test," however, is what often makes the difference between an officer making an arrest and his decision to let the motorist proceed on his way.

Coming soon, what the test actually means. Film at eleven...