Horizontal Gaze Nystagmus or HGN is a key tool in law enforcement’s arsenal to investigate an alleged DUI.

Defined as the “involuntary jerking of the eyes that occurs when eyes gaze to the side” the HGN has been validated as one of the most reliable indicators of driver impairment when compared with all the other field sobriety and investigative tests officers employ while on the roadside.

If HGN is conducted correctly a 2013 San Diego validation study has found that it can be 88% accurate in detecting that a motorists BAC is above .08. The findings in an HGN test alone can give an officer the probable cause they need to arrest a motorist and bring them back to the barracks where an evidentiary breath test is taken.

As is the case with all field sobriety tests however, the key determination in the HGN is if the test was in fact administered properly according to National Highway Safety Administration (NHTSA) standards.

As a certified HGN practitioner who has gone through the certification process of administering the HGN, I can say with conviction that the NHTSA requirements are difficult to administer correctly in a controlled classroom environment, let alone in the wide variety of environmental conditions that an officer would face when conducting an HGN test on the roadside.

Timing, position of the stimulus (object motorist is to follow with eyes), medical conditions (such as head injuries) and the surrounding environment are but a few of the many factors that can play a role in the accuracy of the HGN. If any of these are comprised by failing to follow NHTSA guidelines, the entire test and, in turn a significant portion of the DUI investigation can be compromised, leaving serious doubt as to whether the officer has the evidence necessary to charge a motorist with DUI.