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How the HGN test tests for drunk driving

Pennsylvania residents who are thought to be driving under the influence of alcohol will likely be asked by law enforcement officers to participate in field sobriety testing. There are three standard tests. This week, this column will go cover the horizontal gaze nystagmus test and how it tests for drunk driving.

What exactly is the HGN test? With the HGN, officers focus solely on a person's eyes. One who participates in this test will be asked to roll his or her eyes from side to side. When the eyeball reaches a certain angle, it will naturally start to jerk or bounce. What police are looking for is if the jerking or bouncing motion is exaggerated, as this would be a sign of impairment.

How accurate is the HGN? According to the National Highway Traffic and Safety Administration, this screening is believed to be very accurate -- if administered correctly and if the officer administers the test has been properly trained on what to look for. The three things law enforcement should be looking for when walking a person through this test are:

  • Eye jerking before the eye reaches a 45-degree angle
  • Eye jerking when the eye is held to the side for four seconds
  • Lack of smooth eye movements

Those in Pennsylvania who are charged with drunk driving based on the results of the HGN or other field sobriety tests may not feel they have a leg to stand on in court. This may not be true. As previously stated, this test is only considered accurate if administered and judged correctly. It may be possible to have tests results ruled inadmissible if one's legal counsel can enter sufficient evidence showing errors occurred during administration. An experienced criminal defense attorney can provide more information about this topic.

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