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Field sobriety tests: What they are & how they work, Part I

Summer is a busy time for law enforcement agencies across Colorado. The summer months - and summer holidays in particular - are often associated with higher rates of drunk driving. As such, drivers in many areas may be more likely to get pulled over than they would be at other times of year.

If you are among these unlucky drivers, you should understand your rights ahead of time and understand what an officer may ask you to do if he suspects that you may have been driving drunk. In our next two posts, we'll be discussing Standardized Field Sobriety Tests.

There are many tests that could be used to determine (objectively or subjectively) if a suspect is impaired. But law enforcement agencies across most of the country, including here in Colorado, rely on a set of three tests endorsed by the National Highway Traffic Safety Administration. They are known collectively as Standardized Field Sobriety Tests (SFSTs). They include:

  • Horizontal Gaze Nystagmus testing
  • The Walk-and-Turn test
  • The One-Leg Stand test

Horizontal Gaze Nystagmus

You may not have heard of the HGN test by its official name, but you almost certainly know what it is. When conducting this test, the police officer asks a suspect to follow a moving object (the officer's finger, a pen, etc.) with his eyes.

Horizontal gaze nystagmus is a phenomenon that occurs when a person's eyes are rotated peripherally at high angles. It is an "involuntary jerking of the eye." If HGN occurs at shallower angles or if the suspect has difficulty tracking the moving object with his eyes, it could be an indication of alcohol impairment.

The problem with this test, however, is that there are many things unrelated to alcohol that could impact eye movement. Perhaps the person has a vision problem. Perhaps they are on a medication that affects their eye movements. Perhaps they are still recovering from having a flashlight shined into their eyes during the traffic stop.

Please check back next week as we continue our discussion about the three SFSTs and how they work.

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