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In earlier posts we covered the first two of the three-phase process police officers use to stop drivers they suspect of drunk driving and to determine whether an arrest is justified. This post covers the third phase, the pre-arrest screening. Like the first two, this phase is included in the National Highway Traffic Safety Administration's  "DWI Detection and Standardized Field Sobriety Testing" guidelines.

The three-phase process NHTSA process is not infallible, and it is subject to being carried out improperly. In a future post, we will examine some ways that a skilled New York DUI defense attorney can use to challenge the officer's arrest decision.

Field sobriety tests continue the psychophysical evaluation that the police officer begins during the personal contact phase. The officer seeks to gather impressions of the driver's capabilities of balance, coordination and the ability to process information. The most common of these tests include:

  • The horizontal and vertical gaze nystagmus;
  • The walk-and-turn; and
  • The one-leg stand. 

In the horizontal nystagmus test, supposedly the most reliable of the field sobriety tests, the officer looks for three things: involuntary eye jerking movement while tracking an object to one side, whether that jerking movement begins at less than a 45-degree angle, and whether the jerking continues once the gaze is as far to the side as possible.

The vertical gaze nystagmus is a variation of the horizontal test. It looks for involuntary eye jerking when the subject's gaze is directed as far upward as possible.

The walk-and-turn and one-leg stand are "divided attention" tests. Driving a vehicle requires the driver to perform multiple tasks simultaneously, and alcohol-impaired drivers frequently have difficulty handling more than one task at a time. Both of these tests have two stages, an instruction stage and a performance stage, and each stage requires the subject to handle multiple tasks at once.

On conclusion of the field sobriety tests, the officer may also have the driver undergo a preliminary breath test to gather additional chemical evidence.

Based on observations during one or more of the three phases, the police officer will decide whether probable cause exists to arrest the driver for driving under the influence.

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