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How do police officers determine if a driver is impaired?

On Behalf of | Jan 25, 2024 | Criminal Defense

Driving under the influence of alcohol or drugs is a serious offense that poses significant risks to an impaired driver and to other road users. Law enforcement officers generally use observational skills, standardized field sobriety tests (SFSTs) and chemical tests to determine if a driver is impaired.

An initial assessment often begins with the officer’s observation of a driver’s behavior and how they operate their vehicle. Signs such as erratic driving, inability to maintain lane discipline or unusual behavior during a traffic stop can all be indicators of impairment.

Once a driver is pulled over, officers use specific tests to gauge the level of impairment. These tests are critical in building a case for a DUI charge because they (theoretically) provide measurable and standardized evidence of a driver’s condition.

Standardized field sobriety tests

The standardized field sobriety tests are three tests that officers use to observe coordination and balance.

  • Horizontal gaze nystagmus (HGN) test: An officer observes a driver’s eyes as they slowly move a pen or similar object horizontally to track the involuntary jerking of the eye
  • Walk-and-turn: The driver walks nine steps, heel-to-toe, along a straight line, then turns on one foot and returns similarly
  • One-leg stand test: The driver must stand with one foot approximately six inches off the ground and count aloud until told to put their foot down

The results of the SFST are highly subjective so that they may be questioned as part of a defense strategy. Still, they play a pivotal role in drunk driving arrests because their results can help to give officers probable cause for an arrest.

Chemical tests for blood alcohol concentration

Chemical tests, including breath, blood and urine, measure a driver’s blood alcohol concentration (BAC). The most common is the breath test, which analyzes a breath sample to determine the alcohol level.

Blood tests, though more invasive, are highly accurate in measuring the actual amount of alcohol or drugs in the driver’s bloodstream. Urine tests are less common but can be used to detect the presence of certain drugs.

Every aspect of a drunk driving stop and arrest can influence a defendant’s defense strategy if it hasn’t been approached properly. As such, it is generally a good way to seek legal guidance in the wake of an arrest instead of simply pleading guilty due to a fear that there is no way to overcome charges.