A standardized field sobriety test (SFST) involves a series of assessments used by law enforcement officers to determine if a driver is impaired by alcohol or drugs. These tasks are designed to provide objective evidence for officers to establish probable cause for arrest and are standardized to ensure consistency and reliability in their administration and interpretation.
The National Highway Traffic Safety Administration (NHTSA) backs three standardized field sobriety tests. These tests have been scientifically validated and are widely used by police departments across the United States.
Horizontal gaze nystagmus test
The horizontal gaze nystagmus (HGN) test involves an officer observing the person’s eyes horizontally as they follow a moving object, such as a pen or flashlight. Nystagmus is an involuntary jerking of the eyeball, which becomes exaggerated under the influence of alcohol or certain drugs.
Officers look for three indicators in each eye: the inability to follow the object smoothly, distinct and sustained nystagmus at maximum deviation and the onset of nystagmus before the eye reaches a 45-degree angle.
The walk-and-turn test assesses an individual’s ability to complete tasks requiring divided attention. The individual is instructed to take nine steps, heel-to-toe, along a straight line, turn on one foot and return in the same manner.
Officers look for eight indicators of impairment, including:
- Inability to balance during instructions
- Starting before instructions are finished
- Stopping while walking
- Not touching heel-to-toe
- Stepping off the line
- Using arms to balance
- Making an improper turn
- Taking an incorrect number of steps
The test indicates impairment if the individual exhibits two or more indicators.
One-leg stand test
The one-leg stand test evaluates balance and concentration. The person must stand with one foot approximately six inches off the ground while counting aloud by thousands until told to put the foot down, typically for 30 seconds.
Officers watch for four signs of impairment: swaying while balancing, using arms to balance, hopping to maintain balance and putting the foot down. Exhibiting two or more of these indicators suggests impairment.
Because these tests are subjective, they’re often challenged during the criminal defense process. Working with a legal representative can help defendants to figure out how to challenge them most effectively.