How do police officers determine driver impairment?

On Behalf of | Jan 3, 2024 | DUI |

Police who initiate a traffic stop because they believe that a driver is impaired need to determine if that’s the case. There are two primary methods for doing this – field sobriety tests and chemical tests.

Chemical blood alcohol concentration (BAC) tests and standard field sobriety tests (SFSTs) serve different purposes and are subject to varying degrees of scientific scrutiny and legal acceptability. A comprehensive understanding of these tests is essential for anyone dealing with DUI charges.

Chemical blood alcohol content tests

Chemical BAC tests are designed to measure the amount of alcohol in a person’s bloodstream. These tests are scientifically reliable and widely used as evidence in DUI cases. The most common types of chemical BAC tests include breath, blood and urine tests.

Breath tests are conducted using a special device. These tests estimate BAC by measuring the amount of alcohol in the breath. They are non-invasive and provide immediate results, making them a popular choice for roadside testing.

Blood tests are considered the most accurate method for measuring BAC. They involve drawing a blood sample and analyzing it for alcohol content. Due to their invasive nature, they’re generally used when breath tests are not feasible or when more precise results are required.

Urine tests are less common and considered less reliable. These tests are occasionally used when breath or blood tests are unavailable, or when drugs – not alcohol – are the suspected source of a driver’s impairment.

Standardized field sobriety tests

SFSTs are a series of physical and cognitive tests used by officers at the roadside to assess a driver’s impairment. Approved by the National Highway Traffic Safety Administration, these three specific tests are standardized and include:

  • One-leg stand: This test involves standing on one leg for a set period, testing balance and focus.
  • Horizontal gaze nystagmus: This test involves following an object with the eyes to observe the natural jerking of the eyeball, which becomes exaggerated when a person is impaired.
  • Walk-and-turn: This test assesses a person’s ability to perform tasks requiring attention, balance and coordination.

Some departments may use additional tests like asking a driver to recite the alphabet backward or to count the number of fingers an officer raises. However, these non-standardized tests are generally considered too unreliable to be admissible in court.

Individuals who are facing DUI charges should know these tests’ roles and limitations. Understanding these tests can significantly impact the course of a DUI case, so seeking legal guidance from a professional who can assess their role in a defense strategy is generally wise.