An officer who suspects a driver is under the influence of drugs or alcohol might ask the person to get out of the car and perform some roadside tests. These tests are known as "field sobriety tests," or FSTs for short. Police use FSTs as a tool to determine whether there’s probable cause to arrest a person for driving under the influence (DUI).
In the mid-1970s, the National Highway Traffic Safety Administration (NHTSA) started researching behavioral tests that could be used to determine whether a driver was under the influence. Through its research, the NHTSA eventually developed three “standardized” FSTs for assessing driver impairment:
- horizontal gaze nystagmus (HGN)
- walk-and-turn, and
- one-leg stand.
Collectively, these three tests are often called the standardized FST “battery.” The NHTSA concluded these three tests were reliable tools for determining whether a driver has a blood alcohol concentration (BAC) of .1% or more.
Since the early 1980s, police have used the standardized FSTs to identify intoxicated drivers. An officer’s decision on whether to make a DUI arrest often depends on the driver’s FST performance.
Officers typically must do training and become certified in how to administer FSTs according to NHTSA standards. This training is supposed to teach officers how to properly administer and interpret a suspect’s performance on these tests. Police are trained to look for "clues" of impairment in each test. If the officer notices a certain number of clues, the person is deemed to have failed the test. Failed FSTs make it more likely the officer will arrest the person for DUI.
In addition to supplying probable cause for a DUI arrest, performance on FSTs can also come into play in DUI trials. In most jurisdictions, officers are allowed to testify in court about observations they made during the tests. Poor performance on FSTs can be used to show the person was too impaired to drive safely—proof of an impairment DUI.
Though the NHTSA has sanctioned only the three standardized FSTs as reliable indicators of impairment, police routinely use other FSTs during DUI investigations. Common non-standardized FSTs include the:
- alphabet recitation
- finger to nose
- hand pat
- number countdown
- four-finger count
- vertical gaze nystagmus (VGN), and
- Rhomberg balance test.
Police use these tests for the same purpose as they use standardized FSTs: deciding whether to make a DUI arrest. However—unlike with the standardized tests—there’s no NHTSA research showing non-standardized FSTs are accurate indicators of intoxication. Without a proven correlation between test performance and impairment, courts might be less likely to accept theses unverified tests as proof of inebriation.