Police frequently use field sobriety tests (FSTs) during DUI stops to gauge whether a motorist is inebriated. FSTs are roadside tests designed for evaluating a driver's mental and physical abilities. In theory, the effects of alcohol will cause an intoxicated driver to perform more poorly on these tests than a sober motorist.
Generally, a driver isn't required to take a field sobriety test. (In contrast, a driver arrested for driving under the influence generally must take a blood, breath, or urine test at an officer's request.) In other words, FST participation is voluntary. But an officer's decision of whether to arrest a suspect for driving under the influence often depends on how well the driver does on FSTs.
According to National Highway Traffic Safety Administration (NHTSA) sponsored research the following three FSTs are accurate indicators of when a driver has a blood alcohol concentration (BAC) of .1% or more:
These three tests are commonly called the "standardized" FSTs or the FST "battery." While conducting a DUI investigation, an officer might ask a suspect to complete one, two, or all three of the standardized FSTs. The NHTSA's studies showed that officers were more successful at detecting impairment when they administered all three tests.
Officers are ordinarily required to do training in how to give the standardized FSTs according to NHTSA standards. At these trainings, officers are supposed to learn the procedures for administering these tests and the "clues" of impairment to look for.
FSTs are scored by the number of clues the officer observes: a certain number of clues amounts to a failed test. For instance, a motorist fails the HGN test if the officer notices four or more clues.
Though FSTs are used primarily by police to determine whether there's probable cause for a DUI arrest, prosecutors sometimes use poor FST performance to prove an impairment DUI charge at trial. Most states allow officers to testify in court about observations they made while giving these tests.
There are lots of non-standardized FSTs—which haven't been approved by the NHTSA as reliable—that police commonly use during DUI investigations. For example, an officer might ask a motorist to:
Though police often use non-standardized tests to determine whether a suspect is under the influence, there's no NHTSA research confirming these tests are reliable indicators of intoxication.
With no proven relationship between impairment and test performance, courts might be less apt to accept the results of non-standardized as evidence of a driver's impairment. Some courts might even prohibit officers from testifying in court about observations they made during one of these unverified FSTs.
If you've been arrested for DUI, make sure you get in contact with an experienced DUI attorney right away. DUI law is complicated and varies by state. A local DUI lawyer can talk to about the law in your jurisdiction, let you know if you have any available defenses, and explain the process.