Police and prosecutors in New Jersey must prove that a person charged with driving while intoxicated (DWI) was impaired by alcohol or drugs when they were operating a vehicle. They frequently do this with evidence that a person’s blood alcohol concentration (BAC) was 0.08 percent or higher, but they can also prove impairment in other ways. An officer’s observations of a driver’s behavior and appearance, when combined with evidence of the officer’s experience in assessing intoxication, can serve as evidence for the purpose of a DWI prosecution. Field sobriety tests (FSTs) allow officers to evaluate whether a person shows physical signs of intoxication and then offer testimony about how the person performed. The reliability of FSTs has some scientific support, but they remain controversial and subject to challenge by DWI defendants.
New Jersey’s implied consent law requires drivers to submit breath samples during DWI investigations, but this usually happens after an arrest. An officer who has stopped a vehicle on suspicion of DWI might use FSTs to establish probable cause for an arrest. The earliest examples of FSTs probably date back to the beginning of the automobile era. No uniform standard for FSTs existed in the U.S. prior to the 1970s. Different police departments developed their own FST regimens, with little to no scientific study into their accuracy or reliability in assessing impairment.
In 1975, the National Highway Traffic Safety Administration (NHTSA) retained the Southern California Research Institute (SCRI) to research multiple FSTs in order to identify the ones with the highest accuracy. The SCRI conducted laboratory and field tests, resulting in three Standardized Field Sobriety Tests (SFSTs) that the NHTSA now promotes as a uniform standard for the whole country. They are the Walk-and-Turn test, the One-Leg-Stand test, and the Horizontal Gaze Nystagmus (HGN) test.