Which SFST is considered most effective at determining impairment?

The Horizontal Gaze Nystagmus (HGN) test is regarded as the most effective at determining impairment among the standardized field sobriety tests. This is because it specifically measures the involuntary eye movement known as nystagmus, which becomes more pronounced when an individual is impaired, particularly by alcohol. During the test, a trained officer observes the subject's eyes as they track a moving object, assessing for clues that indicate impairment.

Research has shown that the HGN test has a high level of reliability and validity in detecting blood alcohol content (BAC) at or above the legal limit. It tests how well the eyes can smoothly follow an object, and impairment typically results in jerky movements rather than smooth tracking. The accuracy of this test makes it a pivotal tool in assessing impairment, leading many law enforcement agencies to prioritize it during DUI investigations.

The other tests, while also important, do not measure physiological responses as directly as the HGN. For example, the One Leg Stand and Walk and Turn tests assess balance and coordination, which can be affected by various factors unrelated to alcohol consumption. The Preliminary Breath Test, on the other hand, measures breath alcohol concentration but does not directly gauge an individual’s physical or behavioral impairment at that moment.