Each day, numerous drivers across the state of North Carolina are stopped by law enforcement for suspected driving under the influence. In addition to talking to them, the authorities often ask people to perform a series of roadside sobriety tests to help assess whether they have been drinking. Their performance on such tests could be used to provide probable cause for drivers’ arrests or as evidence of intoxication at trial.

According to FindLaw, the Standardized Field Sobriety Test includes a battery of three tests. Law enforcement employs the walk and turn, horizontal gaze nystagmus and one-leg stand tests to assess whether drivers might be under the influence of alcohol. The SFST is endorsed by the National Highway Traffic Safety Administration.

The walk and turn test assesses drivers’ ability to perform tasks while their attention is divided. For this test, people are asked to walk heel-to-toe in a straight line. They are directed to take nine steps in one direction, turn on one foot, and take nine steps back in the same manner. While those who have consumed alcohol may have trouble with this test, it is deemed easily performable by those who are unimpaired.

As people’s eyes gaze to the side, there is natural involuntary jerking that occurs. When impaired by alcohol, this jerking is exaggerated. Law enforcement watches for indicators such as distinct jerking when the eye is at its maximum deviation or within 45 degrees of center, or an inability to smoothly track a moving object, which may indicate that a person has been drinking.

The one-leg stand tests people’s balance. Drivers are directed to stand on one leg, raising the other approximately six inches off the ground, for 30 seconds. Law enforcement may note any swaying, hopping, toe-touching or use of the arms for balance as possible signs of impairment.

According to the NHTSA, alcohol impairment is accurately indicated in 91% of cases using the three components of the SFST together. Of the tests, a validation study of the SFST in San Diego found that the walk and turn was 70% accurate, the horizontal gaze nystagmus was 88% accurate and the one-leg stand was 83% accurate.