Police conduct various field sobriety tests, but only three tests have been standardized by the National Highway Traffic Safety Administration (NHTSA). Here's a rundown of the three:
Horizontal Gaze Nystagmus (HGN)
In an HGN test, the driver is asked to track a moving object, usually the police officer's finger or flashlight. An impaired driver would normally find it difficult to track the object because the nystagmus—involuntary eye movement—happens at lesser angles. Police also use the HGN test to find out what kinds of drugs the driver may have consumed.
The WAT test is one of two divided attention tests (the other being the one-leg stand test) where the officer instructs the driver to perform simple physical actions. Usually, the driver will be asked to walk on a straight line, heel-to-toe. A driver with a blood alcohol content (BAC) of 0.08 or higher may commit more than one mistake (e.g. falling down or balancing with arms).
One-Leg Stand (OLS)
The OLS test, as the name implies, requires the driver to stand on only one leg and lift the other six inches above the ground. The driver will also count while performing an OLS for 30 seconds but starting at 1,000 (i.e. 1,001, 1,002, 1,003). Just like the WAT test, committing two or more errors suggests a BAC of 0.08 or higher.
The combination of these standardized field sobriety tests accounts for an accuracy rate of 91 percent, according to studies.