Field Sobriety Tests were created to demonstrate a driver's level of cognitive function and coordination, both of which are diminished when under the influence of drugs or alcohol. The test usually involves simple tasks such as standing on one leg or following a moving instrument with your eyes. If a driver fails to successfully complete any one of these tasks, law enforcement officers can consider it probable cause for your arrest or for you to take a chemical test.
These tests are nearly impossible to pass even sober, they are almost always just the key to getting probable cause for you to take a breathalyzer test or a blood test.
There are three FST's that officers utilize during a DUI investigation: the Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn (WAT), and the One-Leg Stand (OLS). These are the only three field sobriety tests endorsed by the National Highway Traffic and Safety Administrations (NHTSA), thus they are referred to as "Standardized" Field Sobriety Tests.
Horizontal Gaze Nystagmus
This is an involuntary jerking of the eye that occurs when the eyes move from side to side. The test requires the driver to look forward at a small object, usually the officer's finger, pen, or a small flashlight and follow the object with their eyes as he moves it side to side. The drivers head is to remain still during the test. Officers look for three indicators of impairment when administering this test: 1) an inability to follow the moving object smoothly; 2) distinct eye jerking when the eye is at maximum deviation (the point at which the eye has moved fully to one side); and 3) whether the jerking of the eye is within 45 degrees of center.
This tests the driver's ability to complete tasks with divided attention. Law enforcement officers believe that the inability to perform a task that requires divided attention is a sign of intoxication. During this test, the driver is instructed to walk, touching heel-to-toe on each step, along a straight line, turn, and walk the straight line back to the start. The officer will look for several indicators of impairment: 1) if the driver cannot keep their balance; 2) walking before the instructions are finished; 3) stopping in the middle of the test; 4) failure to touch heel-to-toe; 5) steps off the line; 6) takes the wrong number of steps; and 7) makes an improper turn.
This is also a divided attention test. The driver is instructed to stand with one leg in the air approximately six inches off the ground while counting to a certain number. The officer looks for indicators of impairment, including: 1) swaying; 2) difficulty counting; 3) using their arms to keep balance; and 4) putting the foot down.