Field Sobriety tests
 The
Standardized Field Sobriety Test, or the SFST, consists of
three tests that are commonly used by police officers to determine
a persons blood alcohol level (BAC) and establish probable cause for arrest. Police officers are trained to conduct the SFST when a person shows any signs of intoxication while driving a vehicle.
The three common tests of the SFST are:
The Horizontal Gaze Nystagmus (HGN)
The Walk and Turn (Divided Attention Testing)
The One-Leg Stand (Divided Attention Testing)
Police officers administer these tests
systematically and evaluate the suspect according to their
response.
The National Highway Traffic and Safety Administration guidelines
were set up to help make these tests more accurate but unfortunately,
many people who are asked to perform these tests fail due to
physical disabilities, health problems, over weight, and other
uncontrolled factors.
The Horizontal Gaze Nystagmus Test (HGN)
Horizontal gaze nystagmus is an involuntary
eye movement or jerking of the eyeball which occurs naturally
as the eyes move from left to right. Under normal circumstances,
nystagmus occurs when the eyes rotate at high peripheral angles
but when a person is impaired by alcohol, nystagmus is highly
exaggerated.
In the HGN test, the police officer instructs
the suspect to follow a moving object such as a pen or flashlight,
horizontally while he observes the movement of the eyes. The
police officer looks for three indicators of impairment which
are:
Eye cannot follow object smoothly
Jerking is distinct when the eyes are at maximum deviation
Eyes are jerking prior to reaching
45 degrees of center
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