To develope clues of impairment against someone suspected of a traffic alcohol offense, officers will likely initiate a combination of National Highway Traffic Safety Administration (NHTSA) Standardized Field Sobriety Tests. These tests include the Horizontal Gaze Nystagmus (HGN), the Walk and Turn, and the One Leg Stand. The tests are used in conjunction with each other to provide circumstantial evidence of your impairment.

Horizontal Gaze Nystagmus (HGN):

The HGN test measures the natural involuntary jerking of the eye that is intensified while under the influence of alcohol. When impaired by the influence of alcohol, studies show there are six indicators of impairment during this test, three in each eye. The officer must find four or more to conclude that the suspect is impaired:

• Subject’s eyes cannot smoothly track the pen.
• Subject demonstrates a distinct and sustained nystagmus at maximum deviation, or approximately 60 degrees from the center.
• Subject’s nystagmus begins before the eyes turn 45 degrees from the center. 

In a highly controlled laboratory setting, the One Leg Stand test scored only an 83% accuracy rate, the Walk and Turn a 79% accuracy rate, and the HGN an 88% accuracy rate at predicting a 0.08 BAC.

Walk and Turn:

The subject must take nine steps along a straight line, real or imaginary, walking heel-to-toe. After taking the steps, the subject must turn on one foot and return in the same manner in the opposite direction. There are eight indicators of impairment during this test, of which the officer must find two or more to conclude that the subject is impaired:

• Subject used arms for balance.
• Subject cannot maintain balance during instructions.
• Subject turns improperly.
• Subject does not take the proper number of steps.
• Subject steps off the line.
• Subject stops to regain balance.
• Subject fails to walk heel-to-toe.
• Subject begins to walk before the instructions are completed.
  

One Leg Stand:

The subject must stand on one foot with the other six inches off the ground. While holding this position, he or she must count by thousands to 30. There are four indicators of impairment during this test, of which the officer must find two or more to conclude that the suspect is impaired:

• Subject used their arms to balance.
• Subject swayed to regain their balance.
• Subject hopped to regain their balance.
• Subject put the raised foot down early.

There are many ways to question the validity of SFST tests. For example, variations in the roadside setting, training of the officers, and the methods of administration. Because of the imperfect nature of sobriety tests, law enforcement officers often use chemical testing of BAC to provide further evidence of impairment.

If the investigating police officer deems it necessary, a chemical examination through the use of a blood, urine, or breath samples may be used to determine the alcohol content of a suspect.  An Intoxilyzer, known colloquially as a Breathalyzer, is the most common means of measuring BAC.