A police officer may ask you to perform certain tests, “just to prove you’re capable of driving safely.” The tests measure varying levels of physical coordination and the ability to listen and to follow what the officer is telling the suspect to do. The tests are not natural, some are open to serious criticism and are often not properly explained by the officer.

The National Highway Traffic Safety Administration (NHTSA) validated a series of Standardized Field Sobriety Tests that are to be performed by police officers. They are the only tests that have been, through field validation studies, shown to have ability to detect legal intoxication. They are the Horizontal Gaze Nystagmus (HGN), the Walk and Turn, and the One Leg Stand. The results of these tests will be entered as evidence against you. But even under laboratory conditions, these tests have only been established to be accurate in predicting blood alcohol content above .10% in 65% to 77% of the time. Additionally, after your arrest, you have the right to request a second, independent blood test.

Standardized Field Sobriety Tests:
Horizontal Gaze Nystagmus
Horizontal gaze nystagmus is an involuntary jerking of the eyeball which occurs naturally. This test theorizes that when a person is impaired by alcohol, nystagmus is exaggerated, and the alcohol-impaired person will have difficulty smoothly tracking a moving object. To administer this test, an officer asks the subject to watch a slowly moving stimulus, such as the tip of a pen. As the officer moves the object horizontally (back and forth), the officer watches the subject’s eyes as they follow the object, watching for six standardized “clues,” up to three in each eye, to intoxication:

  • Lack of smooth pursuit (eye cannot smoothly follow stimulus).
  • Sustained and distinct nystagmus at maximum deviation (nystagmus longer than
  • 4 seconds when eye is moved furthest from center (approximately 60 degrees)).
  • Onset of nystagmus before eye reaches 45 degrees.

The presence of four or more clues is considered a failure of the test. The circumstances of a roadside administration, or a lack of training for the officer administering the test, can significantly affect and/or invalidate test results.


A subject is instructed to take nine steps along a straight line, 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.

The officer looks for eight clues of impairment, specifically, if the subject being tested:

  • Cannot keep balance while listening to officer’s instructions.
  • Begins before instructions are finished.
  • Stops walking.
  • Breaks the heel-to-toe walk by more than ½ inch.
  • Steps offline.
  • Raises arms more than 6 inches from side of body.
  • Improper turn.
  • Takes an incorrect number of steps.

One Leg Stand

The subject is instructed to stand on one leg, with the other foot suspended approximately six inches off the ground and count aloud by thousands (“One thousand-one”, “one thousand-two”, etc.) until instructed to put the foot down. The officer times the person for thirty seconds.

The officer looks for four clues of impairment, specifically if the subject being tested:

  • Puts the raised foot down.
  • Uses arms to balance (raises more than 6 inches from side).
  • Sways.
  • Hops.