Field Sobriety Tests (FST's) are used by police officers to determine if you are driving under the influence. While police officers administer a number of FST's, only three are recognized by the National Highway Traffic & Safety Administration, because only these three tests are associated with data sufficient to indicate their reliability in identifying intoxicated drivers.
Horizontal Gaze Nystagmus Test
The first is the Horizontal Gaze Nystagmus (HGN). In this test, the officer places something like a finger, a pen, or a penlight. The officer then moves the object from side to side, to see at what point the motorist’s eyes begin jerking movements, or nystagmus. Research indicates a correlation with alcohol use, if nystagmus starts before the gaze reaches a 45 degree angle. Because interpretation of this test is often subjective, officers should use other tests, to supplement the HGN.
The 'Walk and Turn' Test
One such test is the “walk and turn”. Part of the test involves the officer clearly explaining the procedure, so that he/she can assess the motorist’s ability to comply. In the first part of the test, the motorist is asked to put one foot in front of the other, in a straight line, with the heel of the forward foot touching the toes of the rear foot. The officer is supposed to demonstrate the technique. In the second part of the test, the driver is instructed to take nine heel-to-toe steps, and after doing so, the driver must turn around, and take another nine heel-to-toe steps, back to the point of origin. The driver is told to keep his/her hands by their side, to watch their feet, and to count each step.
Any two of the following is considered indicative of blood alcohol above .10%:
- Inability to stay balanced, during instructions
- Starting/stopping before instructed to do so
- Failure to touch heel-to-toe
- Stepping off the line
- Using arms to balance
- Improperly turning; and
- Using the incorrect number of steps
High heels, or uneven ground has been held to invalidate this test. Also, some folks (elderly, people with back or leg problems, etc) might not be able to perform this test even when sober.
The 'One Legged Stand' Test
Accordingly, another FST is used, along with eventual breath alcohol testing, to determine sobriety. That third FST is the “one legged stand” test, which has been found to be accurate about 2/3 of the time, in demonstrating alcohol content over the legal limit.
The subject is instructed to keep his/her hands at their sides, and to raise either leg about six inches off the ground; they are instructed to count, “one thousand one, one thousand two…”, and so one until they reach 1,030.
Hints of alcohol use, demonstrated by this test include:
- Marked, or significant swaying, while balancing
- Using arms to maintain balance
- Hopping on the stationary foot to maintain balance
- Resting the raised foot on the ground three or more times during the 30 seconds the test takes
Again, some folks will not be able to do the test even sober. As well, high heels and uneven ground can invalidate the test.