For as long as there has been DWI enforcement, police officers have been making judgment calls about whether to arrest individuals for drunk driving. Part of this process has involved the administration of "sobriety tests" to help police officers decide. These tests developed haphazardly, with different police officers and departments giving different tests. Beginning in the 1970's, two police officers in California initiated a push to standardize this process. A series of studies, which were sponsored by the National Highway Transportation Safety Administration (NHTSA) and conducted by the Southern California Research Institute (SCRI) resulted in the widespread use of the NHTSA Standardized Field Sobriety Test (SFST) battery. According to these studies, which have been challenged and are not universally accepted, the three SFST's are able to predict, with some degree of reliability, whether a person's blood alcohol concentration (BAC) exceeds the .08% legal limit.
Two of the SFST's, the walk and turn (WAT) and the one leg stand (OLS), are physical coordination tests which are designed to test an individual's ability to divide his/her attention. Since driving is a divided attention task, the concept behind these two SFST's is that a person who is unable to successfully complete these two tests is too impaired by alcohol to safely drive a car. The WAT requires a person to listen to instructions, take nine heel to toe steps down a line, turn in a specified manner, and take nine heel to toe steps back. The officer is taught to look for eight clues. These clues include starting too soon, missing heel to toe, stepping off the line, and making an improper turn. If a person displays two or more of the eight clues, the officer is taught to view this as a sign that the individual has a BAC higher than .08. The OLS operates on the same concept of divided attention. The individual is asked to raise one foot six inches off the ground and to count out loud "one-thousand and one, one-thousand and two..." up to thirty. While doing this, the subject is supposed to look at his/her raised foot. The officer looks for four clues and is trained to view the presence of two or more of these clues as evidence that the subject has a BAC greater than .08. The four clues are sways while balancing, uses arms to balance, hopping, and puts foot down.
The other test which is part of the SFST battery is the Horizontal Gaze Nystagmus (HGN) test. Nystagmus is defined as an involuntary jerking of the eyes. This jerking has been compared to a marble rolling over sandpaper or a wind shield wiper moving across a dry windshield. The officer is trained to look for six clues (three in each eye) which are considered to be progressive indicators of an elevated BAC. The first clue is "lack of smooth pursuit." This clue is observed when the subject follows a stimulus (i.e., a pen or the officer's finger) across his/her field of vision. The second clue is called "distinct and sustained nystagmus at maximum deviation." For this clue to be observed, the officer must hold the stimulus at the maximum deviation from the center of the subject's field of vision and must observe nystagmus for a four second hold. Frequently, officers will record this clue against a motorist without holding it for the full four seconds. This is incorrect and can result in errors by officers administering the HGN test. The third and final clue is called "onset prior to 45 degrees." This clue indicates a higher level of alcohol impairment than either of the first two clues. Police officers frequently make errors in detecting this clue. For an officer to detect this clue properly, he/she must move the stimulus slowly across the subject's field of vision and correctly estimate a 45 degree angle.
The NHTSA battery of three SFST's is doubtless an improvement over the random approach that law enforcement had taken to field sobriety testing prior to their development. Nevertheless, none of these tests, either taken alone or in combination, are fool proof indicators of an elevated BAC. The most important thing to remember about these tests is that for the conclusions suggested by SFST's to be valid, the tests must be administered properly and in a standardized manner. This frequently opens up opportunities for attack, since police officers rarely get the administration of these tests completely right. The HGN test in particular takes skill to administer properly. It is really a medical test administered by officers who receive very limited training. Police officers almost invariably fail to follow all of the instructions which are required to correctly administer HGN. The walk and turn and one leg stand test are both, in my view, as much tests of the subjects coordination as they are of his/her level of impairment. For a person with coordination issues, or for a person who is fifty or more pounds overweight, these two tests are difficult to perform regardless of whether the subject has consumed any alcohol.