Scientists claim that there is “not a single study” that links the Standardized Field Sobriety Tests (SFSTs) to driving impairment.
Furthermore, neither National Highway Traffic Safety Administration (NHTSA), United States Department of Transportation (DOT), nor the NHTSA-commissioned researchers have claimed that the standardized tests are actual direct indicators of actual driving impairment. In fact, they specifically state that “Driving a motor vehicle is a very complex activity that involves a wide variety of tasks and operator capabilities. It is unlikely that complex human performance, such as that required to safely drive an automobile, can be measured at roadside.”
One study has stated that “even valid, behavioral tests are likely to be poor predictors either of actual behind-the-wheel driving.”
Some police agencies tend to agree, at least in terms of the nystagmus test , that some standardized field sobriety tests do not indicate a driving impairment. For example, the California Highway Patrol Manual states that the HGN [horizontal gaze nystagmus test] is not a psychophysical test. The clues associated with HGN are not designed to be considered signs of impairment.
It is clear that alcohol, at elevated levels, causes mental and physical impairment that affects the safe operation of a motor vehicle. However, whether a non-chemical field sobriety test can reliably elicit enough reliable information for an expert to make an informed opinion regarding the sobriety or influence of a particular individual at a particular time is controversial.
Non-chemical tests have inherent issues that may make many of the tests unreliable or marginally reliable. Some of these issues include:
In addition, the use of statistics from one set of data to support similar tests that are administered in a different manner (e.g. using standardized field sobriety test statistics of non-standardized field sobriety test administered tests) can be troublesome.
Therefore, scientists must consider all aspects of non-chemical performance before rendering an opinion of driving impairment or sobriety or making a statement regarding the impairment of “all persons” at a particular level.