How valid are these FSTs? Not very, according to Orange County DUI attorney Lawrence Taylor, a former prosecutor and the author of the leading legal textbook "Drunk Driving Defense, 6th edition". The tests are basically "designed for failure". In 1991, Taylor reports, Dr. Spurgeon Cole of Clemson University conducted a study on the accuracy of field sobriety tests. His staff videotaped 21 individuals performing 6 common field sobriety tests, then showed the tapes to 14 police officers and asked them to decide whether the suspects had "had too much to drink to drive." Unknown to the officers, the blood-alcohol concentration of each of the 21 subjects was .00%. The results: 46% of the time the officers gave their opinion that the subject was too inebriated to drive. In other words, the FSTs were hardly more accurate at predicting intoxication than flipping a coin. Cole and Nowaczyk, "Field Sobriety Tests: Are They Designed for Failure?", 79 Perceptual and Motor Skills 99 (1994).
What about the new, improved "standardized" DUI tests? Research funded by the National Highway Traffic Administration determined that the three most effective field sobriety tests were walk-and-turn, one-leg stand, and horizontal gaze nystagmus. Yet, even using these supposedly more accurate -- and objectively scored -- tests, the researchers found that 47% of the subjects who would have been arrested based upon test performance actually had blood-alcohol concentrations of less than the legal limit. In other words, almost half of all persons "failing" the tests were not legally under the influence of alcohol!
According to the Orange County DUI lawyers in Mr. Taylor's Southern California law firm, the fact that these tests are largely unfamiliar to most people, and that they are given under extremely adverse conditions, make them more difficult for people to perform. As few as two miscues in performance can result in an individual being classified as "impaired" because of alcohol consumption when the problem may actually be the result of unfamiliarity with the test.
In short, field sobriety tests are unreliable and can be effectively handled in trial by an experienced DUI lawyer.