FSTs generally consist of a battery of three to five exercises, such as walking and turning in a different direction, standing on one leg, nystagmus (using the eyes to track an object), touching the finger to the nose and reciting the alphabet.

The officer may subjectively decide whether the individual failed the test, or he may decide after applying recently-promulgated federal scoring standards. The NHTSA thinks these tests are a reliable way to gauge sobriety; however, there is mounting evidence that they are anything but.

Many FSTs are based on the concept of divided attention, which occurs when an individual must concentrate on more than one thing at the same time. A driver must divide her attention among various physical and mental activities to safely operate a vehicle, and her ability to do so is significantly reduced by the consumption of alcohol.

Ideally, FSTs evaluate the same mental and physical capabilities that a person needs to drive: information processing; short-term memory; judgment and decision making; steadiness, sure reactions; clear vision; small muscle control; and coordination of the limbs.

The most recently developed of the FSTs is the horizontal gaze nystagmus (HGN) test. HGN does not test a driver's ability to divide his attention, but it is thought to be an even more accurate means to detect whether a person has consumed alcohol because it measures involuntary eye movement.

Nystagmus is an involuntary oscillation of the eye, and it occurs naturally in human eyes when they focus too far off center (straight ahead). If a sober person tracks an object from side to side, the eyes will follow it smoothly up to a certain point, when they will begin to oscillate. If an intoxicated person tracks an object from side to side, alcohol will make nystagmus more pronounced, meaning the eyes will oscillate sooner.

However, alcohol is not the only cause of pronounced nystagmus. Nystagmus can be caused by inner ear problems, flu, strep throat, measles, syphilis, muscular dystrophy, multiple sclerosis, hypertension, glaucoma and epilepsy.

Temporary conditions such as motion sickness, sun stroke, eye strain or fatigue, and changes in atmospheric pressure may also result in nystagmus. Further, the consumption of common substances such as caffeine, nicotine, or aspirin can also lead to nystagmus. An individual's circadian rhythms or biorhythms also affect nystagmus readings.

The integrity of the original research affirming the validity of the HGN test has been called into question, and in 2001 additional research indicated that the test is improperly administered by most police officers. HGN, however, is not the only FST to be called into question. The entire battery of tests appears to offer dubious evidence of intoxication.

The most obvious problem is that the conditions under which FSTs are given almost guarantee failure: usually late at night; along a graveled or sloped roadside; with unsteady lighting from passing cars? headlights, the officer's flashlight and patrol car's strobe and headlights; and sometimes gusts of wind from intemperate weather or passing cars. The test is given to a person who is usually nervous and possibly frightened.

In 1991, Clemson University conducted a study on the accuracy of FSTs. Police officers were shown videotapes of individuals performing six common FSTs and asked to determine whether they were too intoxicated to drive. The officers concluded that almost half of the people were too drunk to drive; however, unknown to the officers, none of them had consumed any alcohol.

Given the questionable reliability of FSTs, it is no wonder they are surrounded by controversy and DUI defense attorneys often advise people not to submit to them.