The 3 standard field sobriety tests (SFST) are the Horizontal Gaze Nystagmus Test (“Eye” test), the Walk and Turn and the One Leg Stand. Although they are called “tests,” you are not told what you are being graded on and you are not allowed to practice. To make things worse, you get no credit for good performance. Instead, you only lose points for improper performance. These “tests” are thus not fairly designed, especially considering the police are supposedly looking for “normal” use of your mental and physical faculties.

Since judges do not always side with defendants on suppression issues, it is critical that your lawyer also understand all methods to impeach the HGN test. If you can afford to fly in an expert witness, that is great, but many people cannot afford this. Consequently, the DWI defense lawyer’s courtroom skill may be all that you have between you and the jailhouse.

There are a number of ways to defeat the HGN test—even if the judge lets it in. See our section on the DWI “Eye” Test for more detailed information.

The other field sobriety tests can also be “invalidated” if the officer does not follow his training manual, which every DWI lawyer owns. This will usually not completely exclude the tests from evidence, but may limit the officer’s testimony or force him to admit that they were “invalidated.”

SFSTs are "the most effective procedure[s] for testing drivers at roadside to determine whether or not they are intoxicated." Many of the most reliable and practical psychophysical tests use the concept of divided attention. Divided attention requires an individual to concentrate on two things at once. Driving is an example of a divided attention task. A driver must simultaneously control steering, braking and acceleration, and react appropriately to changing conditions in order to operate a vehicle safely. Alcohol significantly reduces persons' ability to divide their attention between tasks. Even while under the influence, many people can handle a single, focused attention task fairly well, but cannot satisfactorily divide their attention to handle multiple tasks at once. Field sobriety tests that simulate the divided attention characteristics of driving are being used by police departments nationwide. The best tests use the same mental and physical capabilities that a person needs to drive safely: information processing; short-term memory; judgment and decision making; balance; steadiness, sure reactions; clear vision; small muscle control; and coordination of the limbs. Two divided attention field sobriety tests that have been proven accurate and effective in DWI Detection are the Walk-and-Turn and the One-Leg Stand

An additional test is the horizontal gaze nystagmus ("HGN") or “eye” test. When used in combination with divided attention tests, HGN helps police officers correctly distinguish suspects who are under the influence of alcohol from those who are not. The test is based on the fact that alcohol affects the automatic tracking mechanisms of the eyes. Nystagmus is defined as "an involuntary rapid movement of the eyeball, which may be horizontal, vertical, rotatory, or mixed." Alcohol slows down the eyes' ability to rapidly track objects and causes to eyes to oscillate, or "jerk", before they normally would in a sober person. Alcohol stimulates the nerve endings, making nystagmus more pronounced in intoxicated persons. As a person's blood alcohol concentration increases, the eyes will "jerk" sooner as they move to the side. The HGN test claims to gauge intoxication by measuring the involuntary oscillation of the eyes.

The procedure to be used by police officers is set out by the National Highway Traffic Safety Administration in the DWI Detection and Standardized Field Sobriety Testing Student Manual. Prior to administration of HGN, the eyes are checked for equal tracking ability and equal pupil size. If the eyes do not track together or if the pupils are unequal in size, injuries or medical disorders are likely the cause of the nystagmus. The NHTSA standardized clues include lack of smooth pursuit, distinct nystagmus at maximum deviation and onset of nystagmus prior to reaching a 45 degree angle. Standardized administration procedures include: holding the stimulus 12-15 inches in front of the suspect's nose; keeping the tip of the stimulus slightly above the suspect's eyes; always moving the stimulus smoothly; always checking for all three clues in both eyes; starting with suspect's left eye; checking the clues in sequence: (1)lack of smooth pursuit, (2) distinct nystagmus at maximum deviation, (3) onset of nystagmus prior to 45 degrees; always checking for clues at least twice in each eye. The NHTSA manual indicates that no other "clues" are recognized by the NHTSA as valid indicators of HGN. In particular, the NHTSA does not support the allegation that onset angle can reliably be used to estimate BAC, and considers any such estimation to be misuse of the horizontal gaze nystagmus test. The NHTSA sets forth standardized criteria for evaluating HGN. The maximum number of clues of horizontal gaze nystagmus suspect can exhibit is six, occurring if the suspect exhibited all three clues in each eye. If a suspect exhibits four or more clues, it should be considered evidence that the suspect's BAC is above .10.