In an ideal world, a discussion of field sobriety tests would be irrelevant because everyone would refuse to do them when being investigated for a DWI. THEY ARE TOTALLY VOLUNTARY! However, the vast majority of potential clients that come to talk to us have done the tests. This is because cops will never tell you that they are voluntary tests. In fact, they will tell you to do them in a way that would make everyone think that they ARE required.
So what are field sobriety tests? These are tests (or, more accurately, physical agility exercises) that a suspect performs so that a cop can build his probable cause to arrest you for DWI. Cops would probably tell you that it works the other way too: it could tell them that you are not impaired and they should let you go. I'm still waiting to hear of a case where that has actually happened.
There are standardized field sobriety tests and non-standardized field sobriety tests. Almost always, a cop will have you perform the standardized tests. A non-standardized test is much less credible in nature. It can include the following tests: reciting the alphabet, finger dexterity tests, counting backwards, finger to nose, etc.
Standardized field sobriety tests are the ones developed by the National Highway Traffic Safety Administration (NHTSA). These tests are composed of specific instructions and elements of performance. Each test much be instructed, graded, and performed (in the case of the HGN test) in the EXACT same manner as the testing manual prescribes or the validity of that test will be deemed to be compromised.
There are three (3) standardized field sobriety tests: 1) Horizontal Gaze Nystagmus test (HGN), 2) Walk and Turn test, and 3) One Leg Stand test.
The cop will position an object (usually his finger or a pen) 12-15 inches away from the suspect's face and move the object from side to side while watching the suspect's eyes. The officer is looking for an involuntary jerking of the eye. This involuntary jerking is referred to as "nystagmus." Theoretically, the presence of nystagmus indicates that the suspect has a BAC of .08 or higher. Note the word "theoretically." There are dozens of natural causes of nystagmus that are completely unrelated to alcohol.
The cop instructs the suspect to take 9 heel-to-toe steps along a line, turn (taking a series of small steps), and take 9 heel-to-toe steps back. The cop is looking to see if the suspect can maintain the starting position, begin at the appropriate time, keep their arms at their side, continue without stopping, take the correct number of steps, "touch" heel to toe, stay on the line, and turn appropriately.
The cop instructs the suspect to stand with heels together, arms at the side, then raise one leg 6 inches off the ground while counting out loud until the cop allows the suspect to stop. The cop is looking to see if the suspect raises his arms, sways, hops, or puts his or her foot down. There are informal indicators that the cop is also looking for, such as the inability to stand still, body tremors, muscle tension, and any statements made by the suspect during the test.
These are supposedly tests that are designed to check "divided attention," a critical skill in operating a motor vehicle. However, there are many people who, for many innocent reasons, cannot perform these tests to the officer's satisfaction. They almost always pay the price with a DWI arrest. Also, the cop often sets the supsect up for failure by performing the test on unlevel, rocky, and/or slippery surfaces.
If your DWI lawyer is well versed in field sobriety testing, these tests can often be a fertile ground for cross examination of your arresting cop.