The Science of Field Sobriety Tests
January 22, 2015
dui field sobriety test
Understand the science behind DUI field sobriety tests

Most people have a vague idea of what happens when the police stop a driver they suspect is under the influence of alcohol. The officer asks for the driver’s license and registration. If he smells alcohol, hears slurred speech, or sees that the driver has bloodshot or watery eyes, the officer will ask the driver to get out of the car and do three physical tasks that make up the Standardized Field Sobriety Test (SFST).

The National Highway Transportation Safety Association (NHTSA) has researched these tests and determined that a failure to perform the tasks can accurately predict that a driver has a blood alcohol concentration (BAC) of 0.08% or higher.

What most people do not know is what these tests actually measure. What does being able to follow a moving target with your eyes have to do with how many beers you had that night? What does being able to stand on one leg have to do with your ability to sit in your car while you drive? How can walking in a straight line tell the officer whether you can keep your car between two lines on a road?

To answer these questions, let’s look at the science behind each test and see what is really being examined if the police ask you to step out of your car.

The Horizontal Gaze Nystagmus Test (HGN)

Amazing as the human eye is, it has limitations. Because your eyes are forward facing, objects at the periphery of your visual range lose detail. Without turning your head, your eyes can compensate by following an object, but at the extreme periphery, your eyes will begin to involuntarily shake or bounce as they attempt to keep the object in focus. This shaking while following an object on a horizontal line is known as horizontal (or lateral) gaze nystagmus.

When a person is under the influence of alcohol or any type of depressant drugs, his or her brain’s control over the eye muscles is impaired. Thus, the involuntary shaking of the eyes occurs at angles closer to their centered line of vision, and is more pronounced at the extreme angles.1

To test the degree of the intoxication, the officer will have you follow an object (such as a pen or flashlight) back and forth in a horizontal line without turning your head. The officer will test each eye and look for the following three indicators:

  1. Whether your eyes follow the object in a smooth or jerky manner;
  2. Whether the nystagmus occurs at 45 degrees or less from your center line of vision; and
  3. Whether your eyes distinctly jerk at the maximum angle from center.

After testing both eyes, if the officer observes a combined total of four or more of these conditions, you are likely to have a BAC of 0.08% or higher. The NHTSA’s research indicates that the HGN test is reliable for 88% of drivers suspected of driving under the influence, and it is the most reliable of the three tests in the SFST.2

The Walk and Turn Test (WAT)

Another test that the officer will likely have you perform if you are stopped for a DUI is the Walk-and-Turn test. In this test, the officer will have you take nine heel-to-toe steps (meaning your front foot’s heel is placed directly against your back foot’s toes) on a straight line while your arms remain at your side, turn on one foot, and then take another nine steps back to where you started.

You might think that this test is only about coordination and balance, because those two functions are easy to identify and observe as being impaired by alcohol. However, the WAT also tests a hidden function: your ability to divide your attention between receiving verbal instructions and performing a basic motor skill. While intoxication definitely affects coordination and balance, it also interferes with multitasking, and you may only be able to concentrate on one of the two areas with which your brain is tasked.

The officer will be looking for two or more of these telltale signs of intoxication:

  1. Lack of balance while receiving instructions
  2. Starting to walk before the instructions are finished
  3. Stepping off the line while walking
  4. Lacking the ability to step in a heel-to-toe manner
  5. Improper turn on one foot
  6. Stopping to regain balance
  7. Using your arms to balance
  8. Taking too many or too few steps

The NHTSA’s research indicates that the WAT is 79% reliable, making it the least reliable of the three tests in the SFST.3

The One Leg Stand Test (OLS)

The final test consists of the officer instructing you to stand on one leg with your other leg extended outward, as if you were kicking a soccer ball. You are to hold your foot six inches above the ground, and count out loud by the thousands, as in “one thousand one, one thousand two,” and so on.

The officer will observe you in this position for about 30 seconds, and will be looking for two or more of the following clues:

  1. Do you sway back and forth?
  2. Do you put your foot down before being told to do so?
  3. Do you use your arms to maintain your balance?
  4. Do you hop to stay balanced?

Like the WAT, the OLS is also a divided attention test, meaning that if you are intoxicated, you may find it difficult to keep counting and maintaining your balance on one leg. The NHTSA’s research indicates that the OLS is 83% reliable in identifying persons who have a BAC of 0.08% or higher.4

Contact Wallin & Klarich Today If You Have Been Arrested for DUI

If you or someone you know has been arrested for DUI, you should speak to an experienced criminal defense attorney at Wallin & Klarich today. A DUI conviction is not always an easy case for the prosecutor to make. Though SFST tests are generally considered reliable, there may be reasons why the tests are unreliable in your case. Using an effective defense strategy specifically tailored to the facts of your case, we may be able to get the charges against you reduced or dismissed altogether. We can help you minimize the consequences and help you achieve the best possible result in your case.

Our attorneys at Wallin & Klarich have over 30 years of experience successfully fighting against DUI charges. With offices in Los Angeles, Sherman Oaks, Torrance, Tustin, San Diego, Riverside, San Bernardino, Ventura, West Covina and Victorville, Wallin & Klarich has been successful in defending thousands of clients charged with driving under the influence of alcohol and drugs.

Call us today at (877) 4-NO-JAIL or (877) 466-5245 for a free, no obligation, telephone consultation. We will be there when you call.

1. See, Humphrey Belton, Lateral Nystagmus: A Specific Diagnostic Sign of Ethyl Alcohol Intoxication, 100 N.Z. Med. J. 534, 535 (Aug. 1987).

2. Stuster, J. and Burns, M., Validation of the Standardized Field Sobriety Test Battery At BACs Below 0.10 Percent, U.S. Dept. of Transp., August1998.

3. Id.

4. Id.

Leave a Reply


Contact Us
