Field Sobriety Tests in DUI Cases


Field sobriety tests ("FSTs") are a series of physical and mental exercises that police administer in DUI investigations. Poor performance on FSTs is thought to be a sign of impairment from alcohol or drugs. As a result, law enforcement relies on them a great deal in deciding whether to arrest someone.

The following chart shows the claimed accuracy of the three “standardized” field sobriety tests when correctly administered under proper conditions:1

Field Sobriety Test

Accuracy at determining .08% BAC

Horizontal gaze nystagmus (HGN)


Walk-and-turn (WAT)


One-leg stand (OLS)


But even when conditions are ideal (which they often are not), sober drivers will sometimes appear intoxicated.

Drivers do not have to take California field sobriety tests

In California, field sobriety tests are completely optional. Drivers can decline to take them without any penalty whatsoever.2

This is important because even sober drivers can fail FSTs for reasons having nothing to do with their blood alcohol content (“BAC”). The driver may then have to defend against wrongful criminal charges such as:

To help you better understand field sobriety tests (FSTs), our California DUI defense lawyers discuss, below:

Even “reliable” field sobriety tests are inaccurate at least 12% of the time.

1. What are the “standardized” field sobriety tests?

The National Highway Transportation Safety Administration (NHTSA) is a federal agency within the United States Department of Transportation.3 As part of its mission, it issues police protocols for DUI field sobriety testing.

Of the dozens of field sobriety tests used by various law enforcement agencies, three have been “validated” as reliable by the NHTSA.4 They are:

  1. The horizontal gaze nystagmus test (HGN),
  2. The walk and turn test (WAT), and
  3. The one-leg stand test (OLS).5

These three tests are generally referred to as the "standardized" field sobriety tests.6 The NTHSA considers them to be reliable predictors of driver impairment, when administered correctly.7

How does the NHTSA validate the standardized tests?

In 2018, the NHTSA cited a study of field sobriety tests conducted by the San Diego Police Department. This field testing reportedly established a high correlation between poor performance on the standardized tests and DUI impairment.8

But, as discussed in Section five, below, there are many reasons why the results of FSTs can be deceptive.

First, however, let's take a quick look at each of the standardized field sobriety tests. For a more complete review, please visit our articles on each test, which are linked to above.

The NHTSA has concluded that the three “standardized” FSTs are, when administered correctly, reliable enough to gauge whether an individual is too intoxicated to drive safely.

1.1. The “Horizontal Gaze Nystagmus” (HGN) test

“Horizontal gaze nystagmus” is an involuntary jerking of the eyes as someone moves his/her eyes toward the side.9 In addition to being involuntary, the person experiencing the nystagmus is unaware of its occurrence.

Several different kinds of nystagmus exist, only some of them influenced by alcohol. But the test given at roadside in a DUI investigation is one of "horizontal gaze nystagmus."

During the administration of the HGN field sobriety test, the officer instructs the suspect to follow (with his eyes) a stimulus to the left and to the right. The officer notes the angle at which the pupil starts to exhibit "nystagmus" (the involuntary jerking of the eye).10

An early onset of nystagmus (at or before a 45-degree angle) is associated with a high blood alcohol concentration.11

Based on the San Diego PD tests, the National Highway Transportation Safety Administration claims the HGN test is 88% reliable. This means it believes that 88% of the time the HGN will accurately determine whether a driver has a BAC of .08% or higher.12

In the horizontal gaze nystagmus (HGN) field sobriety test for DUI, California law enforcement officers look for an involuntary jerking of the eyes.

1.2. The “Walk and Turn” (WAT) test

The “walk and turn test” (WAT) is a "divided attention" field sobriety test.13 A divided attention test is one which requires the subject to concentrate on both mental and physical tasks at the same time.14

The walk and turn test is sometimes also referred to as:

  • The “nine-step” test,
  • The “nine-step walk turn,"
  • A “DUI straight line” test, or
  • A “DUI walk the line” test.

As of 2018, the NHTSA claims that there is a 79% correlation between poor performance on the WAT and a BAC of .08% or greater.15

What happens during the “walk and turn” test?

During the walk and turn test, a driver is required to follow and remember instructions while performing the following physical movements:

  1. Taking nine heel-to-toe steps on a real or imaginary line,
  2. Pivoting around, and
  3. Taking nine heel-to-toe steps back.16

During the test, the officer will be watching for eight signs that may indicate impairment. Specifically, the officer is looking to see whether the driver:

  1. Keeps his or her balance during instructions,
  2. Starts too soon,
  3. Stops while walking,
  4. Fails to touch heel-to-toe,
  5. Steps off the line,
  6. Uses his/her arms to balance,
  7. Fails to turn correctly, or
  8. Takes an incorrect amount of steps.17
The “walk and turn” (WAT) and “one leg stand” (“OLS”) DUI field sobriety tests challenge a driver's ability to perform physical and mental tasks at the same time.

1.3. The “One-Leg Stand” test

The one-leg-stand (OLS) DUI field sobriety test is the second "divided attention" test among the three standardized field sobriety tests.18 During the one-leg-stand test, the officer instructs the suspect to:

  1. Raise his/her foot about six inches off the ground,
  2. Hold still in that position,
  3. Count from 1001 - 1030, and
  4. Look down at his/her foot.19

As the driver does this, the officer looks for four clues that the driver is impaired. These include whether the suspect:

  1. Sways,
  2. Uses his/her arms to balance
  3. Hops, and/or
  4. Puts his/her foot down.20

According to NHTSA, there is an 83% chance that someone who displays two or more of these clues during the OLS has a blood alcohol concentration at or above .08%.21

2. What are the non-standardized FSTs?

Only three field sobriety tests have been standardized by the NHTSA. Yet there are a number of other FSTs routinely used by California law enforcement in DUI investigations.22

The problem with these tests is that there is little or no demonstrated correlation between them and DUI impairment.

Worse, procedural administration of the test may vary a great deal from one police officer to the next. So the legitimacy and/or accuracy of these non-standardized FSTs is questionable.

Let's take a quick look at a few of the most popular non-standardized drunk driving FSTs used by California officers.

2.1. The hand pat field sobriety test

The hand-pat field sobriety test is a “divided attention” test for DUI.23During this test, a DUI suspect must pat one side of the hand and then the other while counting.

Specifically, the driver must:

  1. Place one hand extended, palm up, out in front of him/her. The other hand should be placed on top of the first, with the palm facing down. The top hand should then begin to pat the bottom hand.
  2. The top hand should rotate 180 degrees, alternating between the back of the other hand and the palm. The other hand remains stationary.
  3. The DUI suspect must then count out loud, "ONE, TWO, ONE, TWO, ONE, TWO, etc.," in time with each pat.

Law enforcement officials normally keep in mind the following four factors when administering the test. They use these to help them determine whether a suspect is impaired:

  1. Ability to follow instructions,
  2. Ability to count correctly,
  3. Rotation and sequence of the hand patting,
  4. Proper timing of when the subject starts and stops the test.24

2.2. The “finger-to-nose” field sobriety test

The “finger-to-nose” test is one of the oldest field sobriety tests in use in California.25 During the finger to nose test, the DUI suspect is required to:

  1. Bring the tip of the index finger up to touch the tip of the nose while his/her eyes are closed and his/her head is tilted slightly back.
  2. Repeat the foregoing movement six times (three with each hand). The officer should instruct the subject as to which hand to use on each attempt.26

Police will watch for the following seven factors during the finger to nose test for signs of possible impairment:

  1. The subject's ability to follow instructions,
  2. The amount and direction in which the subject sways,
  3. Eyelid tremors and body/leg tremors,
  4. Muscle tone,
  5. Any statements or unusual sounds made by the when performing the test,
  6. The subject's depth perception, and
  7. Whether the subject touches his/her index finger on his/her face.27
The “finger to nose” field sobriety test has not been validated for correlation with blood alcohol concentration (BAC).

2.3. The “Rhomberg balance” field sobriety test

The Rhomberg balance field sobriety test evaluates a driver's internal clock. During the administration of the Rhomberg balance test, a DUI suspect is to:

  1. Stand with his/her feet together,
  2. Have his/her head tilted slightly back,
  3. Have his/her eyes closed, and
  4. Estimate the passage of 30 seconds.
  5. When the DUI suspect believes that 30 seconds has passed, he/she should tilt his/her head forward, open his/her eyes, and say "stop."28

Police officers look at six factors when gauging whether or not a suspect is impaired using the Rhomberg balance test:

  1. The amount and direction in which the suspect sways,
  2. The suspect's estimated passage of 30 seconds,
  3. Eyelid tremors and/or body/leg tremors,
  4. Muscle tone (either more rigid or more flaccid than normal),
  5. Any statements or unusual sounds made by the subject when performing the test, and
  6. The suspect's overall ability to follow instructions.29

2.4. The “finger count” field sobriety test

During the “finger count” field sobriety test, the officer instructs the DUI suspect to:

  1. Put one hand in front of him/her with the extended palm facing upward,
  2. Have the top of the thumb then separately touch the tip of the index, middle, ring and little finger,
  3. Count out loud, "ONE, TWO, THREE, FOUR" in relation to each finger-thumb connection,
  4. Reverse the process, completing a total of three complete sets.30

During the administration of the finger count test, officers look at five factors to determine whether a suspect is intoxicated:

  1. Ability to follow instructions,
  2. Ability to count correctly,
  3. Ability to touch each finger separately and in the correct sequence,
  4. Ability to start and stop the test when instructed, and
  5. Performance of the correct number of sets.31

3. How can field and weather conditions affect the tests?

Adverse conditions can make field sobriety tests unreasonably difficult to perform. Police are supposed to make certain conditions are suitable for testing. But they do not always do so.

The NHTSA requires that the three standardized field sobriety tests be performed under appropriate and safe test conditions. These tests are the walk and turn test, the one-leg stand test, and the horizontal gaze nystagmus test.

There is no uniform set of procedures for police to follow when giving the non-standardized FSTs. But courts and juries will want to see that these tests were also given under fair and reasonable conditions.

Conditions that can affect the validity of field sobriety tests include:

Surface conditions

Field sobriety tests should be given under conditions in which the DUI suspect is not in danger of falling. This means that FSTs should generally be conducted on a surface that is reasonably:

  • Dry,
  • Hard,
  • Level, and
  • Non-slippery.

There should also be sufficient room for the suspect to perform and/or complete the test.

If these guidelines cannot be followed at the location of the traffic stop, the should try to move the suspect to a better location.32

Wet and/or uneven streets can invalidate the results of field sobriety tests for DUI.

Lighting conditions

Officers should make sure there is adequate lighting for the administration of field sobriety tests. The suspect should be able to see both the officer and the ground below fairly well.

If the lighting is not adequate, the officer may use a flashlight to illuminate the ground. Not surprisingly, even sober people have trouble completing some of the FSTs in total darkness.33

Auditory conditions

The subject must be able to hear the officer give instructions during field sobriety tests. If there is disruptive honking, sirens, or other noise disturbances, the officer should take the suspect to a more suitable location.34

4. How accurate are field sobriety tests for DUI?

The NHTSA cites accuracy statistics from three 1990s field studies. The studies are based on tests conducted by police in Colorado (1995), Florida (1997) and San Diego, California (1998).35

According to the NHTSA, arrest decisions based on FSTs were correct (based on a BAC of .08% or higher) in the following percentages of cases36:

FST field study location

Correct arrest/release decisions





San Diego, CA


The NHTSA emphasizes, however, that this high correlation only applies when:

  • Tests are administered in the prescribed, standardized manner,
  • Standardized clues are used to assess the suspect's performance, and
  • Standardized criteria are employed to interpret that performance.37

If any of these elements is changed, the validity of a field sobriety test may be compromised.38

Challenging field sobriety test results as part of a DUI defense

Despite the NHTSA's claims, field sobriety tests are far less accurate than claimed. And even when done properly and under ideal conditions, they result in false “positives” more than 12% of the time.

A skilled DUI defense attorney can help clients take the position that the FSTs in his/her case do not indicate impairment. Eight common reasons why field sobriety tests might not be accurate are:

1. Physical and/or mental conditions

Physical and mental conditions other than intoxication can cause sober drivers to perform poorly on field sobriety tests. Common causes of poor performance on FSTs include:

  • The driver is over 60 years of age,
  • The driver is sick/ill,
  • The driver has any back, foot, or leg problems,
  • The driver has inner ear problems,
  • The driver is overweight by 50 or more pounds,
  • The driver is in pain,
  • The driver is nervous or intimidated,
  • The driver has mild brain damage,
  • The driver suffers from mental disabilities that make it difficult to follow instructions, or
  • The driver suffers from any other condition that makes completing the tests difficult.39

2. Officer movement

The officer is supposed to remain as motionless as possible during many of the field sobriety tests so as not to interfere with it.40 If the officer walks around or exhibits any other forms of distractive behavior, the test results may be deemed tainted.

3. Unsuitable attire

A driver's performance on field sobriety tests can be negatively affected by his or her clothing. Clothing that may make FSTs inappropriate for gauging sobriety include:

  • High heels or dress shoes,
  • Boots,
  • Shoes that are too tight,
  • Tight pants,
  • Baggy or beltless jeans or pants,
  • Gloves, or
  • Any other type of clothing that may have inhibited the driver's ability to effectively maneuver or perform the test.41
High heels can make it difficult for sober drivers to perform well on field sobriety tests for DUI.

4. Improper timing

Timing is critical in several field sobriety tests. Results of these tests might not be accurate if the officer:

  • Does not time the test with a watch, or
  • Starts and/or ends the timing incorrectly.42

5. Environmental conditions

Field sobriety tests are meant to be performed under certain “road” conditions. If these conditions do not exist, the accuracy of the FSTs will be compromised.

Adverse conditions that can invalidate the results of field sobriety tests include:

  • Inclement weather,
  • Poor lighting,
  • Uneven road or sidewalk surfaces, and/or
  • The distraction of traffic, lights, and/or spectators.43

6. Non-standardized tests

The NHTSA has validated just three field tests as accurate: the horizontal gaze nystagmus, the walk-and-turn, and the one-leg stand.44 These are also the only tests for which the NHTSA has provided approved procedures for law enforcement to follow.

The absence of approved procedures for other FSTs means that actual practices followed by officers can vary widely. Ventura County DUI lawyer John Murray explains how he fights these DUI sobriety tests:

"Unlike the standardized FSTs, these other field sobriety tests have no uniform method of administration. Nor do they have any scientific data to support their reliability. The best California DUI defense lawyers will always aggressively attack results from non-standardized field sobriety tests.”45

7. Non-alcohol related causes for coordination failures

During field sobriety tests, officers monitor the subject for physical coordination. Lack of physical coordination can be caused by alcohol and/or drugs.

But lack of coordination can have many other causes. Reasons that a driver might appear physically uncoordinated during field sobriety testing process can:

  • Medications (for instance, medication to control seizures),
  • Lack of sleep,
  • Exhaustion from over-exertion,
  • Dehydration,
  • Pulled or strained muscles, and/or
  • Muscle fatigue due to a hard workout.
Lack of sleep can negatively affect performance on California DUI field sobriety tests.

8. Incorrect/vague Instructions

Officers conducting field sobriety tests must give the driver precise instructions on how to successfully perform them. These precise instructions are required whether given verbally or by visual demonstration.46

If the officer does not correctly instruct the suspect (either orally or visually) on how to properly conduct the test, the results are subject to challenge.

5. Can I refuse to take the FSTs? Should I?

There are no legal penalties for refusing to take any field sobriety test in California.47 Politely declining the FST is, therefore, a valid option for a driver to take.

A driver may think that successfully performing an FST will keep him/her from being arrested. And in theory, FSTs are just a tool to help an officer decide whether a driver is under the influence.

But as noted by DUI expert witness Robert LaPier,48 an officer has most likely already made his decision before requesting a field sobriety test.

In this context, field sobriety tests are generally "designed for failure.” They are simply one more way for an officer to validate the traffic stop and gather further evidence against the driver.

We generally recommend, therefore, that drivers politely decline to take any field sobriety tests when requested by an officer.

Charged with DUI in California? Call us for help…

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Whether or not you took field sobriety tests, an experienced California DUI lawyer can help you fight your case.

Call us at 855-LAWFIRM for a free consultation to discuss the best DUI defenses for your situation.

We have local offices in Los Angeles, the San Fernando Valley, Pasadena, Long Beach, Orange County, Ventura, San Bernardino, Rancho Cucamonga, Riverside, San Diego, Sacramento, Oakland, San Francisco, San Jose and throughout California.

We also have offices in Las Vegas and Reno if you need to challenge field sobriety tests in Nevada.

Legal References:

  1. The National Highway Transportation Safety Administration's 2018 DWI Detection and Standardized Field Sobriety Testing Instructor Guide claims the following accuracy percentages: horizontal gaze nystagmus (HGN) 88%; Walk-and-turn (WAT) 79%; One-leg stand (OLS) 83%. These percentages are based on field testing done by the San Diego Police Department.

  2. The only non-optional tests in California are (1) a post-arrest breath, blood and/or urine test to determine blood alcohol concentration, and (2) if the driver is under 21 or on DUI probation, a pre-arrest preliminary alcohol screening (PAS) breath test. See Vehicle Code 23612 [post-arrest chemical tests], Vehicle Code 13388 VC [requiring PAS breath test for DUI suspects under 21], and Vehicle Code 13389 [requiring PAS breath test for suspects on DUI probation].

  3. See

  4. Same.

  5. NHTSA Instructor Guide, endnote 1.

  6. NHTSA Instructor Guide, endnote 1.

  7. NHTSA Instructor Guide, endnote 1.

    See also Stuster &Burns, “Validation of the Standardized Field Sobriety Test Battery at BACs Below .10 percent,” US Dept. of Transportation Rep. No. Dot-HS-808-839 (1998), at 33; Burns & Moskowitz, “Psychophysical Tests for DWI Arrest,” U.S. Dept. of Transportation Rep. No. DOT-HS-802-424 (1977) (recommending the three-test battery of one-leg stand, walk and turn, and HGN to aid officers in discriminating BAC level); Anderson, Schweitz & Snyder, “Field Evaluation of Behavioral Test Battery for DWI,” U.S. Dept. of Transportation Rep. No. DOT-HS-806-475 (1983) (determining that standardized field test battery as tested by police officers from four jurisdictions was approximately 80% effective in determining BAC above and below .10 percent).

  8. Same.

  9. See, e.g., American Optometric Association, “Nystagmus.”

  10. NHTSA Instructor Guide, endnote 1.

  11. Wilkinson, Kime & Purnell, “Alcohol and Human Eye Movement,” 97 BRAIN 785 (1974) (oral dose of alcohol impaired eye movement of all subjects); Lehti, “The Effect of Blood Alcohol Concentration on the Onset of Nystagmus,” 136 BLUTALKOHOL 414 (West Germany 1976) (noteing a statistically significant correlation between BAC and the angle of onset of nystagmus with respect to the midpoint on the field of vision).

  12. NHTSA Instructor Guide, endnote 1.

  13. Same.

  14. Same.

  15. Same.

  16. Same.

  17. Same.

  18. Same.

  19. Same.

  20. Same.

  21. Same.

  22. See, e.g., California Highway Patrol (December 2007). Memo Re: Highway Patrol Manual (HPM) 70.4, Driving Under the Influence (DUI) Enforcement Manual. (“The Department recognizes five alternative FSTs and accepts that additional FSTs may be given if approved and authorized by the local district attorney.”)

    Although the Hand Pat test has not been tested under scientific conditions, experienced officers have indicated that it is a reliable FST. he hand pat test was among the six optimal DUI field sobriety tests that were examined during the initial 1977 study conducted by SCRI. The hand pat test was also included in a Finnish DUI study conducted in 1974 and was implemented by the LAPD during the formation of their DRE program. The hand pat test is noted for its divided attention qualities and depth perception issues.

  23. Same.

  24. In its December 2007 memo (endnote 22), the CHP claims that “Many of the same necessary exercises used to drive a vehicle are tested with the Hand Pat FST.” These include:

    (a) Information processing.

    (b) Short-term memory.

    (c) Judgment and decision making.

    (d) Steady, sure reactions.

    (e) Clear vision.

    (f) Small muscle control.

    (g) Coordination of limbs.

  25. It was implemented by the Los Angeles Police Department (LAPD) during the formation of their Drug Recognition Evaluator (DRE) program and accepted by NHTSA “due to its unique divided attention qualities.” See CHP memo, December 2007, endnote 22.

  26. CHP Memo, endnote 22. Note that the finger-to-nose test differs from the other DUI psychophysical tests in that the examiner must continue to give instructions to the subject throughout the test.

  27. Same.

  28. Same. According to the CHP, the Rhomberg balance test is an accurate and effective field sobriety test even though it has not been tested under approved scientific conditions.

  29. Same.

  30. Same. According to the CHP, the finger count test is an accurate and effective field sobriety test even though it has not been tested under approved scientific conditions.

  31. Same.

  32. NHTSA instructor guide, endnote 1.

  33. Same.

  34. Same.

  35. Same, p. 12.

  36. Same, pp. 14-17.

  37. Same, p. 17.

  38. Same.

  39. Same.

  40. Same.

  41. Same.

  42. Same.

  43. Same.

  44. Same.

  45. John Murray is a DUI criminal defense attorney qualified by the NHTSA to administer field sobriety tests in connection with DUI roadside investigations. Mr. Murray defends clients in court and at the DMV in Ventura, Los Angeles, San Bernardino, and Riverside Counties.

  46. NHTSA instructor guide, endnote 1.

  47. See endnote 2.

  48. Robert “Bob” LaPier is a DUI defense expert witness and former police officer. He has instructed thousands of polices officers and hundreds of attorneys (both DUI defense lawyers and prosecutors) in the proper administration of field sobriety tests.

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