Field sobriety tests (FSTs) are a series of physical and mental exercises that California 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 you.
The following chart shows the claimed accuracy of the three “standardized” field sobriety tests when correctly administered under ideal conditions (which often they are not):1
Field Sobriety Test | Accuracy at Determining .08% BAC |
Horizontal gaze nystagmus (HGN) | 88% |
Walk-and-turn (WAT) | 79% |
One-leg stand (OLS) | 83% |
In California, field sobriety tests are completely optional. You can decline to take them without any penalty whatsoever.2
This is important because even if you are sober, you can fail FSTs for reasons having nothing to do with your blood alcohol content (“BAC”). You may then have to defend against wrongful criminal charges such as:
- Driving under the influence – Vehicle Code 23152(a),
- Driving with a BAC of .08% or higher – Vehicle Code 23152(b),
- DUI of drugs (“DUID”) – Vehicle Code 23152(f), or
- Underage DUI – Vehicle Code 23140.
To help you better understand field sobriety tests (FSTs), our California DUI lawyers discuss, below:
- 1. Standardized Field Sobriety Tests
- 2. Non-Standardized FSTs
- 3. Field and Weather Conditions
- 4. Accuracy
- 5. Should I refuse to take the FSTs?
- Additional Reading
1. 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 tests (FSTs).
Of the dozens of FSTs used by various law enforcement agencies, three have been “validated” as reliable by the NHTSA and always used in California.4 They are:
- The horizontal gaze nystagmus test (HGN),
- The walk and turn test (WAT), and
- 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
Though, 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 FSTs. For a more complete review, please visit our articles on each test, which are linked to above.
The “Horizontal Gaze Nystagmus” (HGN) Test
“Horizontal gaze nystagmus” (HGN) is an involuntary jerking of the eyes as you move your eyes toward the side.9 In addition to being involuntary, nystagmus is something you are unaware of when it happens.
During the administration of the HGN field sobriety test in California, the officer instructs you to follow (with your eyes) a stimulus to the left and to the right. The officer notes the angle at which your pupil starts to exhibit nystagmus.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 you have a BAC of .08% or higher.12
The “Walk and Turn” (WAT) Test
The “walk and turn test” (WAT) is a “divided attention” field sobriety test.13 It requires you to concentrate on both mental and physical tasks at the same time.14
In California, 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, you are required to follow and remember instructions while performing the following physical movements:
- Taking nine heel-to-toe steps on a real or imaginary line,
- Pivoting around, and
- Taking nine heel-to-toe steps back.16
During the test, the officer will be watching for eight signs that may indicate impairment. Specifically, California police are looking to see whether you:
- Keep your balance during instructions,
- Start too soon,
- Stop while walking,
- Fail to touch heel-to-toe,
- Step off the line,
- Use your arms to balance,
- Fail to turn correctly, or
- Take an incorrect amount of steps.17
The “One-Leg Stand” Test
The one-leg-stand (OLS) DUI field sobriety test is the second “divided attention” test among the three standardized FSTs in California.18 During the one-leg-stand test, the officer instructs you to:
- Raise your foot about six inches off the ground,
- Hold still in that position,
- Count from 1001 to 1030, and
- Look down at your foot.19
As you do this, the officer looks for four clues that you are impaired. These include whether you:
- Sway,
- Use your arm to balance
- Hop, and/or
- Put your foot down.20
According to NHTSA, there is an 83% chance that displaying two or more of these clues during the OLS signals a blood alcohol concentration at or above .08%.21
2. 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.
The Hand Pat Field Sobriety Test
The hand-pat field sobriety test is a “divided attention” test for drunk driving in California.23 During this test, you must pat one side of your hand and then the other while counting.
Specifically, you must:
- Place one hand extended, palm up, out in front of you. Your other hand should be placed on top of the first, with your palm facing down. Your top hand should then begin to pat your bottom hand.
- Your top hand should rotate 180 degrees, alternating between the back of your other hand and your palm. Your other hand remains stationary.
- You 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 you are impaired:
- Ability to follow instructions,
- Ability to count correctly,
- Rotation and sequence of your hand patting,
- Proper timing of when you start and stop the test.24
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, you are required to:
- Bring the tip of your index finger up to touch the tip of your nose while your eyes are closed and your head is tilted slightly back.
- Repeat the foregoing movement six times (three with each hand). The officer should instruct you 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:
- Your ability to follow instructions,
- The amount and direction in which you sway,
- Eyelid tremors and body/leg tremors,
- Muscle tone,
- Any statements or unusual sounds you make when performing the test,
- Your depth perception, and
- Whether you touch your index finger on your face.27
The “Rhomberg Balance” Field Sobriety Test
The Rhomberg balance field sobriety test evaluates your internal clock. During the administration of the Rhomberg balance test, you:
- Stand with your feet together,
- Have your head tilted slightly back,
- Have your eyes closed, and
- Estimate the passage of 30 seconds.
- When you believe that 30 seconds have passed, you should tilt your head forward, open your eyes, and say “stop.”28
California police officers look at six factors when gauging whether or not you are impaired using the Rhomberg balance test:
- The amount and direction in which you sway,
- Your estimated passage of 30 seconds,
- Eyelid tremors and/or body/leg tremors,
- Muscle tone (either more rigid or more flaccid than normal),
- Any statements or unusual sounds you make when performing the test, and
- Your overall ability to follow instructions.29
The “Finger Count” Field Sobriety Test
During the “finger count” field sobriety test, the officer instructs you to:
- Put one hand in front of you with your extended palm facing upward,
- Have the top of your thumb then separately touch the tip of your index, middle, ring and little finger,
- Count out loud, “ONE, TWO, THREE, FOUR” in relation to each finger-thumb connection,
- Reverse the process, completing a total of three complete sets.30
During the administration of the finger count test, California police look at five factors to determine whether you are intoxicated:
- Ability to follow instructions,
- Ability to count correctly,
- Ability to touch each finger separately and in the correct sequence,
- Ability to start and stop the test when instructed, and
- Performance of the correct number of sets.31
3. Field and Weather Conditions
Adverse conditions can make field sobriety tests unreasonably difficult to perform. California police are supposed to make certain conditions are suitable for testing. However, they do not always do so.
The NHTSA requires that the three standardized FSTs 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. However, 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 FSTs include the following:
Surface Conditions
Field sobriety tests should be given under conditions in which you are 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 you to perform and/or complete the test.
If these guidelines cannot be followed at the location of the traffic stop, California police should try to move you to a better location.32
Lighting Conditions
California police should make sure there is adequate lighting for the administration of FSTs. You 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
You must be able to hear the officer give instructions during FSTs. If there is disruptive honking, sirens, or other noise disturbances, the officer should take you to a more suitable location.34
4. Accuracy
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 |
Colorado | 86% |
Florida | 95% |
San Diego, CA | 91% |
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 your 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. Even when done properly and under ideal conditions in California, they result in false “positives” more than 12% of the time.
A skilled DUI defense attorney can help you take the position that the FSTs in your case do not indicate impairment. Eight common reasons why FSTs might not be accurate are:
Physical and/or Mental Conditions
Physical and mental conditions other than intoxication can cause you to perform poorly on field sobriety tests. Common causes of poor performance on FSTs include:
- You are over 60 years of age,
- You are sick/ill,
- You have any back, foot, or leg problems,
- You have inner ear problems,
- You are overweight by 50 or more pounds,
- You are in pain,
- You are nervous or intimidated,
- You have mild brain damage,
- You suffer from mental disabilities that make it difficult to follow instructions, or
- You suffer from any other condition that makes completing the tests difficult.39
Officer Movement
California police are supposed to remain as motionless as possible during many of the FSTs so as not to interfere with it.40 If the officer walks around or exhibits any other forms of distracting behavior, the test results may be deemed tainted.
Unsuitable Attire
Your performance on field sobriety tests can be negatively affected by your 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 your ability to effectively maneuver or perform the test.41
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
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 FSTs include:
- Inclement weather,
- Poor lighting,
- Uneven road or sidewalk surfaces, and/or
- The distraction of traffic, lights, and/or spectators.43
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. California DUI lawyer John Murray explains how he fights these 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 drunk driving defense lawyers will always aggressively attack results from non-standardized FSTs.”45
Non-Alcohol Related Causes for Coordination Failures
During field sobriety tests, California police monitor you for physical coordination. Lack of physical coordination can be caused by alcohol and/or drugs.
However, lack of coordination can have many other causes. Reasons that you might appear physically uncoordinated during FST process are:
- 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.
Incorrect/Vague Instructions
California police conducting field sobriety tests must give you 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 you (either orally or visually) on how to properly conduct the test, the results are subject to challenge.
5. Should I refuse to take the FSTs?
There are no legal penalties for refusing to take any field sobriety test in California.47 Politely declining the FSTs is, therefore, a valid option to take.
You may think that successfully performing an FST will keep you from being arrested. In theory, FSTs are just a tool to help an officer decide whether you are under the influence.
Though as noted by DUI expert witness Robert LaPier,48 an officer has most likely already made their decision before requesting a FST.
In this context, FSTs are generally “designed for failure.” They are simply one more way for California police to validate the traffic stop and gather further evidence against you.
We generally recommend, therefore, that you politely decline to take any field sobriety tests when requested by an officer.
Additional Resources
For more in-depth information on field sobriety tests, refer to the following:
- National Highway Traffic Safety Administration (NHTSA) manual on standardized field sobriety tests – Detailed information about how police test suspected drunk/drugged drivers
- Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial – A
- Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication – National Institute of Justice article on the unreliability of FSTs
- Gaze‐evoked nystagmus induced by alcohol intoxication – A J Physiol. article on how alcohol affects eye movements
- The Drug Evaluation and Classification (DEC) Program – A course for members of the National District Attorneys Association about how drug recognition experts (DREs) work
Legal References:
- 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.
- 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].
- See NHTSA.gov.
- Same.
- NHTSA Instructor Guide, endnote 1.
- NHTSA Instructor Guide, endnote 1.
- 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).
- Same.
- See, for example, American Optometric Association, “Nystagmus.”
- NHTSA Instructor Guide, endnote 1.
- 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) (noting a statistically significant correlation between BAC and the angle of onset of nystagmus with respect to the midpoint on the field of vision).
- NHTSA Instructor Guide, endnote 1.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- See, for example, 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. The 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.
- Same.
- 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.
- 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.
- 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.
- Same.
- 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.
- Same.
- 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.
- Same.
- NHTSA instructor guide, endnote 1.
- Same.
- Same.
- Same, p. 12.
- Same, pp. 14-17.
- Same, p. 17.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- Same.
- 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.
- NHTSA instructor guide, endnote 1.
- See endnote 2.
- Robert “Bob” LaPier is a DUI defense expert witness and former police officer. He has instructed thousands of police officers and hundreds of attorneys (both DUI defense lawyers and prosecutors) in the proper administration of field sobriety tests.