Field Sobriety Tests in Colorado DUI Cases

officer arresting businessman

Colorado field sobriety tests: horizontal gaze nystagmus (HGN), walk and turn (WAT), and one-leg stand (OLS)

Field sobriety tests (FST) are a tool to help law enforcement officers determine whether a driver is under the influence of alcohol and/or drugs. In Colorado, officers use three standardized field sobriety tests:

  1. Horizontal gaze nystagmus (HGN), 
  2. Walk-and-turn (WAT), and
  3. One-leg stand (OLS).

These tests have been standardized by the National Highway Traffic Safety Administration (NHTSA) and are considered the most reliable of the FSTs.

But even under perfect conditions, standardized FSTs are only accurate predictors of inebriation 65-77% of the time. And conditions are seldom perfect.

What's more, Colorado field sobriety tests are completely optional. If asked to take them, you may politely refuse without penalty.

To help you better understand Colorado's standardized field sobriety tests and their consequences, our Colorado DUI defense lawyers discuss the following, below:

1. What are Colorado field sobriety tests?

A field sobriety test (FST) is a test an officer may ask you to take during a traffic stop to help determine if she should arrest you for:

While dozens of FSTs have been devised, the National Highway Traffic Safety Administration (NHTSA) has produced standardized procedures and results for just three:

  1. Horizontal gaze nystagmus (HGN),
  2. Walk-and-turn test (WAT), and
  3. One-leg stand (OLS).1

The results have been independently validated in a 1995 report by the Colorado Department of Transportation (CDOT). In the report, CDOT concluded that the NHTSA's standardized field sobriety tests serve as accurate indicators of impairment by alcohol and/or drugs.

In fact, however, these tests are far from conclusive.

(See our article about Colorado DUI checkpoints/sobriety checkpoints.)

1.1. Horizontal gaze nystagmus (HGN)

officer giving horizontal gaze nystagmus test

The first Colorado field sobriety test you will be asked to take is the horizontal gaze nystagmus (HGN) test. "Nystagmus" refers to an involuntary jerking of the eyes. 

Horizontal gaze nystagmus is an involuntary motion that occurs when the subject moves his or her eyes to the side. A person with horizontal gaze nystagmus cannot control it. In fact, he or she will be unaware that it is happening.2

To test you for horizontal gaze nystagmus, the officer will instruct you to stand with your feet together and your hands at your sides. You will then be told to hold your head still and follow the motion of a small stimulus with your eyes only – usually the tip of a pen or penlight. The officer will conduct two “passes” with each eye, beginning with your left eye.

The officer is looking to see if you exhibit any of these signs that could indicate impairment by alcohol and/or drugs:

  • Your eyes are unable to smoothly track the stimulus,
  • Your eyes exhibit sustained jerking when you look to the side, or
  • The onset of nystagmus occurs before your eyes have tracked 45 degrees.3

While horizontal gaze nystagmus can be caused by drugs and/or alcohol, it can also be caused by medical and atmospheric conditions such as:

  • Changes in barometric pressure,
  • Extreme temperatures,
  • Your biorhythms,
  • Viral or bacterial infections,
  • Inner ear conditions,
  • Eye diseases,
  • Albinism,
  • Vitamin deficiencies,
  • Glaucoma,
  • Sunstroke,
  • Neurologic disorders (such as multiple sclerosis or epilepsy),
  • Endocrine conditions (such as thyroid disease or diabetes mellitus),
  • Cardiovascular disease, 
  • Some prescription medications, and
  • Innocuous substances such as caffeine, nicotine or aspirin.

In addition, fatigue and stress can make nystagmus worse.

1.2. Walk and turn (WAT)

The second Colorado field sobriety test is the walk and turn (WAT) test. It is a “divided attention” task. Divided attention tasks require you to concentrate on more than one thing at a time – usually something physical and something mental.

Driving is itself a complex divided attention task. Drivers must simultaneously steer and accelerate or brake, while at the same time reacting to a constantly changing environment. Alcohol and drugs reduce a person's ability to divide their attention between tasks. The walk and turn looks for an inability to do this by having you try to perform two tasks at once: walking heel-to-toe and counting.

In the WAT test, the officer will instruct you to stand with your feet in a heel to toe position, with your arms at your sides. You will then be asked to take nine heel-to-toe steps, turn in a prescribed manner, and take nine similar steps back, while counting the steps out loud. This tests your ability to divide your attention between listening, understanding and carrying out the instructions.

The Walk and Turn test is interpreted in a standardized manner – that is, the same way every time. The officer will look for eight clues that might indicate you are intoxicated:

  1. You can't keep your balance while listening to the instructions,
  2. You start too soon,
  3. You stop while walking, 
  4. You do not touch heel to toe,
  5. You step off the line,
  6. You use your arms for balance,
  7. You make an improper turn, or
  8. You take an incorrect number of steps.

Many factors other than alcohol and/or drugs can, however, affect your ability to walk and turn as instructed. In particular, you might find the WAT difficult even when sober if:

  • You are overweight,
  • You have mobility problems, 
  • You have difficulty balancing,
  • You have problems with your feet, or
  • You are ill or injured.

1.3. One-leg stand (OLS)

Officer giving driver a one leg stand (OLS) field sobriety test
The one-leg stand (OLS) is a "divided attention" Colorado field sobriety test.

Like the walk and turn, the one-leg stand test (OLS) is a divided attention test. The officer will instruct you to stand with your feet together, with your arms at your sides. You will then be told to raise one foot six inches off the ground while keeping both legs straight. You will be asked to look at your elevated foot and count "one thousand one", "one thousand two", “one thousand three,” etc., until told to stop 30 seconds later.

The OLS divides your attention between balancing on one foot and counting out loud. Accuracy of the test depends on it being given for at least 30 seconds. While many impaired subjects are able to stand on one leg for up to 25 seconds, few can do so for 30 seconds.4

The officer is looking for four specific signs of intoxication during the OLS:

  1. You sway while balancing,
  2. You use your arms to balance,
  3. You hop, or
  4. You put your foot down before the 30 seconds are up.

As with the other standardized FSTs, the test may not be accurate if you have certain medical problems or issues with balance and/or mobility.

2. How accurate are FSTs?

The NHTSA claims that the three standardized tests can accurately predict a blood alcohol concentration (BAC) of .10% or more approximately 65-77% of the time. More recently, the NHTSA has reported that the tests can also detect impairment at .08% BAC (Colorado's “legal limit” for DUI per se) and .04% (the legal limit for drivers of commercial vehicles).5

More specifically, the Administration claims accuracy levels for the three individual tests as follows:

  • HGN – 77%.
  • WAT – 68%.
  • OLS – 65%.

3. Problems with FSTs

The reported accuracy of field sobriety tests is based on tests in a controlled environment. The actual conditions under which you will given the tests will, in actuality, seldom be as reliable.

Even the NHTSA admits that accuracy levels of FSTs can decrease based on certain factors in addition to those set forth above. These include (but are not limited to):

  • The officer is not well-trained in detecting the signs of impairment,
  • The impairment is caused in whole or in part by drugs,
  • Inadequate lighting in the test area,
  • Uneven ground,
  • The subject's height restricts the officer's ability to clearly see the subject's eyes, or
  • The subject is wearing clothing that makes the test difficult.

4. Do I have to take Colorado field sobriety tests?

Unless and until you are arrested, there are only two things you must do during a Colorado traffic stop:

  1. Tell the officer your name, and
  2. Show the officer your license and registration.

You are under no obligation to answer any other questions or to take any field sobriety tests. In addition, if you are 21 or older, you have no obligation to take a preliminary (pre-arrest) breath test on a handheld device.6

These tests are simply tools to help the officer decide whether there is probable cause to arrest you. You are under no obligation to incriminate yourself and there are no consequences if you refuse to take any FSTs. If you do agree to take the FSTs, however, the results can be used against you in court.7.

Accordingly, if an officer asks you to take FSTs, it is generally best to politely decline them.

5. How to challenge Colorado FSTs in court

The best Colorado DUI lawyers know that field sobriety tests are just one component of a Colorado DUI, Colorado DWAI or Colorado UDD case. At best, they are evidence of impairment -- not proof.

And as we have seen, Colorado field sobriety tests are subject to a wide variety of errors. Even under the best of circumstances, they are accurate only 65-77% of the time. And conditions in the field are seldom perfect. An experienced DUI attorney will explain this to the jury. We will also offer alternative explanations for your performance on FSTs. This may include evidence that you performed poorly because of:

  • A medical condition,
  • Prescription medication that didn't affect your driving,
  • Your weight,
  • Mobility issues,
  • Injury,
  • Illness,
  • Uneven ground,
  • Poor visibility at the test site,
  • The weather,
  • You were tired,
  • Your eyes were strained,
  • The officer was not properly trained to perform the test(s),
  • The officer was inexperienced,
  • The officer couldn't see your eyes clearly,
  • The instructions were not clearly given, or
  • You were wearing high heels, tight clothing or other attire that affected your ability to stand and/or walk.

Call us for help…

Did you "fail" a Colorado field sobriety test? Were you arrested as a result for Colorado DUI, DUI per se, DWAI or UDD?

Our caring Colorado DUI attorneys have decades of experience challenging arrests based on field sobriety test results. We know that there are many reasons you may not have performed them well. And we know how to challenge them to create reasonable doubt.

If you were charged with a Colorado drunk driving offense or an offense involving drugs, we invite you to contact us for a free consultation. We will respond quickly so that you can tell us what happened. Don't let a field sobriety test deprive you of your liberty or your right to drive.

Fill out the form on this page, or call us at our Denver home office:

Colorado Legal Defense Group
4047 Tejon Street
Denver, CO 80211
(303) 222-0330

Arrested in Nevada? See our article on Nevada FSTs in DUI cases.

Legal references:

  1. See DWI Detection and Standardized Field Sobriety Testing, March 2015 Edition, Participant Guide.
  2. See NHTSA No. 207, September 1998: A resource guide describes the science and the law about horizontal gaze nystagmus.

  3. See end note 1.
  4. Same
  5. See NHTSA No. 196, March 1999: Standardized field sobriety test (SFST) validated at BACs below 0.10 percent.
  6. If you are under 21 and the officer reasonably suspects you have been drinking, you can be required to take the preliminary breath test. 42-4-1301 (6)(i)(I), C.R.S.
  7. If you are at least 21, however, results from a preliminary breath test are inadmissible other than in a hearing outside the presence of the jury to establish whether the officer had probably cause to suspect of you of DUI, DWAI or UDD. 42-4-1301 (6)(i)(III), C.R.S.


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