Field sobriety tests, explained

There are three standardized tests most officers are trained to use, and each is scored against a specific checklist rather than a general impression.

Knowing what is actually being measured helps make sense of what happened during a stop, whether or not it changes anything about your situation now.

The three standardized tests

These three were developed and studied by the National Highway Traffic Safety Administration, and they are what most agencies train officers to use.

Horizontal Gaze Nystagmus. The officer asks you to follow a small object, usually a pen or a flashlight, with your eyes only, without moving your head, while they move it from side to side. They are watching for involuntary jerking of the eye, called nystagmus, at defined points in the movement. This is the test people are least likely to realize is happening, because nothing is being asked of your body.

Walk and Turn. You walk a straight line heel to toe for a set number of steps, turn in a specified way, and walk back. The officer is scoring defined indicators: whether you keep your balance during the instructions, start too soon, step off the line, use your arms for balance, take the wrong number of steps, or turn incorrectly.

One-Leg Stand. You stand on one leg with the other foot raised a set height and count aloud for a defined period. Again the officer scores specific indicators: swaying, using arms for balance, hopping, or putting the foot down early.

The instructions phase is part of the test in the last two. Losing balance while being told what to do counts, before you have taken a step.

Non-standardized tests are a different thing

Officers sometimes ask for other tasks. Reciting the alphabet from a letter other than A, counting backwards, touching finger to nose, tilting your head back and estimating time.

These are not part of the standardized battery, they have not been validated in the same way, and they are not scored against the same defined criteria. That distinction is worth knowing, because it can matter later in how much weight the task carries.

What the scoring actually looks like

These are not pass or fail the way an exam is.

Each test has a defined number of indicators the officer is trained to look for, and reaching a certain count is associated, in the validation research, with a statistical likelihood of a particular blood alcohol range. The officer is counting specific behaviors against a script, not forming a general impression.

Two things follow from that.

It means the test can be examined. A script that was not followed produces a score that means less than it appears to.

And it means performance can be affected by things unconnected to alcohol.

Things that affect performance and have nothing to do with drinking

• Fatigue. Many stops happen very late.
• Medical conditions affecting balance, the inner ear, the back, the knees or the legs.
• Age and weight. The training material itself acknowledges that certain people may have difficulty with the balance tests regardless of impairment.
• Footwear. Heels, boots, sandals, or no shoes at all.
• The surface. A sloped shoulder, gravel, a painted line, wet ground.
• Lighting, weather, and cold.
• Passing traffic, both the noise and the wind from it.
• Anxiety. You are being watched by an officer, at the roadside, in flashing light, in a situation you have never been in before.
• Simple unfamiliarity. Almost nobody has ever walked heel to toe in a straight line on command.

None of this makes the tests meaningless. It makes them one piece of evidence, scored against criteria, rather than a measurement.

Are they voluntary?

In many places, yes, in the sense that you are not legally compelled to perform them the way you may be compelled to submit to chemical testing after arrest under implied consent law.

But voluntary and consequence-free are not the same thing.

Declining does not prevent an arrest. An officer can proceed on other observations: the driving that prompted the stop, your speech, odor, appearance, and what you say.

Declining may be noted, and in some places may be mentioned later.

And the rules genuinely differ by location, including for holders of commercial licenses and drivers under 21, who are sometimes treated differently.

This is a separate decision from the breath test after arrest, which is governed by different rules and carries much heavier consequences. Confusing the two is common and costly.

More on the breath test decision →

The handheld device at the roadside is not a field sobriety test

Worth separating out, because they happen in the same few minutes.

The portable breath device an officer may offer at the roadside is a chemical test, not one of the three physical tests above. In many states it is treated differently from both the standardized battery and the evidentiary test at the station.

Knowing which of the three things you are being asked to do is the difference between an informed decision and a guess.

Can the results be challenged?

Yes, and this is one of the areas where a lawyer's specific knowledge earns its cost.

The standardized tests are valid, according to the training material itself, only when administered under specific conditions with specific instructions. Relevant questions include:

• Was the surface reasonably level, dry and firm?
• Was there adequate lighting?
• Were the instructions given in full, and demonstrated?
• Was the test scored on the defined indicators, or on impression?
• Did the officer know about a medical condition, injury, or footwear issue, and was it accounted for?
• Was there video, and does it match the written report?

That last question matters more than any other. Dashcam and bodycam footage frequently exists, and what it shows does not always match what a report describes. Video is normally retained for a limited period and then overwritten, so requesting it early is one of the genuinely time-sensitive steps in a case.

This is not something to argue at the roadside. Arguing with an officer about test conditions during a stop helps nobody, least of all you. It is a conversation for afterward.

What to write down afterward

While you still remember it.

• Where exactly you were standing, and what the surface was like.
• What the lighting and weather were.
• What you were wearing on your feet.
• Any injury, medical condition, or medication that affects your balance, and whether you mentioned it.
• How long you had been awake.
• What instructions you were given, and whether they were demonstrated.
• Which tasks you were asked to do, in order, including any non-standardized ones.
• Whether you were asked to blow into a handheld device, and at what point.

Photograph the location in daylight if you can. Details like a slope or a broken surface are much easier to show than to describe.

What this means for you now

If you have already been through this, understanding the tests does not change what happened. What it does is let you have a much more useful conversation with a lawyer about whether the administration matched the standard procedure, and help you understand what the report probably says.

If you are thinking about a future stop, no article can prepare you for the moment itself. What helps is knowing that these are specific, scored tests rather than a judgment call, and that performance on them is one part of a larger picture rather than the whole case.

More on whether you need a lawyer →

This is general information about how standardized field sobriety tests are typically administered and scored, not legal advice about your situation. Rules about voluntariness and the consequences of declining differ by location. Confirm anything you rely on with a lawyer licensed in your state.