Almost every DUI stop in San Diego includes the same scene. The officer asks you to step out of the car, then has you follow a pen with your eyes, walk an imaginary line heel to toe, and balance on one foot in the dark on the shoulder of the road. These are field sobriety tests, and by the time you are cuffed, the officer has already decided you failed them.
Here is what most people never get told: these tests are far less reliable than they look, and the way they get scored gives the officer a lot of room to interpret ordinary human behavior as signs of impairment. Understanding what these tests actually measure, and what they do not, is the first step in fighting the case.
First, You Can Usually Say No
Let me clear up the biggest misconception right away. In California, field sobriety tests are voluntary for adult drivers who are not on DUI probation. You are not legally required to do them, and there is no license penalty for politely declining. That is different from the chemical test after arrest, which is covered by the implied consent law and does carry consequences for refusal. If you want the details on that distinction, read our page on implied consent.
Why does this matter? Because these roadside tests are designed to gather evidence against you. You are being asked to perform physical and mental gymnastics on the side of a freeway, at night, possibly in traffic and cold, while nervous and watched. Most people do not realize they can decline. Now you do.
The Three "Standardized" Tests
Law enforcement leans on three tests that the National Highway Traffic Safety Administration, or NHTSA, calls standardized. Standardized means there is a specific way they are supposed to be given and scored. When officers deviate from that procedure, and they often do, the results lose much of their value.
Horizontal Gaze Nystagmus
This is the pen test. The officer moves an object side to side and watches your eyes for nystagmus, which is an involuntary jerking of the eyeball. Alcohol can make that jerking more pronounced, so the theory is that visible nystagmus points to impairment.
The problem is that nystagmus has dozens of causes that have nothing to do with alcohol. Fatigue, certain medications, caffeine, inner ear problems, some neurological conditions, and even natural nystagmus that a person is simply born with can all produce the same eye movement. Worse, this test is almost impossible for you to check. You cannot see your own eyes. You are taking the officer's word that they saw the jerking, that they saw it at the right angle, and that they moved the stimulus at the correct speed. When the procedure is done sloppily, the results are close to meaningless.
Walk and Turn
Here the officer has you take nine heel-to-toe steps down a line, turn in a specific way, and walk back, all while counting out loud and keeping your arms at your sides. It is called a divided attention test because it splits your focus between a physical task and a mental one.
The officer is scoring you on things like whether you step off the line, start too soon, use your arms for balance, or take the wrong number of steps. Sounds objective. It is not. Think about who is being asked to do this. If you are over 65, more than 50 pounds overweight, have any back, hip, knee, or ankle problem, or are wearing anything other than flat shoes, NHTSA's own guidance says the test may not be valid for you. Add an uneven shoulder, gravel, poor lighting, and headlights in your face, and a completely sober person can rack up "clues" of impairment.
One Leg Stand
You stand on one foot, hold the other about six inches off the ground, and count out loud until told to stop. The officer watches for swaying, hopping, putting your foot down, or using your arms to balance.
Try standing on one leg in your living room right now, sober, for 30 seconds. Plenty of people wobble. Now do it roadside at midnight after a stressful traffic stop with an officer's flashlight on you. Age, weight, footwear, medical conditions, and plain nerves all affect balance. This test measures balance under stress at least as much as it measures anything about alcohol.
The Non-Standardized Tests
Sometimes officers throw in extras like reciting the alphabet from a certain letter, counting backward, or the finger-to-nose test. NHTSA never validated these as reliable indicators of impairment. They carry even less weight, and a good cross-examination can expose them for what they are.
Why Sober People Fail
Put it all together and you can see the pattern. Field sobriety tests do not directly measure your blood alcohol level. They measure how you perform a set of awkward physical tasks under bad conditions, and then an officer interprets that performance. Nerves, medical issues, age, weight, footwear, the road surface, the weather, the lighting, and the officer's own coaching or scoring all feed into the outcome. A person who has had nothing to drink can absolutely "fail," which tells you these results are opinion evidence dressed up to look scientific.
How We Challenge Them in Court
When I take a DUI case, the field sobriety tests are one of the first things I dig into. There are several angles.
We compare what the officer did against the NHTSA manual step by step. If they gave the instructions wrong, moved the pen too fast, skipped required questions about your medical conditions and footwear, or scored a clue the manual does not recognize, that undermines the reliability of the whole test.
We look hard at the bodycam and dashcam footage. Very often the video does not match the report. The report says you were swaying badly, and the video shows you standing more or less still. Juries notice that. For more on how that footage can help, see our post on the San Diego DUI court process, where video evidence often comes into play at pretrial and trial.
We put your medical history, age, weight, and any injuries on the record, because those are legitimate reasons a sober person struggles with these tests. And we remind the jury that these are subjective judgment calls, not lab results.
None of this is a guarantee. Every case depends on its own facts, the strength of the other evidence, and the county's approach. But challenging the field sobriety tests can shrink the prosecution's case, and when it is combined with a challenge to the breath or blood evidence, it can change the outcome entirely.
What If You Already Took the Tests
Most people reading this already did the roadside tests, because nobody told them they could say no. That is fine. Doing the tests does not sink your case, and it certainly does not mean a conviction is inevitable. The tests are still just the officer's interpretation of your performance, and that interpretation can be challenged the same way whether you "passed," "failed," or something in between.
What matters now is preserving everything about how those tests were given. That means getting the bodycam and dashcam footage before it is overwritten, pinning down the road and weather conditions that night, and documenting any medical issue, injury, or footwear that would have affected your balance and coordination. The sooner someone starts gathering that, the stronger the challenge tends to be.
How This Fits With the Rest of Your Case
Field sobriety tests almost never stand alone. They are usually paired with a preliminary breath test at the roadside and a chemical test after arrest, and the prosecution tries to weave all of it into one story of impairment. Part of the defense strategy is refusing to let those threads support each other. If the field sobriety tests were unreliable and the breath reading is questionable, the whole narrative starts to come apart.
That is why I look at the field sobriety tests, the traffic stop, and the chemical evidence together rather than in isolation. A weakness in one often exposes a weakness in another. If you were pulled over at a checkpoint, there are even more questions to ask about whether the stop and the screening were handled properly, which we cover on our page about DUI checkpoints.
What To Take Away From This
Field sobriety tests feel official and scientific in the moment. In reality they are voluntary, subjective, and easy to fail even stone sober. Failing them does not mean your case is lost, and it certainly does not mean you should walk into court and plead guilty before anyone reviews the evidence.
This article is general information, not legal advice, and every situation is different. If field sobriety tests played a role in your arrest, the specifics of how they were given can matter a great deal.
If you were arrested after roadside tests in San Diego and you want someone to look honestly at what happened, call Crudo Law at (858) 622-7280 or reach out through our contact page. The consultation is free and confidential, and Joe Crudo will walk you through where you stand.