TLDR: Field sobriety tests are one of the most commonly challenged pieces of evidence in DUI cases, and for good reason. They’re subjective, officer-administered, and easy to fail even when sober. Courts treat them as supporting evidence, not proof on their own, and a skilled defense attorney can often pick apart how they were conducted. Understanding what these tests actually measure, and where they fall apart, matters if you’re facing a DUI charge.
What Officers Are Actually Looking For
When a police officer pulls someone over on suspicion of drunk driving, the field sobriety test is usually the next step before an arrest is made. There are three tests recognized by the National Highway Traffic Safety Administration: the horizontal gaze nystagmus test, the walk and turn test, and the one leg stand. Each one is designed to measure balance, coordination, and the ability to follow multi step instructions while divided attention is required, which is the theory behind why alcohol impairment shows up in these tasks.
The problem is that “designed to measure” and “accurately measures” are two very different things. A person with a bad knee, an inner ear condition, or even a pair of uncomfortable shoes can fail the walk and turn test stone sober. I’ve seen cases where the driver was wearing heels on a gravel shoulder at 11pm, and the officer never accounted for that in the report.
How the Horizontal Gaze Nystagmus Test Works
This is the one most people have never heard of by name but have seen in movies, where the officer moves a pen or finger side to side and asks the driver to follow it with their eyes. The officer is checking for involuntary jerking of the eyeball, which alcohol can cause at certain blood levels.
Why This Test Gets Challenged Often
HGN results depend almost entirely on the officer’s training and how precisely they administer it. Some jurisdictions require the test be performed exactly according to NHTSA protocol, including specific timing and positioning, and any deviation can be enough to get the results thrown out or heavily discounted at trial.
The Walk and Turn and One Leg Stand
These two tests get the most attention because they’re the ones jurors actually picture when they think “field sobriety test.” The walk and turn asks a driver to take nine heel to toe steps along a straight line, turn on one foot, and walk back. The one leg stand asks the driver to hold one foot about six inches off the ground for thirty seconds while counting out loud.
What Officers Are Trained to Score
Officers are trained to look for specific “clues,” things like starting too soon, stepping off the line, using arms for balance, or putting the foot down early. Even two or three clues can be enough for an officer to note “signs of impairment” in a report, and that language carries weight with a jury even when the underlying test conditions were far from ideal.
Why These Tests Aren’t Automatically Reliable in Court
Here’s the part most people don’t realize until they’re sitting across from a defense attorney. NHTSA’s own research on these tests shows accuracy rates that are decent under controlled, ideal conditions, but far less impressive out on a dark roadside with traffic passing, uneven pavement, and a driver who’s nervous and already assuming the worst about how the encounter will end. Fatigue, weather, medical conditions, and anxiety alone can all produce the same “clues” officers are trained to associate with intoxication.
Body Cam Footage Often Tells a Different Story
This is where dashcam and body camera footage becomes so valuable. I’ve reviewed footage where the written report claimed a driver “swayed noticeably” and the video showed almost no movement at all. When the video and the report don’t match, that gap becomes one of the strongest tools available for challenging the charge.
Using This Evidence to Build a Defense
None of this means field sobriety tests are worthless as evidence, courts do allow them, and prosecutors lean on them heavily. But they’re rarely the whole case. A breath or blood test result, the officer’s initial observations, and the sobriety tests all get weighed together, and each piece can be challenged individually rather than accepted as a package deal.
If you’re facing a DUI charge and field sobriety tests were part of your stop, the report itself is only one version of events. Requesting the video, checking the exact administration procedure against NHTSA standards, and looking at any physical or medical factors that weren’t considered are usually the first moves worth making before deciding how to respond to the charge.
