DUI Evidence Collection: How Illinois DUIs Are Won (or Lost)

If you’ve been charged with driving under the influence (DUI), the evidence against you (or lack thereof) is a major deciding factor in the outcome of your case.

At our Chicagoland law firm for drivers, we’ve handled DUI cases ranging from the straightforward to the complex, each with a unique set of evidence, ranging from 911 calls to bodycam footage.

Typically, the less evidence the State has to support its case, the more likely we can secure a favorable outcome. But that doesn’t mean that a large body of evidence inevitably means you’ll be found guilty. How the evidence was collected is just as critical.

In this blog, one of our top Chicago DUI lawyers breaks down the different types of evidence the police gather during their investigation and how an experienced defense attorney might challenge it.

Why Is Evidence Important for Proving an Illinois DUI Charge?

Under Illinois law, the prosecution must prove beyond a reasonable doubt that you were driving (or in actual physical control of a vehicle) while impaired, or that your blood alcohol concentration was at or above the legal limit.

This is a high burden (in fact, the highest burden in our legal system) to meet. If a judge or jury has a “reasonable” doubt as to whether the evidence proves you committed the crime you are charged with, they must deliver a not-guilty verdict.

This proof rarely comes from a single source. Instead, prosecutors “stack” evidence.

Here’s an example:

If the only DUI evidence against you is the police officer’s account of your traffic stop and their bodycam footage showing you acting nervously, this can be challenged in multiple ways. Firstly, a defense attorney could challenge the officer’s bias or suggest that they made a bad judgment call. As for the bodycam footage, being anxious is not a crime. It’s actually a very normal reaction. You might have seen videos of drivers being arrested even after blowing a zero on a breathalyzer. You might be worried about your license being suspended if you’re charged. You might be distrustful of law enforcement, knowing that they might twist your words or ask you trick questions to try to catch you out. Or, you might simply be intimidated by authority figures.

So, it’s easy to see how a DUI attorney can present evidence: in this case, the bodycam footage doesn’t suggest guilt, only that you were nervous. A jury might even be able to empathize if they’re nervous around the police or know someone who’s had a negative experience with law enforcement. But more importantly, this introduces doubt.

Now, if the police have this same evidence, but they also have a breathalyzer test showing that you were over the limit, an admission that you had been drinking, empty beer cans on the passenger seat or in the footwell of your car, and a failed field sobriety test, it becomes harder (but not impossible) to overcome.

In addition to introducing doubt by challenging specific evidence, an attorney might also attack the DUI evidence collection process.

The police must follow a specific legal procedure when investigating a DUI and collecting evidence, including:

• Having a valid reason to conduct a traffic stop
• Administering field sobriety tests correctly
• Conducting breathalyzer tests correctly
• Explaining your rights if you are under arrest or in custody
• Having probable cause to search your vehicle
• Getting your consent for a search if they do not have probable cause.

If an officer fails to follow procedure at any point in the process, it may be grounds to exclude specific evidence or even get your DUI charge dismissed entirely.

For example, if the police search your car and arrest you based on evidence they find stored in the glove box or another hidden compartment, the search may be illegal, and that evidence might be inadmissible in court.

If the officer who pulled you over did so randomly, and not because they had reason to believe you were committing a violation, your attorney can argue for your case to be dismissed, as the evidence that led to your arrest would not have been gathered because you wouldn’t have been stopped in the first place.

So, now we can appreciate just how important DUI evidence collection is, not only in terms of volume but also in the way it is gathered.

But what types of evidence might be used to build a case against you? Let’s break them down, along with how a skilled DUI attorney might challenge them.

The Different Types of Evidence in a DUI Case

Evidence can range widely in a DUI case, depending on the circumstances. For example, 911 call records can be brought in as evidence, but not every DUI arrest will come as a result of a bystander calling the police. Similarly, the State can bring in a breathalyzer result showing you were over the legal limit, but only if you consent to the test.

911 Calls

Some DUI arrests begin with a 911 call from a bystander. The dispatcher sends a patrol car, and before you know it, you find yourself at the station under arrest for suspected drunk driving.

911 call transcripts and recordings can be used by the State to paint a picture of your alleged behavior and corroborate other evidence.

However, this DUI evidence can also be challenged in several ways:

The first thing to establish is what exactly was reported. If the police respond to a report of reckless driving, the 911 call can provide reasonable suspicion for a stop. If officers observe signs of impairment (odor of alcohol, bloodshot eyes, slurred speech, admissions, etc.), they can expand the encounter into a DUI investigation, which might lead to an arrest.

But what if the 911 call is about something else? A callout for another alleged crime can shift into a DUI investigation if the police encounter was lawful.

But what does that mean?

Let’s say police are dispatched after a bystander calls 911 to report a fight at the side of the road. When officers arrive, they encounter a group of people standing next to a couple of cars.

Because the police are responding to a 911 call, they are lawfully present at the scene. This means they can:

• Secure the area
• Identify individuals present
• Ask questions
• Determine what happened.

If, based on this interaction, they have a reasonable suspicion that a person is under the influence, they can expand their investigation.

For example, they might speak to one individual and observe bloodshot eyes, slurred speech, and the smell of alcohol.

But, even then, this DUI evidence introduces plenty of room for doubt. The Illinois law on driving under the influence states that a person must be driving or in actual physical control” of a vehicle (625 ILCS 5/11-501). In our scenario, we end up with more questions than answers, including:

• Was the person driving?
• Was the engine running?
• Were they behind the wheel, or did witnesses place them there?

If officers arrive at the scene of a suspected assault and a person who appears intoxicated is standing outside of the vehicle, and the keys are not in the ignition, it might be reasonable to suspect that one of the other people at the scene was driving.

Another way to challenge a 911 call is to question the caller’s credibility. Vague reports can open the way for mistaken identity, especially if they cannot provide specific details, such as a description of the vehicle or individual.

Without strong corroborating evidence, a 911 call is only evidence that a report was made. Context is everything.

Body Worn Camera and Dashcam Videos

Video footage can be powerful DUI evidence, but it can also be flawed. Body cams (worn by officers) and dashcams (onboard cameras that record the view from the patrol car) are meant to provide an unbiased picture, but they don’t always capture the full context.

Officers might not capture the full interaction (including the reason for the traffic stop, any field sobriety tests conducted, and the arrest itself) because:

• Technical issues mean the cameras activate late or cut early
• An officer did not set their camera to record
• Environmental factors (like wind or traffic) distort the audio
• An officer steps into frame, blocking key moments.

When footage is missing or obscured, the State will likely rely on supporting evidence to “fill in the gaps.”

For example, if the body-worn camera does not record a walk-and-turn test, the State might put the officer on the stand to testify that you did not follow the instructions correctly. This is why it’s important to hire a lawyer if you’re charged with DUI. An experienced attorney can question the officer’s credibility and recollection.

Bodycam footage can also work in your favor, especially if there are inconsistencies between the video footage, the officer’s testimony, and written reports.

Reports are written after the fact, and an officer may misremember what happened or even exaggerate or frame a stop to justify their arrest or increase the likelihood of a conviction. If these types of DUI evidence don’t match up, it can undermine the State’s case (and strengthen yours). For example, a written report might state you were confused, when the bodycam footage might show you acting calmly.

Videos and Audio from the Police Station

Your interactions with the police after your arrest should also be recorded. This can include further testing (such as an evidentiary breath test, if you consent) or interrogation. Like bodycam footage, this DUI evidence is seemingly objective, but it can also be subject to bias and misinterpretation.

Field Sobriety Tests

Officers typically administer three standardized field sobriety tests (FSTs) during an Illinois DUI investigation:

• Horizontal Gaze Nystagmus (HGN): Evaluates eye movement
• Walk-and-turn: Tests balance, coordination, and ability to follow instructions
• One-leg stand: Measures balance.

These tests were developed by the National Highway Traffic Safety Administration (NHTSA) to gather evidence for a potential arrest, but they are highly subjective.

FSTs must be conducted properly, yet we often see:

• Instructions given too quickly or explained poorly
• Demonstrations performed incorrectly
• Officers interrupting before a person is finished
• Tests conducted in poor conditions, such as low light, on an uneven pavement, or on a damp surface during rainfall.

Even if the correct evidence-collection procedure is followed, the results of FSTs can be influenced by factors such as age, weight, physical injuries or medical conditions, fatigue, and nerves.

Because of this, field sobriety test results are not the most reliable DUI evidence for securing a conviction.

It’s also important to know that you can refuse these tests without consequence. An arrest can still be lawful if there is other evidence of impairment, but you will not incur a mandatory license suspension like you will if you refuse a breathalyzer test at the police station.

Preliminary Breath Tests

There are two types of breath tests administered in Illinois for determining a driver’s blood alcohol content. The first is the roadside breath test, and this is probably what comes to mind when you think about a breathalyzer.

Preliminary breath tests are not typically admissible in court, so the State cannot admit your result as evidence that you were over the limit. Their primary purpose is to screen drivers in the field, and a positive result can give the police probable cause to arrest you.

These handheld devices are not always accurate and can result in false positives due to calibration issues, improper administration, and the presence of residual mouth alcohol.

The police don’t need a positive preliminary result to arrest you for DUI. You can be below the limit or refuse the test and still be arrested if an officer has a reasonable belief you are impaired.

However, if the preliminary test is the primary reason you were arrested and there were calibration or procedural errors, it can give you grounds to challenge the legality of the arrest.

Evidentiary Breathalyzer Tests

Evidentiary breath tests are conducted at the station after an arrest using a machine called an Intoxilyzer.

This test is more accurate than a preliminary test and is also admissible in court as DUI evidence. However, specific procedures must be followed, including:

• Regular maintenance
• Proper calibration
• Administration by a certified operator
• A mandatory observation period of at least 20 minutes before conducting the test.

You also have the right to refuse an evidentiary breath test. This will result in a one-year license suspension for a first DUI (under Illinois’ implied consent rules), versus six months if you take the test and have a blood alcohol content over .08%.

We typically recommend refusing the test, as breathalyzer results are often heavily relied upon by prosecutors to secure a conviction. Without it, they have a weaker case.

Blood and Urine Tests

Blood and urine samples are considered more reliable than breath samples, but they are not always conducted. They also come with their own set of potential issues, which can be used to weaken the State’s case.

We will always question who administered the test, where the sample was stored, how long before it was tested, and who handled it at each stage to identify any issues with the chain of custody, contamination, testing delays, and documentation.

How It All Comes Together: A Real Client Story of DUI Evidence Collection Gone Wrong

In our years of DUI defense, we’ve encountered numerous cases where the evidence presented in the police report didn’t match the reality captured on body-worn camera footage.

One particular case stands out in our minds, as it perfectly showcases the importance of scrutinizing every piece of evidence.

Our client, Michael, was a middle-aged professional with no prior run-ins with the law. One evening, after attending a dinner party, he was pulled over for a minor traffic violation. The officer suspected intoxication and conducted standard field sobriety tests. The police report painted a picture of Michael failing these tests miserably, leading to his DUI arrest.

However, when we obtained the body-worn camera footage of the traffic stop, a very different story emerged. While reviewing the video, we identified not one, not two, but at least seven instances when the officer’s instructions during the sobriety tests deviated from the standard protocols taught at the police academy. These deviations were not minor; they were significant enough to compromise the validity of the entire testing process.

For instance, during the walk-and-turn test, the officer’s instructions were unclear and contradictory. Michael was told to start before the instructions were completed. During the one-leg stand test, the officer failed to provide essential details about how long Michael was supposed to hold the position. These discrepancies completely contradicted the standardized procedures and raised serious questions about the accuracy of the officer’s observations and report.

Armed with this evidence, we developed a strategy around these inconsistencies. During cross-examination of the arresting officer, we systematically highlighted every deviation from protocol and exposed the gaps between the report and the actual events.

The turning point came when the judge expressed doubts about the officer’s credibility after reviewing the footage and hearing our arguments. It became clear that the sobriety tests, as they were conducted, were not reliable indicators of our client’s sobriety. Consequently, the judge found the evidence insufficient to support the DUI charge.

The judge ultimately ruled in favor of our client, declaring him not guilty of the DUI charge.

Incorrectly interpreted or flawed evidence can lead to severe consequences, from wrongful convictions to avoidable penalties.

If you’ve been charged with a DUI in Illinois, you might think that the State has a mountain of evidence against you, but we’ve seen how quickly a case can fall apart if proper DUI evidence collection procedures aren’t followed.

The key is to pay attention to every detail, and that’s what our Chicagoland DUI attorneys do. What’s unique about our firm is that we collaborate on every case. You get the eyes of every one of our DUI attorneys on your evidence, and that means we can easily spot the inconsistencies and opportunities most other firms would miss.

Contact Driver Defense Team today for a free consultation and see how we can help you.