Many who have recently been arrested for DWI feel defeated or discouraged from the initial moments of their cases. This is an understandable response to what is always a very stressful, unlucky, and shocking event in their lives that has jarred them to their core. However, much of this sense of doom people feel is their own personal belief that there is overwhelming evidence against them. Before any evidence has been released to their defense attorney, and before they have seen any of it, a sense of doom pervades. There are probably various reasons for this. Sometimes, these ideas come from lawyer television shows like Crime Scene Investigation and from movies, and perhaps from anecdotal stores of DWI cases from their friends and families. These sources probably inform how they feel about the state of their case. Probably this also comes from their own ideas about the technology involved in DWI cases. Scientific evidence appears to be infallible and irrefutable, but something people don’t often consider is the human element that is present at every stage of breath tests and especially blood tests. But technology cannot remove the human element from blood or breath testing in DWI cases, and will not be able to for a long time to come. This means that there is the possibility of error at every stage. There is always room for a defense lawyer to stir up reasonable doubt when there is the potential for human error. This page surveys the different kinds of evidence will surface in DWI cases, from scientific evidence to video, and witness statements.

What Kinds of Evidence Are In a DWI Case?

Criminal cases are made up of different kinds of evidence. What evidence is allowed to come before the jury can go a long way in determining a win and a loss in a given case. The rules that govern the admissibility of evidence are very complex, but the rules are nuanced legal topics. Mastering the rules of evidence is a crucial skill for any criminal defense lawyer and any lawyer that practices any litigation. However, that is not what this article is about. This page is about what to expect from a DWI case file and how evidence in a DWI case can influence the outcome of a case, and how it can determine the outcome of a jury trial. As mentioned above, the types of evidence in a DWI case range from video to written statements to forensic alcohol tests. Forensic alcohol tests, i.e. blood or breath test results, are present in almost every DWI case. These are discussed below.

Blood Tests

Blood tests can be some of the most powerful and persuasive evidence against someone in a DWI case, and need to be skillfully handled. Convictions in DWI cases have risen since the proliferation of blood tests across Texas. Jurors believe blood tests and will believe whatever result the prosecutor has typed on a piece of paper as truth, unless a defense attorney can poke holes in it and undermine the test. Likewise, defense attorneys have had to step up their game and learn about blood testing, the underlying science, and how it is performed to effectively challenge the evidence against their clients and provide full and effective representation. Undermining the test requires several skills: knowing what to look for and explaining complex scientific issues and problems to the jury in as simple and as easy terms as possible. This is not an in-depth page about blood testing and how it works or how to challenge blood tests, but a criminal defense attorney must be able to understand how blood testing works and how to challenge it to practice effective DWI defense.

Gas Chromatography

Gas Chromatography is the method used to test and analyze compounds in blood samples across Texas and the United States. It is the premier analytical tool to separate and measure ethanol in human blood. The process can be very precise and accurate. When conditions are scientifically sound, gas chromatography is a great and reliable tool for separating and measuring ethanol content in blood. If the result is reliable, this means a number of conditions have been met. This means—among other things—that the blood is drawn and stored according to procedures that prevent contamination, and the blood is mixed and placed into a vial with the proper ratio of other compounds for testing, and that the gas chromatograph is well-calibrated and well-maintained. There are a large number of variables that need to be measured and controlled. There are a large number of precautions that must be taken. This entire process is complicated and because—again—a human being prepares and analyzes the samples, there is always the potential for human error. It does not matter how accurate and reliable gas chromatography may be, when a person performs the analysis it is always subject to many legal challenges from a criminal defense attorney who knows what to look for.

The Lab Analyst

Blood tests are the most powerful evidence in a DWI case, and there are two ways to challenge and undermine the result. The first way is for the defense to hire an expert to review the evidence and testify in court that the resulting score is unreliable for one reason or another. This is ideal, but this can be expensive, and many people cannot or do not want to spend the money to retain a defense expert. IF money cannot be found to hire an expert, the other alternative is a thorough cross examination of the state’s lab analyst. This is person who tested the blood that produced the result. The lab analyst is a very important feature in a criminal defense trial. The U.S. Constitution requires that criminal defendants be able to confront witnesses who will bear testimony against them. The importance of this cannot be overstated. Every lab analyst who analyzes blood evidence in a DWI trial must testify in court for the evidence of the blood test is to be admitted. Without the analyst to appear in court in testify, the evidence of a blood test cannot come before the jury. The lab analyst must testify and be accountable to defense questions and challenges regarding all the policies and procedures of the laboratory, and the reliability of the machine. The criminal defense attorney must take full advantage of this opportunity to undermine the result and chip away at the armor of the blood test.

Breath Tests

If someone takes a breath test in a DWI case, this will also be a huge piece of evidence for the state, and the defense must strategize and account for it. Breath tests require that the person consent to the test. Unlike a blood test, a person must choose to take a breath test. A police officer cannot obtain a warrant to get a breath test. Breath tests were once more common than blood tests, although that has changed. Some police agencies do not even have breath testing programs anymore. However, they are still alive and well and many police agencies still use them. The breath test uses a technology called infrared spectroscopy, discussed briefly below. For a breath test to be admitted as evidence, both the breath test operator who conducted the test and the technical supervisor in charge of the breath testing program must testify in court for the result to be admitted. In Texas the breath test machine that is used is called the Intoxilyzer 9000, a name which sounds like a bad video game from the 1980s. The person taking the test must blow into the machine, and the machine takes two samples several minutes apart. The two samples must be within .02 of each other for the result to be considered acceptable and reliable, although this can still produce wildly varying results that are still considered by the state to be scientifically valid. There are different ways to challenge the reliability of breath tests and different things to look for. The goal is to cast doubt on the machine’s ability to produce an accurate and reliable result.

Infrared Spectroscopy

The technology that qualifies and measures the alcohol in someone’s body is called infrared spectroscopy. It measures the quantity of a molecule, ethanol (alcohol), by how much infrared light the ethanol molecules absorb. Then, the machine compares the amount absorbed to a reference sample and provides a BAC reading. The machine measures the relative the amount of ethanol in the reference sample. The reference sample is prepared by the technical supervisor and gets measured every time there is a test. The reference sample solution is how the machine gets calibrated, and so it is very important, and even though the solution was prepared to be a known quantity of .08 BAC, the machine will often read the reference sample differently, such as .078, or .074. Regardless, there are many ways in which the breath test evidence can be undermined and questioned. Some of these are outlined below in a discussion regarding the technical supervisor, who like the blood test analyst, must testify to satisfy the Confrontation Clause of the U.S. Constitution.

The Technical Supervisor

The technical supervisor must testify at every DWI trial for the score of the breath test to come before the jury as evidence. The technical supervisor must testify that infrared spectroscopy is a reliable method for testing BAC that is accepted in the scientific community, and that the machine was working properly on the day in question. Because breath tests are scientific evidence, the court must be satisfied that the test meets the basic threshold for admissibility. Generally this threshold is achieved by the technical supervisor and the prosecutor, and the jury gets to see the breath test score. Creating doubt is essential for the defense to prevail. What does a number mean if a prosecutor cannot rule out a reasonable possibility that it may not be accurate? This can mean everything in a breath test DWI case. It is up to the state to prove that the breath test score is reliable. Sometimes they can and sometimes they cannot, and the only thing standing in the way is the defense lawyer.

Dash Camera Video

Besides the scientific evidence in a DWI case, the other keystone piece of evidence will be a police video from the scene. Although body-worn cameras (discussed below) are becoming more prevalent as more police agencies acquire them, all police agencies still have cameras installed in their dashboards. These cameras record key information such as the initial encounter between a driver and a police officer. The camera will also record the field sobriety tests, which are a key piece of evidence in any DWI case. The standard block of tests that any police officer will have a driver suspected of DWI perform are three tests. They are the walk and turn test, the one leg stand test, and the horizontal gaze nystagmus test. The rules and procedures for these tests have been implemented and designed by a federal agency called NHTSA and they are utilized by police officers in every state. Both the walk and turn and the one leg stand tests have long instructions that can seem complicated in the stress of the moment of the arrest. Police officers will often downplay the impact of stress while performing these tests, but jurors can understand this. The horizontal gaze nystagmus test is familiar to most people with the image of a police officer waving a pen or similar object in front of a person’s face as they follow it with their eyes. Dash camera evidence can also capture important evidence about how a person walks and stands and moves around.

Body Camera Video

Body camera footage is much more common now than it has been. In a typical DWI case, there may be three to five officers on the scene. The number of officers varies, but if they are all wearing body cameras then there will be variations of the same event from different angles. Usually, only footage from one of these cameras will come before the jury in a case as evidence. Body camera footage is more intimate than dash camera footage. It also has audio, which sometimes dash camera footage lacks. On a body camera, you can clearly hear how a person talks and make close-up observations and judgements about whether a person is intoxicated or not based on how clearly they are speaking and if they are slurring their words. Most police agencies in larger cities like Fort Worth and Dallas have body camera footage, while smaller cities might not.

Witness Statements

Witness statements are common evidence in DWI cases. When a DWI arrest occurs, there may be a variety of people who observed the driver and gave an account to police about their observations. Generally, the witness must be present in court to testify for their impressions and judgments to come before the jury as evidence. The witness can be someone in the car as a passenger, or it can be someone who was with the driver earlier in the evening. The witness can also be another driver. It is not uncommon for someone driving to call in a suspected intoxicated driver. When this happens, there is a 911 call and sometimes also a witness statement if the caller also appears at the scene of the stop. On the other hand, the witness could be someone who will contradict what the officer says. It is reasonable for two people to arrive at differing conclusions about what may or may not have happened in a case. Witnesses can provide powerful first-hand testimony for the state or for the defense. However, what appears to be powerful eyewitness testimony can also fall apart on cross examination from a skilled defense lawyer.

Police Reports

In every DWI arrest, the police officer who made the arrest will write a narrative account of what happened. Sometimes, there are multiple reports that flow from one arrest. A police report in a DWI case will outline the circumstances that led to the officer following and eventually deciding to pull over the driver. The reasons for stopping a driver are often speeding, or another traffic violation, such as failing to signal a turn or failing to come to a complete stop at a stop sign. The report will then detail the officer’s initial impressions of the driver upon approach of the vehicle. These impressions will almost always detail the odor of alcohol, and commonly also list slurred speech and bloodshot eyes as signs of intoxication. The report will give a rundown of how the driver then performed on the field sobriety test and how they scored on each given test. There will probably be other details written into the report as well that are specific to the case. Police reports are not evidence in DWI cases and the jury cannot see them. However, the specific statements and observations in a police report are very important, and the jury will hear them when the officer testifies.