In any Driving While Intoxicated (DWI) case, the prosecution must prove two primary facts:
1) the person being charged (the "defendant") drove a vehicle, and
2) at the same time, the defendant was "under the influence"—meaning that the person's ability to drive safely was affected to an appreciable degree by drinking alcohol, taking a drug, or a combination of alcohol and drugs. A DWI defense can be anything that proves one of these two elements wrong and prevents the prosecution from proving its case. A valid defense might also prevent the prosecution from introducing evidence at trial.
A significant part of the evidence against you in a DUI or DWI case will usually consist of the arresting officer's observations and impressions as to whether or not you were drunk. For example, the officer may testify about the way you were driving (uneven speeds, weaving, crossing the center line, running a red light, or hesitation going through a green light), how you looked and acted once your vehicle was stopped (bloodshot eyes, slurred speech, stumbling), or how you performed on field sobriety tests. If you can challenge the officer's observations or present evidence that might refute those observations, you may be able to knock a big hole in the prosecution's case.
In some cases, a defendant may be able to introduce witnesses who can refute the officer's "opinions" by testifying that you didn't drink anything before you got in the car, that you appeared to be sober, or that there were other circumstances involved; i.e., you ran a red light because you were distracted by conversation, not because you were drunk. You may also be able to counter the officer's decision that you had been drinking by offering valid explanations for how you looked or acted when you were pulled over. For example, you may be able to prove that you did not perform well on field sobriety tests because of physical impairments or because the instructions were confusing.You might claim that your eyes were bloodshot because of lack of sleep or allergies, or your speech was slurred because of lack of sleep or medications you take.
Most DUI and DWI cases start with a driver getting pulled over, so there usually isn't much argument over whether the defendant was actually driving. But if a police officer didn't actually observe you driving; i.e., the officer approached your idle car while you were behind the wheel in a parking lot, the issue might be debatable.
If the officer did not have legal justification to stop your vehicle and arrest you in the first place, or if the officer failed to follow proper legal procedures during the arrest, any evidence gleaned from the traffic stop or the arrest might be deemed "inadmissible" and kept out of a court case. This could leave the prosecution with no real case (for example, because breathalyzer results and the arresting officer's testimony couldn't be used as evidence), and the charges against you could be dropped.
"Probable Cause" means that the facts support an objective belief that the person to be arrested has committed a crime, or that a place or item to be searched bears evidence of a crime. The obvious question in probable cause is "How likely must it be that the defendant committed a crime, or that the place or item contains criminal evidence?"
If an officer did not have probable cause to stop your vehicle, detain you, or arrest you for drunk driving, then you may be able to keep any evidence obtained during the arrest from being admitted at trial. For example, if you believe you were stopped simply because of your race or ethnicity rather than because you were driving erratically or appeared to be intoxicated, you may be able to challenge the arrest.
In Texas, an adult who drives with a blood alcohol content (BAC) level of .08% or above is guilty of Driving While Intoxicated. For drivers under the age of 21, almost all states set the limit at .01% or .02%.
The police administer chemical tests to measure the BAC in your breath, blood, urine, or saliva, and the results of these tests are usually the main evidence entered in the prosecution's case against you. If you can successfully challenge the accuracy of these tests, the test results may be deemed inadmissible at trial. Vials used in collecting medical blood samples may lack anticoagulant and preservatives, the antiseptic used to cleanse the area may contain alcohol, and any disease process affecting the liver, such as hepatitis, will impair blood test results.
Also, when alcohol is consumed it can take from thirty minutes to one and a half hours to be absorbed by the body. This is influenced by the last time a person consumed food. This is a viable defense when your BAC would have been below the legal limit when you were driving, but by the time the breath or blood test was performed the level had increased beyond the legal limit.