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Prosecution Expert in a Drunk Driving Trial

During California DUI criminal trial, the second prosecution witness called, after the arresting officer, is usually a chemist from the crime lab - the prosecution expert witness. The evidence presented by the prosecution expert always points to a defendant's guilt, but an experienced criminal DUI defense attorney will cross-examine the expert and call defense experts to contradict the prosecution toxicologist's testimony.

Prosecution experts nearly always make presumptions about the defendant's level of intoxication based on general scientific principles and assumptions. These assumptions don't take a defendant's size, alcohol tolerance level, metabolism, or meals eaten prior to drinking into account. The expert will give details about the chemical test the driver took, such as how it was performed and what the result says about the level of alcohol in the defendant's system.

The prosecution usually asks the expert to testify about how alcohol affects people mentally and physically, and to provide specific examples of behavior that demonstrates a person is under the influence. The expert may then offer an opinion as to how the defendant performed on the field sobriety test and what this says about the driver’s blood alcohol content (BAC).

Finally, the prosecution expert may make statements about hypothetical "situations or drivers" and use those conclusions to support the charge that the defendant in this specific DUI trial also was under the influence. Remember that experts can rely on hearsay evidence - out of court statements offered to prove the truth of the matter asserted - to form opinions and conclusions.

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Attorneys for the firm represent clients accused of driving under the influence of alcohol or driving with a blood alcohol level at or above the legal limit in San Francisco, San Mateo, Marin, Alameda, Santa Clara, Contra Costa, Sonoma and Napa counties.

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Once the prosecutor has finished directly examining his or her expert, the DUI defense attorney can cross-examine the witness. A skilled criminal defense lawyer will use this chance to discredit the expert and punch holes in the prosecution's DUI case. Just like cross-examination of the arresting officer, grilling the prosecution's expert in a DUI case is a chance for the defense attorney to testify. The attorney's leading questions are designed to elicit a "yes" or "no" response. This allows the defense lawyer to tell jurors what he or she wants them to hear, and getting the prosecution's witness to agree with that testimony.

An experienced California DUI defense lawyer knows it's far more effective to score points with the prosecution's expert than with his or her own expert witness. The jury knows the defense expert is hired by the defense to help the defense. The prosecution's expert is in opposition to the defense, so to persuade that witness to agree with the defense, or support something that helps the defense in a DUI case, is much more valuable than anything a defense expert could say.

Cross-examination is an excellent opportunity in a California DUI criminal trial to point out that the prosecution expert isn't familiar with the defendant's drinking patterns, level of tolerance, or how his or her body absorbs alcohol. The expert does not know how much the defendant drank the night he or she was arrested. The expert is unaware of any injuries or illnesses that would impact the defendant's performance on field sobriety tests, or affect the sample provided during the chemical tests.

Without cross-examination from criminal defense attorney who specializes in DUI defense, the prosecution expert will only testify about the information that will help to support the prosecution DUI case and convict the driver. With a skilled cross-examination, the impact of a prosecution expert’s testimony can be diminished or completely eliminated.

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