The second prosecution witness usually called in a Florida DUI / DWI trial is a chemist from the crime lab – the prosecutor’s “expert.”  The prosecution expert always gives testimony that points to a defendant’s guilt, but an experienced drunk driving criminal defense attorney will perform an aggressive cross-examination and call independent experts to the stand to counteract the prosecution toxicologist’s testimony.

Prosecution experts nearly always make assumptions about the defendant’s level of intoxication that don’t take the driver’s size, metabolism, or tolerance level into account.  The expert will testify about the driver’s chemical test, including how it was performed and what the result said about the level of alcohol in the defendant’s system. 

The expert is usually asked to testify about the mental and physical affects of alcohol on driving skills, and to provide examples of behavior that demonstrates a person is under the influence. 

The prosecution expert might make conclusions about hypothetical drivers and use those assumptions to support the allegation that the defendant also was under the influence.  It’s important to keep in mind that experts can rely on hearsay evidence – out of court statements – to support opinions and conclusions. 

After the prosecutor finishes directly examining his or her expert, the defense lawyer begins cross-examination. An experienced defense attorney will seize this opportunity to discredit the expert. The defense attorney will ask leading questions designed to draw a yes or no response.  Just as in the cross-examination of the arresting officer, grilling the prosecution’s expert in a drunk driving case gives the defense attorney an opportunity to testify. This allows the defense attorney to tell jurors what he or she wants them to hear.

A skilled DUI / DWI defense attorney knows that it’s far more powerful to score points with a prosecution toxicologist than with his or her own expert. To persuade a witness called by the prosecution to agree with the defense, or support something that helps the defendant’s case, is much more valuable than anything a defense expert could say. 

During cross-examination, a savvy defense attorney will point out that the prosecution expert has little or no information about the defendant’s drinking patterns, level of tolerance, or how his or her body absorbs alcohol.  The defense toxicologist has no information about how much the defendant drank before being arrested and is unaware of any injuries or illnesses that would affect the driver’s performance on a field sobriety test.