Banner for The Criminal Justice Systems: A Guide for Law Enforcement Officers and Expert Witnesses in Impaired Driving Cases

THE SCIENTIFIC EXPERTS: Toxicologists, Crash Reconstructionists,
Optometrists, and Other Medical Personnel

Scientifically-based evidence can be critically important in impaired driving cases. However, oftentimes the evidence is only as good as what is understood by the trier-of-fact (judges or jurors).

Jurors tend to come from all walks of life—school teachers, store clerks, factory workers, college professors, and stay-at-home parents. Even college students can end up on a jury.

The key to expert witness testimony is to make it understandable to those who need to absorb the meaning of what you are saying. Scientific testimony is, by its very nature, complex in both language and meaning. It is imperative that whenever possible you use a common, everyday word instead of the specialized language of your field. It is also imperative to illustrate the scientific concepts with simple analogies.

General Areas of Inquiry for Expert Witnesses

  • You will be asked to explain your training and background, including memberships in professional organizations. Be prepared to discuss the requirements for membership—do you merely sign up and pay a fee or does the organization require some minimal qualifications? What qualifications are required? Do you have more than the minimum qualifications? If yes, don’t hesitate to list them.
  • Have you testified as an expert witness in other court cases? If yes, know how many, in what cities, and whether civil or criminal (for defense or prosecution).
  • Have you written any articles? If yes, were they published?
  • If published, were they peer-reviewed journals?
  • Please explain what is meant by “peer-reviewed journal.”

At this point the prosecutor will ask the court to accept you as an “expert witness,” someone who is capable of giving an opinion about what happened in this case to help the fact finder (judge or jury).

The defense attorney may object to your being accepted as an expert witness and is entitled to ask questions of you—a process called voir dire—before the judge decides whether or not to allow you to testify as an expert. The defense attorney’s questions will generally center on your professional experience and any bias you might have for the prosecution and against the defendant. Assuming you are then allowed to continue testifying, the prosecutor will ask you to briefly explain the work you did on this case.