Why Do You Need to Hire an Expert Witness to Fight a DUI Case?



If you want to win a jury trial on your driving under the influence case, or have real leverage for plea negotiations, you need to hire a Defense Attorney and an Expert Witness. You probably knew the Defense Attorney part already. In this article, we explain why you need an Expert Witness as well.

The Government Collects Evidence for The Prosecution Expert Witnesses.

When a person gets charged with a “Driving Under the Influence” offense (DUI or in some states “DWI”), the Government (in the form of police and prosecutors) collects a lot of evidence in preparation for a jury trial, just in case the Defendant decides to plead “Not Guilty” in court.

In all criminal prosecutions in the US, it is the Government’s burden to prove the Defendant is guilty if he/she demands a trial. The government prepares for that possibility of a trial by collecting the evidence they expect to need to get the jury to vote “Guilty”.

What kind of evidence can you expect in a typical DUI trial? It really doesn’t matter what state a Defendant is in. The evidence in a DUI trial is pretty universal in the US. In general, a DWI Defendant can expect the prosecutors to offer the jury:

1. Testimony from the Investigating Officer(s);
2. Video of the field sobriety tests, dash cam, and/or jail booking facility;
3. Blood Alcohol Content Test Results from the side of the road and from the hospital or nurse station;
4. Testing machine records;
5. Defendant’s own statements (if any);
6. Other evidence such as empty beer cans, cell phone pictures, bag of weed found under your butt;

AND (here’s the kicker) near the end of the government’s case:

5. Expert Testimony and Expert Opinions and Conclusions from a Forensic Toxicologist employed by the Department of Justice (DOJ).

Department of Justice!?? Sound unfair?

Well, it’s not unfair. And in a typical DUI case, the most important prosecution witness is the “DOJ Forensic Toxicologist”.


The Government Will Use a Forensic Toxicologist At Trial to Introduce and Explain the Evidence.

The government prosecutor prepares their evidence for a trial by sending much of it off to a “Forensic Toxicologist” employed by the Department of Justice or its local equivalent for review and analysis. This prosecution oriented “toxicologist” as they are called, are the government’s Expert Witness. Their job is to review the evidence and to produce “Expert Opinions” to be used as further evidence at the trial for the jury. They explain the evidence to the jury.

The rules of evidence usually prohibit the OPINIONS of witnesses. Opinions are not facts. Everyday people cannot offer Opinions as evidence in court, because opinions of lay people are unreliable. But a qualified “Expert Witness” is allowed to offer “Expert Opinions” as evidence in court. And once tagged as an “Expert Opinion” by the court, to the jury, they are facts.

If needed, this DOJ government expert will testify at trial about the defendant’s intoxication “indicators”, the effects of alcohol on the brain and reflex times, the dangers of driving under the influence, the accuracy of the breath testing machines, and their records, and how the Police Officer complied with state regulations and professional standards when collecting test results, observations, etc.

Usually, the prosecution “Experts” are qualified as expert witnesses by the court in advance of the trial, and allowed to give opinion evidence based upon scientific observations because of their education, training and experience. That means at trial they can offer a “scientific opinion” as to whether or not you were impaired at the time of driving, even though they were not present, and that “expert” opinion is admissible evidence.

The prosecution Experts are educated and licensed in their field. You should expect them to be super well prepared, and experienced at testifying in front of a jury. They have been trained by the government to work in court. They know exactly what to say to help get a conviction, and the question and answer testimony in the trial between the DOJ expert and the prosecutor is well rehearsed. Choreographed. And they have heard all of the Defense Attorney’s cross examination questions before. They know how to answer those questions also. They are in the trial as part of the prosecution’s team working to get a conviction.

A properly prepared Defense Attorney would be ready for a well prepared Prosecution Expert in a DUI trial. The Defendant needs to understand that when consider options.

The Government Uses a Department of Justice Forensic Toxicologist to Verify the Test Results

When shopping for a Defense Attorney, most defendants will hear Attorneys claim the test results are always wrong, the errors can be shown at trial. But the government has a plan for authenticating the test results at trial if necessary.

During a DUI jury trial, the Defendant can expect the prosecution’s Forensic Toxicologist to testify on the stand about the manner in which the tests were administered by the officers and/or hospital personnel. You can expect to hear trial testimony something to the effect of:

Question from District Attorney: “Do you have an Expert Opinion on whether or not thetests were accurate?”

Answer from Expert: “I have reviewed all of the evidence, and records, and it appears to me that the testing machine(s) were properly used, maintained and were in good working order at the time of Defendant’s test. There is no reason to suspect the test results are wrong.”

When a jury hears this evidence, it is very powerful. The Defendant must have a way to counter this expert opinion that the jury is likely to accept unless contradicted.

The Government Will Use an Expert Witness to Explain and Verify the Field Sobriety Tests

Neary all DUI Defendants think they have passed the Field Sobriety Tests. I’m not sure if it is denial, or just a lack of understanding about how the test work that causes this phenomenon, but its real. Everyone thinks they passed, but the expert is going to testufy that they failed miserably. A well preapred Defense Attorney will be ready for that testimony, and a Defendant with a well designed defense will have an Expert Witness of their own to testify to an Opposing Expert Opinion.

The Judge Will Instruct a DUI Jury That Experts May Have Differing Opinions, and They Can Choose Who to Believe.

Jury instructions are important in a DUI trial. The instructions are the rules the jury will use for making a decision.

When there are multiple Experts with differing opinions, the jury gets a special instructions. They are instructed to make their own decision on who to believe. This instruction give a trial defense attorney a great foundation for a final argument stating something to the effect of:

“The jury instructions from the judge tell you that you have the right to choose which expert to believe. Experts may disagree. In this case, the Defense Expert is the one to believe. Protect the innocent. The prosecution’s expert is a government employee. You can reject that expert’s opinion. Opinions are not facts. The presumption of innocence is a fact.”

If the prosecution expert’s testimony is not contradicted by a defense expert, this instruction cannot help you, and your foundation of defense will be made of defense attorney flavored jello.

Defendants Have the Right to Their Own Counter Expert Witnesses.

Defendants have a right to use their own expert Forensic Toxicologist at trial also. A defendant with money can search and hire the best defense expert they can find. An indigent Defendant with a Public Defender, may not be able to hire the expert of their choice, but they still have a right to an expert witness to testify in their defense if they can talk the judge into providing the money for it.


How Do You Get a Good DUI Expert Witness?

Most experienced defense attorney already have contacts for good defense oriented expert witnesses. Some experts are better for certain issues, some are better for women defendants, etc. A quality defense attorney can help you sort through the candidates to find the best expert for your facts.

Are Expert Witness Fees Included in Attorney’s Fees?

No. An expert witness must be hired separately from the attorney. The expert witness is a independent professional who is not an employee of the attorney. They are essentially hire by you to give an independent assessment of the evidence. And to offer independent scientific opinions at trial.


Defense Experts Can Produce Reports That May Help in Plea Negotiations.

Even if the defendant does not want a trial. There is work for a defense attorney and even an expert witness to do. The defense attorney is always trying to negotiate a settle of the case in teh form of a plea agreement. In most courts, this happens when the prosecutor makes an offer to end the case. Frequently, this might just be as simple as “plead guilty to a standard penalty to first offense DUI charges like everyone else.”

But if the Defense Attorney has in her hand a Report from a Qualified Expert Forensic Toxicologist, it may help. For example, the Defense Attorney can use the report to convince the prosecutor that there are triable issues – there were problems with that machine that night that the jury might want to hear about. And maybe that justifies enough doubt in the prosecutor’s mind to get a better offer.

And if the report from the expert finds a big error – such as a failure to test the defendant on time – it can lead to dismissal of a case that otherwise would have resulted in a guilty plea for a defendant without an expert.

Questions? Leave a Comment.


About Attorney Christopher Dort

Public Interest Attorney. CA State Bar #196832. Licensed to practice law in California 1998, and then the US Federal District Court in 2000. Civil Litigation Trial Attorney (Insurance Defense Firms) 2000-2003. Private practice 2003 - present. First Solo Criminal Jury Trial 1998 (Santa Cruz County). First Civil Jury Trial 2002 (Orange County). Santa Cruz County Public Defenders Office 1996-1999 (law clerk). BA in Politics from UC Santa Cruz, 1995. JD, University of California, Hastings, 1998.
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