Facing Drug-Related DUI Charges: Can You Call an Expert Witness to Challenge the Validity of Field Sobriety Tests?

DUI ― Driving Under the Influence ― is commonly thought of as a crime involving alcohol. But in Pennsylvania, a DUI conviction can arise from the use of street drugs, prescription medication, alcohol, or some combination of these. So, what happens when police try to use the results of field sobriety tests to prove a drug-related (rather than alcohol-related) DUI in court? Can the driver present an expert witness to attack the scientific validity of the tests for detecting drug impairment?

The Superior Court of Pennsylvania recently said “yes.”

Adderall and Xanax

In Commonwealth v. Taylor, a driver was driving her car about 20 mph over the speed limit with her 18-month-old child secured in a car seat. When the traffic light turned red, she had to brake abruptly and nearly rear-ended a stopped vehicle in front of her. When the light turned green, she rapidly accelerated her car over a nearby curb and crashed into a utility pole located about 100 feet from the road.

Another motorist saw the accident and approached the car on foot to offer help. The driver got out of her car and told the motorist that she and her child were not injured. While speaking with the motorist, the driver attempted to shut her car door, but the motorist stopped it from shutting because it could have hit the child’s outstretched arm.

A police officer arrived at the scene a few minutes later. He observed that the driver had bloodshot eyes and slurred speech, but he did not smell alcohol. The driver also appeared to be confused and very tired. The officer had the driver perform two standard field sobriety tests ― walking in a straight line and walking and turning 180 degrees. According to the officer, the driver performed poorly on the tests due to trouble balancing herself and following directions.

The officer arrested the driver on suspicion of DUI and Endangering the Welfare of a Child (EWOC). While in custody, the driver admitted to taking Adderall and Xanax, but could not say the amounts or how long before the accident they had been ingested. She denied having any injuries or medical conditions that could have affected her ability to operate a motor vehicle.

Expert Witness Testimony Barred

At trial, the central point of dispute was whether the driver was impaired by a controlled substance when she crashed her car. The Commonwealth did not allege that she was impaired by alcohol and did not admit any blood test into evidence.

The Commonwealth relied heavily on the arresting officer’s testimony about the accident and how the driver performed on the field sobriety tests to prove its case. The officer testified at length regarding his expertise in administering those exercises. Other than describing the scene of the car accident, almost all of the officer’s testimony was focused on how poorly the driver performed on the tests. He testified that the driver’s performance indicated impairment due to drug use.

The driver attributed her poor performance to a possible head injury from the accident. She attempted to rebut the officer’s testimony with the opinion of her own expert witness, who was a medical toxicologist and physician. The witness planned to testify, in part, that there is no scientific basis to rely on field sobriety tests to detect drug impairment because they have only been validated to reveal intoxication from alcohol.

But when the witness attempted to testify to that effect, the Commonwealth objected, and the court refused to allow the testimony.

Without the benefit of her expert witness’s testimony, the driver was convicted of DUI and EWOC. She appealed to the Superior Court.

Heart of the Central Jury Question

Rule 702 of the Pennsylvania Rules of Evidence provides that a witness who is “qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise” if their knowledge is “beyond that possessed by the average layperson,” the knowledge “will help the trier of fact to understand the evidence or to determine a fact in issue,” and “the expert’s methodology is generally accepted in the relevant field.”

Here, the Superior Court found that the driver’s witness was qualified as an expert and should have been permitted to testify that field sobriety tests have never been scientifically validated as reliable indicators of drug impairment.

Moreover, the court found that if the testimony had been admitted, it could have changed the outcome of the trial:

[The expert] would have opined that field sobriety tests are not scientifically proven methods of detecting drug impairment. If admitted into evidence and accepted by the jury, this expert opinion would have rebutted the officer’s conclusion that [the driver] was impaired by drugs. It was for the jury to weigh that evidence, but it never got the chance.

Since the excluded testimony went to the heart of the central jury question, the Superior Court ruled that the driver was entitled to a new trial.


The driver will have the opportunity to present her defense, which is good news for her. For the rest of us, this decision is significant because it provides all drug-related DUI defendants with a potential strategy for fighting the charges if no bloodwork is taken.