An expert witness who hammered home to the jury his view that a car accident was the defendant’s fault may have cost the plaintiff her recovery.

The expert “usurped the function of both the court and the jury when he repeatedly testified that defendant’s conduct was the proximate cause of the accident,” a New Jersey appeals court ruled Tuesday in ordering a new liability trial.

As Tammy Wilkey was crossing Market Street in Saddle Brook on Sept. 24, 2007, she was struck by a car driven by defendant Elaine Mayer. Wilkey was 38 at the time and Mayer, who admitted to having cataracts, was 53 at the time.

Witness accounts differ as to whether Wilkey had run into the street. Mayer, who was driving west on Market, said she suddenly saw Wilkey out of her left eye and unsuccessfully tried to swerve.

Wilkey testified that the only thing she remembered of that day was saying goodbye to her cat and then waking up in the hospital. As a result of the accident, her right arm was broken in three places and she sustained multiple fractures to her left ankle.

John Desch, an engineer and traffic accident reconstruction expert, was hired by the plaintiff. Based on police reports, Desch opined that Wilkey had been walking at 5.8 feet per second and was hit 42 feet from the curb when the impact occurred. Desch said that when Wilkey stepped off the curb Mayer was 300 feet away, giving her ample time to see Wilkey and stop her car.

“The proximate cause of this accident, in my opinion, was the fact that [defendant] never saw this woman crossing for the seven seconds that she was out into the roadway,” Desch testified at one point. “She never saw her.”

The jury returned with a verdict in Wilkey’s favor and awarded the plaintiff $600,000 in damages. The jury said that while Wilkey was negligent, that negligence was not a proximate cause.

On appeal, Appellate Division Judges Paulette Sapp-Peterson and William Nugent said Desch’s testimony regarding proximate cause amounted to an improper net opinion because the term is an “expression of legal significance.”

“We agree that it was error to permit the expert to testify in the manner that he did on the issue of proximate cause,” the judges said in Wilkey v. Mayer.

“Although Desch was qualified by his education, training, and experience to reconstruct such things as [the car's] rate of speed and path of travel, plaintiff’s pace and path of travel, and the times it would have taken the parties to traverse certain distances, nothing in his background gave him any special ability to apply legal concepts of proximate cause and comparative negligence to the facts that he had reconstructed,” they said. “That was, exclusively, the jury’s task.”

Mayer’s lawyer, Floyd Lombardi, says it was clear that Desch’s testimony was inappropriate and improper.

“There can always be more than one proximate cause to an accident,” says Lombardi, of Jersey City’s DeSevo Lombardi. When Desch said Mayer’s actions were the sole probable cause, “that confused the jury,” he says.

“I have no choice but to accept the decision,” says Wilkey’s lawyer, Charles Lorber, of Mandelbaum Salsburg Lazris & Discenza in West Orange. “We’ll be ready to try the case again.”

Neither side contested damages, so the next trial will be for liability only. If Wilkey’s actions are found to be a proximate cause, it could affect how much of the $600,000 she ultimately receives.