A 17-year-old boy with autism is standing trial in the South Australian Youth Court over a triple fatal crash on the Princes Highway in the state's south-east.
Millicent residents Ned and Nan Walker and their daughter Sue Skeer died in the head-on collision at Suttontown on November 28, 2020.
The boy — who was 16 at the time of the crash — has pleaded not guilty to three counts of causing death by dangerous driving.
Prosecutor Aimee Winra described the crash as a "catastrophic tragedy" in her opening address on Tuesday.
She told the court the teenager, who had a learner's driver's licence, had been "thinking about school" and "not been concentrating" when he drifted into the opposite lane and collided with the oncoming car.
Ms Winra said the boy's "gross inattention" had led to the crash.
The court heard the teenager was on medication at the time which made it difficult for him to concentrate.
Ms Winra told the court the boy may have "zoned out" while the car was in cruise control, but he had braked in the seconds before the crash.
Mistake 'immediately rectified'
The boy's defence lawyer Bill Boucaut QC said any drifting into the incorrect lane was "momentary in the extreme" and "immediately rectified by the defendant's mother" who was supervising.
Five witnesses gave testimony in the first day of the trial, including the boy's aunt, who was in the car at the time of the crash.
She told the court it was the oncoming car, driven by Nan Walker, that was on the wrong side of the road.
She said "it happened so fast" and she "didn't see them [the oncoming car] until they were there".
"They crashed into us," she said.
The judge will visit the scene of the crash on Wednesday.
The trial is expected to finish on Friday.
