Nicola Armstrong should have been celebrating her son David's 21st birthday last month.

Instead, she sat in a courtroom listening to the harrowing details of how he died on a sunny morning two years ago when he and another motorist failed to merge on a passing lane.

That motorist, 36-year-old Lucilla Brunt, was today found guilty of causing his death on State Highway 2 near Upper Hutt.

"The cause of this crash is that neither slowed to allow the other in," District Court Judge Mike Mika said this morning while handing down his decision.

"I find that a reasonable and prudent driver would have reduced their speed to let Mr Armstrong pass.

"Failing to do so put Mr Armstrong, Mr and Mrs Lancaster, and other road users at risk."

David was just 19 when he died. His mother recalls a young man who gave his all to the community he loved.

David Armstrong's mum Nicola said her son would do anything for anyone. Photo / Hazel Osborne

From children to the elderly, he was someone who would do anything for anyone his mother told Open Justice.

"Everyone was drawn to him and everybody was engaged by him. He would go out of his way to make you feel like you were his friend even after just one encounter," she said.

During the judge-alone trial, Judge Mike Mika heard how David and Brunt were driving side by side at 103 km/ph for 400 metres in what one witness described as looking like an act of road rage or two vehicles racing.

Brunt's vehicle eventually clipped David's - sending him spinning into the oncoming lane and colliding with motorists Caron and Mark Lancaster.

He died at the scene on a clear sunny morning in November 2020.

More than 500 people attended his funeral days later. His mum didn't even know he knew that many people but said it was clear he had made an impact on a community he loved.

"He donated a lot of his spare time and his death affected the whole Upper Hutt community," she said.

A youth leader in his church, an Oranga Tamariki summer camp volunteer, a son, a boyfriend, an identical twin to his brother Cain. He was a guy who would put his hand up for anything.

The two cars drove side by side through the intersection of Gibbons Street and State Highway 2, moments later disaster struck. Photo / Google Earth

David's family sat through every minute of the two-day trial.

Hearing the expert witness, Senior Constable Lisa Toseland, go over the crash that claimed David's life was difficult for his loved ones, but what hurt even more was the lack of acknowledgement that a young man had his life claimed by a crash that was completely preventable.

For Nicola Armstrong, that lack of recognition was what disappointed her the most.

"What I really wanted was for her to be remorseful, for her to admit her bad decisions as well and acknowledge that... that she at least felt something about her involvement in what happened.

"Whilst David may have made a dumb decision that day, he was a caring, kind boy that was a really upstanding member of the community."

During the trial, Crown Prosecutor Grant Burston said both drivers made bad decisions that morning when David was rushing to help a family friend with a cash job.

But it was the stance of the defence that incensed Nicola Armstrong.

"Someone else's bad decision doesn't make yours obsolete," she said. "Just because someone else has made bad decisions doesn't absolve yours... obviously he made some bad decisions as a driver but that doesn't negate hers."

At today's decision, Judge Mika said that although David's driving could have been negligent, "it could not have been a defence in this case".

The pain for David's mum extends further than just her own family, she believes the couple who collided with her son that day are the real victims.

Both suffered extensive injuries, both physically and mentally, from that split second in time when their car collided with David's.

David was described to have lit up any room he walked into by a family member outside the hearing, and today's verdict has given a small sense of closure for the group.

- Hazel Osborne, Open Justice