Allan Michael Dyson, 61, was behind the wheel of the Kenworth truck when it hit the rear of a Subaru station wagon being driven by Jasmine Payget on the Hume Highway in regional NSW just after 9.30pm on November 26, 2004.
The collision ended up killing Ms Payget's six-year-old son Rian Strathdee and seriously injuring her husband Laurie Strathdee and an 11-year-old boy.
Dyson is on trial in Goulburn District Court and has pleaded not guilty to one count of dangerous driving occasioning death and two counts of dangerous driving occasioning grievous bodily harm.
The family were on a trip from their home in the Blue Mountains to visit Ms Payget's sister in Canberra when they stopped at the Shell service station at Sutton Forest, in the Southern Highlands, crown prosecutor Nerissa Keay told a jury on Thursday.
They bought ice creams and swapped drivers before pulling back onto the highway, accelerating to around 100 km/h.
When Mr Strathdee leant back to offer ice cream to the boy in the back seat he noticed large, bright headlights close to the back of their car, the jury heard.
From the witness box, driver Ms Payget said she felt like the car was "filled with light" moments before the impact.
"Simultaneously I remember a horn blowing that was clearly a truck horn and I was like, 'God, that's close'," she told the jury.
She said she did not remember the moment of the collision but remembered being in the upside-down car after it had rolled down the embankment.
She was told that Rian had died on the scene and she signed the documentation there and then as he lay in an ambulance by the road.
Ms Keay told the jury that a witness who was driving nearby and listening to his UHF radio heard someone with a strong Australian accent say, "I have sorted out the Suburu. Now they are in the paddock."
The crown case is that the man heard there was Dyson, a claim the truck driver denies.
The now-61-year-old did not stop his truck and instead fled the scene to avoid responsibility for the impact, the prosecutor said.
While he has admitted being the driver of the truck, Dyson disputes that he was driving dangerously at the time of the collision.
Defence barrister Harry Maarraoui said Ms Payget's driving would be a major issue during the trial.
As he questioned her, she said she did not remember looking at the speedometer but argued she felt she was going at the right speed.
She also admitted that the vehicle was loaded down with luggage and her rear view was partially obscured with three mountain bikes secured to the back of the Subaru.
However, she insisted that she looked in the side mirror when merging onto the highway.
"I don't recall seeing any vehicles. I understood it was safe," she said.
Earlier on Thursday, Ms Keay said witnesses did not see the truck brake or change lanes before hitting the Subaru.
The evidence would show Dyson must have been travelling at over 100km/h but would have had at least 20 seconds to see the vehicle and react, she told the court.
Analysis of the damaged car after the impact showed there were no defects that could have caused the incident.
The trial before Judge Ross Hudson continues on Friday.
