Mr Atchinson looked up before the impact, but was not able to get to safety, Mr Frost said.
The driver of the pick-up truck was travelling about the speed limit, which was 90 kilometres an hour. He tested negative for drugs and alcohol and was not using a cell phone, but the investigation was ongoing.
"The driver is devastated ... it's an unfortunate tragedy."
Mr Atchinson was working alone and police were "not aware" of any precautions he took to warn motorists he was working on the road.
Stanislaus County spokesman David Jones referred comment to Highway Patrol when he spoke to the Modesto Bee last week.
Mr Atchinson's sister Lynley Atchinson said family had few details of what had happened.
"He's not the kind of person to not do things by the book."
Her brother was "devoted" to family, she said.
"Ellen and Clara were his world, but also me and my brothers, and extended family and friends, were important to him. He always missed New Zealand."
Brother Terry Atchinson said his brother was a bubbly person who got along with everybody.
The Waiuku College old boy had lived in the United States for more than 20 years, starting as a ski patroller, but spent some time living in New Zealand in the early 2000s.
He remained connected to his homeland, especially local motorsport, Terry Atchinson said.
"I didn't even have to worry about motorsport. He would call up and tell me what was going on here."
Mr Atchinson's wife was "pretty cut up" and was planning a memorial service in the United States for her husband.
His New Zealand family was likely to hold their own memorial service at a later date, and some of his brother's ashes were likely to be returned to New Zealand, Terry Atchinson said.
"[Ellen] talked about that, he must've said he wanted some of his ashes were to come back here."
