John Campbell, 66, told officers investigating the accident which claimed the lives of John and Emma Grier that he had stopped at a red light then moved on when it changed.
Mr Campbell, of Glasgow, said it was only when a bus driver stopped him and said there had been an accident that he realised something was wrong.
A tape recording of an interview between Mr Campbell and police was played to a jury at Hamilton Sheriff Court where he is on trial.
He has denied causing the deaths of the Griers by driving without due care or attention.
The Griers, who were both 76 and from Motherwell, Lanarkshire, had been crossing a busy road when they were struck by the lorry in Hamilton in January 2011.
The court heard Mr Campbell tell police how he had been alerted to the incident.
He said: "The lights changed from red to green and I didn't see anybody crossing the road so I moved off. After that I don't know what happened.
"The bus next to me stopped me and I asked the driver what had happened. I thought that I had somehow clipped his wing mirror or something.
"He then told me about the two people and I went down to the road to see them."
The trial also heard from Police Scotland PC Ewan Thomson, 48, who said he believed the Griers would have been visible from the truck driver's position because the bus driver had claimed to have seen them.
The trial continues.