Marc Bell, 38, of Gateshead, said his Scania lorry started to slide on ice as soon as he touched the brakes.
He said he was trying to slow down because he was afraid the driver of the car would walk out in front of him.
Mr Bell denies causing the death of Dr Mahmoud Abdel-Fattah, of Dunfermline, by dangerous driving in January 2010.
The doctor had earlier skidded into a snow bank and was in his car phoning for help when Mr Bell's lorry went out of control on the ice-covered road and hit his car.
Mr Bell told the High Court in Livingston how he was trying to slow down when he saw what he thought was a gritting lorry with flashing lights ahead of him.
When he braked, his 44-tonne vehicle headed towards the lights of what in fact was a breakdown truck attending the doctor's car.
He tried to correct the skid, but the trailer pushed his tractor unit sideways and he hit the stationary car.
Mr Bell stated in his evidence that when he hit his brakes, his vehicle just seemed to shoot off, and he could not do anything.
"I shot back in the chair and I was just sliding," he told the court.
"As I was looking up it was heading for the salt wagon and I tried to bring the trailer back in line."
It was only after the truck came to a halt that he realised Dr Abdel-Fattah had been sheltering from the sub-zero temperatures in his car.
The trial later heard how Mr Bell had set the cruise control on his lorry to a steady 52mph (84km/h) even though conditions were "treacherous".
Other motorists had cut their speed down to 20mph (32km/h) or 30mph (48km/h).
Mr Bell told the court the setting was not excessive because he was driving up a gradient at the time.
He said there was "no chance" of the lorry reaching a speed faster than the 50mph (80km/h) limit going up the hill to the road junction at Tower Farm in the Borders where the accident happened.
He admitted that tachograph evidence proved that he had been travelling at 46mph (74km/h) when he braked as he approached the doctor's car.
However, he told the court there was nothing warning of ice on the road and he thought the route had just been treated with salt.
