Henry Bett, 26, of Hall Lane, Thornham, Norfolk, was driving a tractor involved in a collision with a Fiat car driven by Rebecca Brown, 43, who was killed.
Mr Bett, a farmer and the son of Norfolk PCC Stephen Bett, denies causing death by dangerous driving.
The trial at Peterborough Crown Court, sitting in Huntingdon, continues.
Mrs Brown, of Castle Acre, Norfolk, died when the Fiat people carrier she was driving was crushed in the collision with Mr Bett's Fendt tractor on the West Acre Road at about 15:10 GMT on 4 December 2013.
Mr Bett described rushing to the car to find Mrs Brown "catastrophically" injured and her son, who had been in the passenger seat of the car, shouting and trying to help her.
Asked how he felt about being involved in a fatal collision, he said: "You can't put it into words. Horrifying."
But he added: "I do not believe I was driving dangerously."
Police found evidence the defendant had previously taken cocaine.
Asked by Lawrence Bruce, defending, if he had been under the influence of cocaine or its after effects at the time of the incident, he said: "No."
The maximum speed of the tractor was 40mph (64km/h), but on public roads it is illegal to drive tractors above 20mph (32km/h).
Mr Bett said he had set the vehicle's speed limiter to comply with the law.
Asked why this was at the maximum level when inspected by officers, he said: "I didn't set it at that level."
He explained in interview that the dial must have been pushed forward as he was thrown on to the dashboard in the collision.
The trial, due to end next week, will be heard by 11 jurors after one member was discharged on Tuesday when concerns were raised about her conduct.
The son of a police and crime commissioner (PCC) accused of killing a woman in a road crash has told a court he was horrified by the crash.