Michael Johnson, 50, of Tiverton, Devon knifed director Carl Watson in the chest after their cars clipped each other, Truro Crown Court was told.
The attack happened after both men pulled into a disused quarry entrance, the jury heard.
Mr Johnson, of Redvers Way, denies stabbing Mr Watson, 42, on the A30 near Launceston in Cornwall on 2 June.
Mr Watson was airlifted to hospital with a single stab injury to the chest.
A surgeon who treated him at Derriford Hospital in Plymouth, said the 12cm (4.7in) deep wound came within 5mm (0.2in) of his heart.
Mr Watson was discharged from hospital the next day.
He told the court he had been returning from a business trip in north Devon when a car driven by Mr Johnson pulled out to overtake but clipped his vehicle.
He said the vehicles then pulled over and he was "calm", but as the men exchanged insurance details the attack happened.
The case continues.
A driver was stabbed within millimetres of his heart in a road rage row, a court has heard.