17 May 2016 Last updated at 08:45 BST
Matthew Daley, 35, killed Donald Lock on the A24 near Worthing in July 2016 after the 79-year-old ran into the back of his vehicle.
Daley, who was convicted of manslaughter, told how he had attacked Mr Lock on "auto-pilot".
[Warning: Some viewers may find the description in this clip disturbing]
A man who stabbed a retired solicitor 39 times after a crash between their cars described the attack in interviews with detectives.