A lorry driver was threatened with a knife after confronting the occupants of a van which swerved in front of his vehicle and almost caused him to crash.
Surrey Police said the "road rage" incident happened at traffic lights in Albert Road, Bagshot, on Thursday.
The articulated lorry nearly collided with a Mercedes car because of how the blue Renault van was being driven.
During an argument that followed, the passenger in the van pulled a knife on the male driver of the lorry.
The man with the knife was described as white, about 5ft 11in tall and of skinny build.
The van driver was also white, about 30 years old and of large build. He was wearing a hooded top.
They drove off in the direction of the M3 after the altercation.
Insp Mark Adams said: "This is a rare incident and we will not tolerate such anti-social behaviour on our roads."