Two suspects were arrested in connection with the incident last week, and police want to find the member of the public, as they may be able to assist with their inquiries.
Around 5pm on Monday November 12, a man was driving his BMW car and turned right from the A1237 onto Boroughbridge Road, before coming to a stop due to traffic.
A man got out of a Renault car that stopped directly behind the BMW. This man opened the door of the BMW, and there was a brief verbal exchange, before he hit the BMW driver.
A physical altercation followed, at which point the member of the public intervened trying to separate the two men.
The female driver of the Renault car then got out of the car and spat at the BMW driver.
Following inquiries, a man and a woman were arrested last Friday. They have been released under investigation while enquiries continue.
Contact Police Investigator Katie Swift on 101, selecting option 2 and stating collar number 6437, quoting incident reference 12180211921.