The 62-year-old victim was driving a white Nissan van eastbound on the A89 at Dechmont, in West Lothian, at about 11.30am on Tuesday when he was involved in a minor disagreement with the motorcyclist.
The motorcyclist, who had been travelling in the opposite direction, turned around to follow him, drawing up alongside at the junction with the B8046.
He struck the van’s wing mirror before punching the driver in the head and shouting abuse. He then sped off westbound on the A89.
The suspect is described as white, tanned, heavily built, 5ft 9in tall, with a local accent. He was wearing an open-faced scrambler style helmet. The bike is described as a scrambler-style off-road dirt bike, with a registration including the numbers five and two.