Four Asian men in a Ford Focus on the M1 in Northamptonshire were flashed by the driver of a Rover 200 and had missiles thrown at them.
The Rover then stopped alongside three other vehicles blocking the northbound carriageway near junction 15.
The Ford Focus driver and his front-seat passenger were assaulted and received minor injuries, police said.
A Northamptonshire Police spokesman said they believe about 15 black men, some armed with knives, baseball bats and bottles, were involved in the attack which was described as "unprovoked".
Isolated incident
Five of the men were wearing black clothing and gold jewellery.
Police believe the cars - an Audi A3, a Toyota Prius and possibly a red Audi A3 - involved in the incident which happened at about 1300 BST on Monday, may have joined the motorway in London.
Police want to talk to anyone who saw "incidents" on the M1 through Hertfordshire, Bedfordshire or Buckinghamshire before the attack.
A police spokesman said: "This is an unusual incident and has been reported to us as an unprovoked attack on this car and its occupants.
"We are following several lines of inquiry to trace these attackers but this appears to be an isolated incident and we are confident that there is no risk to other motorists from this group of offenders."
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