Officers were called to the M5 slipway at junction 21 at Weston-super-mare at 10.10pm on Friday to reports of a 26-year-old man having being stabbed.
Avon and somerset police said the man “suffered stab injuries” after a “dispute” with another driver.
The victim was taken to hospital with police adding his injuries are “not life-threatening”.
Avon and Somerset Police said in a statement: “We’ve launched an investigation after a man was seriously assaulted on the southbound slip road of the M5 at junction 21 (Weston-super-Mare) on Friday night.
“At about 10.10pm, we received a call from the ambulance service to attend this location after a 26-year-old man suffered stab injuries.
“He was taken to hospital for treatment to injuries which are not life-threatening.
“The victim was a passenger in a car which had just exited the M5 at this junction and was at a set of traffic lights.
“A dispute then ensued with the driver of another car, following which the victim was stabbed.
“The offender was in a dark coloured car and we’re making urgent enquiries to locate this vehicle.
“One lane of the slip road remains closed so we can carry out a further examination of the scene.
If you saw any part of this incident and haven’t yet spoken to the police, please contact us on 101 and give the call handler the reference number 5220152683.
The exit slip road on the M5 remains closed as police launch an investigation into the stabbing.
Highways England has warned “congestion may build throughout the day” as a result of the closure.
The authority tweeted: “#M5 Southbound EXIT slip at J21 #WestonSuperMare, There is a lane 1 closure in place on the slip road for a @ASPolice investigation.
“Congestion may build throughout the day here as a result. Pass with caution.”
In a further tweet, Highways england warned the closure of the exit slip road is likely to remain in place throughout this morning.
They said: “#M5 J21 Southbound exit slip for #WestonSuperMare there is one lane closed on the slip road for an @ASPolice incident.
“This lane closure is likely to remain throughout the morning.”
This is a breaking story. More to follow…