The two drivers were heading southbound on Sunday driving dangerously, the police added.
They stopped at the Bombarral tolls on the A8 and got out of their vehicles.
One of them went to attack the other, but the son of the second man stood in front and was stabbed in the chest.
The attacker fled and the victim was taken to hospital with non-life-threatening injuries.
The attacker was later found and arrested.