Police are appealing for witnesses following an assault in an A14 lay-by next to the Catthorpe Interchange.
The incident happened between 4.45pm and 4.55pm on Thursday.
The victim was driving a DAF tractor unit. He was travelling off the M6 and joining the A14 when a white lorry overtook him, the driver sounding his horn as he did so.
A spokesman for Northampontonshire Police said: “Both lorries pulled off at the next lay-by and an altercation followed. The victim was verbally abused and the offender also spat in his face.
“Officers would like to speak to anybody who witnessed the incident, and saw the two men in some sort of altercation.”
Witnesses to this incident, or anyone with information, should call Northamptonshire Police on 101 - alternatively, information can be given anonymously to Crimestoppers on 0800 555 111.