The victim, who was driving a Toyota Verso, was travelling from Northampton Road in Roade village along the A508 towards Northampton, when the driver in the van behind allegedly began sounding the horn and flashing lights, tailgating and driving erratically.
When the Toyota driver stopped in a layby to allow the van to pass, the van driver also stopped, got out and assaulted the Toyota driver by slamming the car door onto his legs. He then pushed him into the car, took the keys, got back into his van and drove off.
The incident happened on Thursday, October 6, between 1.50pm and 2.10pm.
The van driver is described as white, with a stocky build, short, light brown receding hair. He was wearing a dark-coloured polo shirt and grey combat-style shorts. The van was white and there was a woman in the passenger seat.
Witnesses or anyone with information are asked to contact Northamptonshire Police on 101. Officers are appealing for witnesses and would particularly like to speak to the driver of a Honda that stopped nearby and may have witnessed the incident.