Crash investigators believe the man, from Wellingborough, may have opened the passenger door of a Vauxhall Grandland travelling westbound on Nene Valley Way.
The carriageway was closed both ways for around four hours from just before 4pm while emergency services dealt with the incident.
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A spokesman for Northamptonshire Police confirmed no other vehicles were involved, adding: “The front seat passenger of a blue Vauxhall Grandland car is believed to have opened the door of the moving vehicle and fallen on to the carriageway between between Riverside and Barnes Meadow.
“The passenger was taken to University Hospital Coventry. His injuries are serious, although not believed to be life-threatening or life-changing at present.”
Anyone with information including dashcam footage which could help police can call 101 using incident number 22000416633.