A man was rushed to hospital with a suspected neck injury after a three-vehicle smash closed the northbound M1 between Junctions 8 and 9.
Early reports to police suggest that debris was scattered across the highway after one of the vehicles collided with the central reservation.
Police and paramedics were called to the smash, about a mile south of the Junction 9 turn-off for Redbourn, just after 1.05pm on Tuesday.
Police spokesman Dan Phillips said a silver Ford Focus, a blue Volvo V70 and a white lorry were involved in the smash.
The motorway was reopened at 2pm.