A 25-year-old moped rider died after coming off his bike when he hit the central reservation on the A2 in Kent.
Police said a witness reported the man was hit by a lorry as he was lying in the road but it is believed the driver may not have realised what happened.
The coastbound carriageway of the A2 was shut for more than four hours after the accident, near Shelvin, on Friday.
Police checked lorries parked on the A2 and at the Port of Dover, but have been unable to trace the lorry involved.
It is believed the vehicle had a foreign registration.
Anyone who saw the accident at 1910 GMT or the moped beforehand is being asked to contact police.