Police have identified the body found on the M20 as a 32-year-old woman from Kent.
A man arrested on suspicion of murder remains in custody, Kent Police said on Thursday afternoon.
Officers discovered the body on the London-bound carriageway near Ashford at 6.05pm on Tuesday. The road was closed between junctions 9 and 10 for nearly 24 hours while the scene was investigated.
The incident caused hours of delays with between six and seven miles of queues stretching back to junction 11 and a diversion was in place.
The road reopened shortly before 3pm on Wednesday.
The 21-year-old, who asked not to be named, said: “As I entered the motorway I expected to see a crash, when I suddenly realised that it wasn’t and unfortunately it appeared to be a body lying in the central lane with police and ambulance staff around it.
“Because of the torches being shone by police (I could see) it appeared that the body had been struck by vehicles.
“It was very shocking. It is not a sight I would want anyone to witness.
“My condolences are with the family of the deceased.”
Police are still appealing for drivers who were on the road at the time to check their vehicles for signs of a collision, and for anyone with dash cam footage or who saw the Nissan Qashqai with the registration KY15 WWX at the time of the incident to come forward.