If your vehicle is traveling 55 miles an hour how fast will the unbelted occupant will still be going at he moment of impact?

55 mph. If the car stops (impact) people inside are still moving at 55 mph. They will hit the inside of the car- at 55 mph. That has the same effect as if you were standing still, and were hit by a car moving 55 mph. Messy. Wear your seatbelt.