Do people wear seatbelts to protect themselves from speed or acceleration?
Both. The acceleration in a crash is high because one or both vehicles had a high velocity to begin with.
Seat belts actually provide the force necessary to decelerate the body. It is better to have the force applied to the body by belts than to the head by a windshield.
I don't see what this question has to do with astronomy, and consider it a poor question. There is no simple "right" answer.