Answer
A vehicle in those circumstances not only bounces up and down, but rotates about an axis near its CM. The CM is generally near the center of a vehicle.
The farther one sits from the rotational axis, the greater the tangential velocity (similar to sitting near or far from the fulcrum of a seesaw), as discussed on page 134. Thus, sitting near the middle of the vehicle, near the CM, provides a relatively smoother ride.
Life pro tip: if you are prone to seasickness, having a cabin near the center on your cruise ship might make you happier.
