Why do we say that traffic is relative?

According to the definition: mechanical movement is a change in the position of the body relative to other bodies. The body, with respect to which we consider the change in the position of the investigated body, is called the reference body.
With respect to different bodies, the investigated body can change its position or not. One and the same body relative to different bodies changes its position in different ways, or does not change its position at all.
The passenger is sitting in a flying plane. It does not change its position in relation to the plane, seat, passengers, that is, it does not move. And relative to the ground, clouds, bridges, it changes its position, and changes in different ways. He approaches some clouds, he recedes from other clouds.