You get the horizontal distortion when the camera's framerate and the motion of the subject aren't exactly in sync.
- If the subject is moving faster than one pixel-width per frame-time-length, then it'll be horizontally compressed, as in the vehicles shown in the first post.
- If the subject is moving exactly one pixel-width per frame-time-length, then it'll not be compressed or stretched, as in the walking people in the first photo.
- If the subject is moving slower than one pixel-width per frame-time-length, then it'll be horizontally stretched.... as the subject lingers in the slit's field of view, the slit will capture the same part of the subject over and over, and they'll be stretched horizontally.
Regarding the direction of travel:
If the subject is moving away from the sun, the shadow will enter the slit's field of view before the subject. If the subject is moving toward the sun, the subject will enter the slit before the shadow. We can tell that we have some cars moving left-to-right, and some cars moving right-to-left, but without knowing the relative location of the sun, we can't identify which set is moving left-to-right, and which set is moving right-to-left.
... however, if the photos were taken in America, we know that the cars in the far lane were moving right-to-left, and the cars in the near lane were moving left-to-right.
Martin, thanks for posting, this is a great discussion. Give the ol' noggin a workout.