Lines drawn to emphasize the velocity of a person or object.
When used directly on objects, esp. in the foreground, these may be accompanied by motion blur effects.
When used in (or in lieu of) the background, speed lines indicate that the persons or objects in the foreground are moving past the background at high speed.
Not to be confused with emphasis lines, which do not indicate motion, but rather are used dramatically to draw attention to a specific part of the image.