Sky crossing

While airplanes are required to keep many miles from one another at the same altitude, they can cross paths even quite closely as long as they are at least 1000 vertical feet apart.  For the observer in the airplane, this means that you sometimes get treated to the sudden appearance of a flight blasting away from under or over you at a tremendous rate of speed, reminding you just how fast you're really moving to create that slow visual crawl of the ground so far below.