David Mann is just what his name suggests: an everyman with a mediocre job who has trouble standing up for himself. While driving through the desert to an important appointment, he passes a slow-moving, rusty tanker truck. The driver proves to have a severe case of road rage and takes offense at this action, devoting the rest of his day purely to killing Mann. The malevolent driver is never seen, giving the impression that it's the truck itself that is the agressor.
This short story from Matheson was rewritten by him as the screenplay of the 1971 movie that launched Steven Spielberg's career.