On the highways or in the cities, sometimes traffic jams extend as far as the eye can see. You snail along at a glacial pace until all of a sudden traffic just clears up for no reason. Self-driving cars aim to fix that.
Traffic usually is the result of human reaction time. The “accordion effect”—where one car slows down, and the car behind that slows down, which causes all the cars behind them to slow down—is the cause of most slowdowns.