On the fourth leg of our road trip, we figure out where traffic comes from and what it would take to make it finally go away. We learn how far back in history traffic jams were happening (spoiler: very far) and how "phantom jams" occur. We visit a room deep underground Los Angeles, the traffic capital of the US, where engineers are trying to ease the city's traffic woes by synchronizing traffic lights. Finally, we explore how, if ever, we can make traffic jams disappear. Are self-driving cars the answer?