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?

Chinese motorists get out of their cars
Chinese motorists get out of their cars to check on a massive traffic jam at a toll booth on the outskirts of Beijing on October 2, 2010. The number of vehicles on roads in the capital -- which currently stands at around 4.5 million -- is expected to hit five million by the end of 2010, while in August, a traffic jam stretching more than 100 kilometres (60 miles), consisting mainly of trucks, was reported to have been at a standstill for over a week on a highway linking the capital with Inner Mongolia.
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Los Angeles Institutes Traffic Surveillance Mechanisms
LOS ANGELES, CA - OCTOBER 16: Computer monitors showing the traffic flow of intersections help traffic engineers manage traffic problems from inside the Los Angeles Department of Transportation's Automated Traffic Surveillance and Control Center (ATSAC) where a press conference was held to announce a $150 million effort today to synchronize thousands of traffic signals across the city on October 16, 2007 in Los Angeles, California. The project was funded by proposition 1B, proposed by the Governor and approved by voters last year, and is projected to result in 20 percent faster travel times, reduced traffic congestion of 30 percent, and to lead to cleaner air by reducing the number of vehicles idling on city streets. Los Angeles has some of the worst traffic congestion in the nation.
David McNew/Getty Images