In this video, a guy explains the root cause of all traffic congestion. Obviously, road work and accidents are a main cause, but the video shows how sometimes you get stuck in traffic for no explainable reason. Apparently, traffic circles are much more efficient than stop signs and traffic lights, but the US has been slow to adopt them.
There are some other good tidbits, for example, reducing the number of total cars driving on roads in Boston by just 1% can reduce a typical commute by 18%. That means increasing public transportation can have a significant impact on traffic. Self-driving cars might also reduce traffic by taking the human element out of following the rules of the road.
Basically, the three things everyone can try to do to reduce traffic is to stop looking at your phone while driving, keep pace with the car ahead of you, and follow traffic signs.