The CGP Grey video on Traffic is a decent explainer on how traffic happens to begin with and how it gets relieved, kind of like a traffic snake that grows and shrinks, travelling in the direction opposite to traffic.
This City Nerd Video explains how traffic gets exponentially worse as it increases.
It usually starts with someone making a dumb move at a merge, changing lanes or another person forgetting to brake until the very last moment. That’s part of the reason I don’t see much benefit in adding more lanes to a highway save for very few exceptions, since you’ll just have more changing lanes leading to slowdowns and extending a section that was a bottleneck often just shifts the bottleneck somewhere else.
So anyways, I’ll keep preaching to the choir: trains, trains, we need more buses and trains.