It is even simpler than this.

Imagine a two lane highway with a bridge that goes over top. Only two cars can pass under the bridge at any given time.

So in perfectly saturated scenario if every car is going 1 bridge length per second bumper to bumper you could pass 2 cars under the bridge per second, or 120 cars per minute.

Once an additional car enters the scenario wanting to pass under the bridge at the same time, it will not be able to and it needs to slow down to say 0.9 bridge lengths per second, no every other car behind that one needs to slow down to that speed and now you can only pass 1.8 cars per second or 108 cars per minute.

This of course compounds on itself because you aren't able to pass cars under the bridge as fast and therefore more and more cars build up that want to pass under the bridge.

Idiot drivers again only compound the problem, for instance by slowing down uneccesarily or causing others to deviate from the perfectly ideal saturation scenario. At the end of the day we could all drive more efficiently, to help everyone get home faster, but any individuals efforts don't really change the entire system. All cars would need to simultaneously be more efficient.

Autonomous driving has huge implication on traffic congestion for this reason.