How does it solve traffic/congestion issue? Because they drive in a way that is more efficient than human-driven cars?

I envision many years or decades where having both self-driving and human-driven cars on the road at the same time is going to be a nightmare for everyone...

Also humans regularly drive slightly over the speed limit (at the flow of traffic). I wonder if self-driving cars would be programmed to drive exactly at the speed limit, which means they'd actually be slower.