To add to this, there is literally a term "the Pittsburgh Left"[1]. Informal etiquette does really help prevent things like a lane getting blocked for three lights when someone needs to make a left at a busy intersection. However, humans definitely aren't perfect and they likely add a lot of congestion on their own from stuff like following to closely, not maintaining speed, etc.
I feel like this is going to be a huge problem if we have mixed human operated and self-driving vehicles on the road at the same time. If we ever move to entirely self driving cars we could move to some sort of protocol for cars to communicate about ambiguous self driving actions.
It's really common to see the Uber Volvo's driving around, but it definitely causes me to tense up when driving around them. I have no idea why, I realize there is significant risk when around any other car on the road. It's new. Hopefully these issues get worked out in a reasonable time frame, but it is software.