I'd edit again, but it's too late. This question kept nagging at me. I figured this must be a studied phenomenon.

https://en.wikipedia.org/wiki/Fundamental_diagram_of_traffic...

This is super interesting! As kind of expected, there's a middle zone that balances out density and velocity. In the context of what we were talking about, the fourth "Basic Statement" jumps out at me: "If one of the vehicles brakes in unstable flow regime the flow will collapse."