It's a scarce good, which loses most of its value if overused.

The standard Econ solution is to set a price that maximizes throughput. At least some toll roads are attempting to hit that price.

In the US, toll roads are required by law to maintain a certain minimum speed. The price is set based on that.

What law?

I suppose "regulation" would be more accurate, and it only applies to HOV lanes that receive funding from a specific source:

https://ops.fhwa.dot.gov/freewaymgmt/hovguidance/chapter4.ht...

This covers pretty much all of the lanes that people complain about so I'll stand by what I said.

That's interesting, but also a significantly narrower category than all toll roads.

Sure, but if the complaint is "this was a public road and now you're adding a toll lane" then this is what applies.

On this note, the "rich people super freeways" model actually does exist and works quite well, when implemented as a totally separate tolled highway that runs parallel to the toll-free one. See: https://en.wikipedia.org/wiki/Ontario_Highway_407