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.