As I understand it, “superroute” was originally the result of a miscommunication in the JOSM issue tracker. The superrelation documentation actually has always recommended nesting a route relation inside a route relation, but this is no longer clear because of the “superroute” page. Even so, that page also alludes to the fact that “superroute” relations are unsuitable for networks such as the USBRS, which is not structured like EuroVelo in the real world. (A USBR is still a USBR within each state.)

A route relation such as 8852715 ideally needs to contain one relation per state, which in turn contains one relation per signposted direction. This would be consistent with the standard for Interstate and U.S. Routes, which are also designated by AASHTO in the same manner. The claimed benefits of “superroute” relations would require the topmost relation to be something else like type=superduperroute…