I've seen signs (like the one below in Southern Tyrol) in street view of "Radroute" bicycle paths around Germany but haven't been able to find any information in English. The signpost below seems like it is part of a decent bicycle path going for at least 50km. What is Radroute?
"Radroute" is just a generic term pointing to a major way for bicycles.
In this place near Reschen (South Tyrol) it most likely points towards a stretch of Via Claudia Augusta that follows the valley of the Etsch river towards Meran: http://www.viaclaudia.org/en/introduction.html
Another map with the route can be found here: https://cycling.waymarkedtrails.org/#route?id=3879552&map=11!46.7027!10.9357