Originally Posted by
reppans
Try
Strava Heat Maps which shows you route popularity/density among other [Strava] cyclists but keep in mind sheer rider volume might still lean toward major roads despite being less pleasant to ride. Then use google Street View to have a look at the actual road/shoulder layout, and maybe Satelitte View to gauge traffic (although highly dependent upon time of Sat. pix).
+1
Note that Germany and Austria don't allow street view, so some of the roads have to be taken on faith.
Once I have a rough outline, I contact people on warmshowers.org who live in the area I will be riding through and ask them about the roads I've chosen. Often, this results in offers of hospitality, as well.