Oldie's question has not been aswered yet and I'm interested as well. Anyone?
I have the idea that when a motorway is normally set to 130 (or whatever) and the road is tagged as 100 (along cities and the like), MNF should calculate with that speed setting, and warn when overspeeding.
But I'd like to know, just like Oldie, whether that's correct or not.
As I know OSM roads have info about average speed but it is often incorrect. So we have road restrictions which overwrite OSM average speed with user defined speed for defined road category. But I do not know exact math formula for route calculation...