I agree that it should be a singular key. I have explained my concept in the other thread already. I propose moving the importance-based values into importance=* while all of the highway=* values would depend on functionality, meaning there would be highway=expressway. I don’t want to stop here and I’d introduce more values such as arterial, collector, local, which will correspond to different sets of parameters of a road. I have explained the concept more throughly here. This of course cannot be implemented right away so another option would be highway=road + road=motorway/expressway/etc. (at first without highway=road).