Just a quick ‘2 pare’ :wink: :

Seems there is precedent for alpha-2 country code here [1] (uppercase please as prescribed by ISO 3166), and also to keep with the ‘official’ classification [2] maybe we should choose something like

RS:class_I
RS:class_II

although the ‘popular’ classification

RS:national
RS:regional

will work just as well…

[1] http://wiki.openstreetmap.org/wiki/Route#Road_routes
[2] http://en.wikipedia.org/wiki/Highways_in_Serbia