Shields of Denmark

[claysmalley]

Denmark has two numbered route networks of its own: primary and secondary. Routes with one or two digits are posted with a black-on-yellow rectangular "primary" shield. Routes with three digits are considered secondary, and have the same shield, but with a white background. There are also some ring roads, with an O prefix in their ref, belonging to both networks.

Shields of Denmark were initially supported, but removed in #702 (see #652). Before we can restore shield support, the Danish mapping community must come to a consensus on how to tag relations for primary and secondary roads.