I think 'traffic_sign' is totally correct here. In most legislatures "traffic signs" are not only the metal plates on poles, but any type of road marking are defined in the same part of law and also named traffic signs. 

I'd suggest to use 'traffic_sign' and in addition some kind of 'location' tag specifying its position and type, e.g. 'on_street'. In this particular case this is not even necessary, because HK:604 already is the code of a sign that always is painted and not on a physical sign.


I strongly recommend not to use road_marking:id. First of all, this resembles the syntax for tags in Indonesian language and second it seems to be based on 'traffic_sign:id' which was recently proposed and heavily downvoted.