I found several reference tags in Turkish roads contradicting each other. Unfortunately the road signs are also not consistent. For example I have seen “D-100”, “D_100”, “D 100” and “D100” used for the same road.
At least for motorways (otoban) I have seen consistent use of “O-1” etc.
I couldn’t find a tagging guideline on the https://wiki.openstreetmap.org/wiki/WikiProject_Turkey page.
What’s the recommended tagging practice for road reference numbers in Turkey?
Following the “map what’s on the ground” (==road signs) rule of OSM we should actually use the “D.100” and “O.3”, but it feels like the dash notation for motorways and no space (since non breaking spaces are very hard to tell apart from normal space) for state roads.
I would suggest to document something on the OSM wiki Turkey project page.