The Thai speed limit regulation defines a ทํางขนําน as
So, for example the secondary road here: https://www.openstreetmap.org/#map=16/14.3938/100.5916
For these kinds of roads, the same default speed limit (if nothing else is signed) applies as for urban roads, regardless if they are in an urban area or not. (see http://www.ratchakitcha.soc.go.th/DATA/PDF/2564/A/077/T_0001.PDF → ข้อ ๗ (Article 7))
Question: Is there any way to designate such a road as a ทํางขนําน? E.g. via *designation=**, or maybe the guideline is to tag them always with zone:maxspeed=TH:urban regardless of whether they are actually in an urban area?