Thanks for your input Noen! I was not aware that the highway=busway tag was fro Bus rapid transit systems. I will change the road segments to «highway=tertiary» + «motor_vehicle=no» + «bus=yes» as suggested

I changed these roads to "highway=busway", as only buses in route are allowed. Busway does not show in OSM Carto however. Therefore i wonder if it is better to set these ways back to "highway=tertiary" and use tags "access=no" and "bus=designated".