After reading through all replies I want to say thank you for the fruitful discussion. I see some disagreement, and maybe it is too big to reach the 75% mark to reach a consensus. But maybe it is not.
I think the frequency of a bus line can be mapped with interval.
What does not — to my knowledge — have a tagging in OSM is the speed of the vehicles (buses)**. I dont mean maxspeed, but average speed including (scheduled) stops, stops at traffic signals and stops/slower movement due to other traffic (mainly cars).
If regular stops are short (few seconds) and traffic lights and (car) traffic do not slow down, then the bus has a special quality which I would like to be able to map.
I am not sure, how this quality shall be named. Maybe the native speakers can help. My ideas are
- undisturbed_vehicle_flow=yes
- independent_vehicle_flow=yes
- stopping_only_at_bus_stops=yes
- no_stopping_between_bus_stops=yes
**Bus only ways or lanes give a hint about it, but I know many buses with such lanes still being slow because they wait at red lights all the time. On the other hand, when a street has almost no car traffic, or car traffic is kept out of the buses’ way through intelligent traffic signals, a bus only way is not needed at some places to reach the quality.