Who is the authority on whether a line qualifies as BRT service or not? It is a global standard, or do we leave some leeway for local pecularities? Is the purpose of this tag for users to merely identify self-styled BRT services, or for data consumers to automatically treat these services differently?
ITDP publishes a Bus Rapid Transit Standard and scores existing systems on a 100-point scale. This is probably as rigorous as it gets. However, sort of like the various walkability scores out there, I wonder whether this information would belong in OSM. It’s not as though the lack of a tag has stopped anyone from compiling these scores externally.
@Nate_Wessel once created this transit map of my hometown, which focuses on frequency over branding – hard facts, classically OSM. Even if the city were to introduce a “BRT” service, this map would treat it like any other, by design. The various characteristics associated with BRT are mostly intended to improve reliability and frequency, so if those characteristics prove to be a success, that success would be apparent on this kind of map.
On the other hand, maybe there is a desire to record different relative classes of service as designated by each operator. service=* seems like a good place for that, even as we lament the continued overloading of that key. Just don’t tag it on ways, because then we’d have to call it service=busway all over again. 