I found the Loading figure to be unfortunately a bit too vague. I would love to see more of the
kind of information provided by Vancouver TransLink, although it's
not perfect either. TransLink also measures bus pass-ups, which is something we really ought to do too, as it's a surefire indicator of capacity shortage (either because of bunching or just raw geometry) and a really obnoxious customer service failure.
Here's another
very thorough critique of performance indicators (KPIs in the Queen's English, apparently).
In a way, TransLink's 99 route is like the Silver Line, in that it is extremely busy (#1 in US/Canada) and should be rail rapid transit. I would say that the Silver Line's main problem right now is that it gets stuck in Chinatown, and it needs all-door boarding desperately. Once they get it up to the level of NYC's Select Bus Service, and it still can't handle that, then it's time to think about really pushing for a Tremont Street subway link and light rail. Arguably they should do that anyway, because it's already built, but I think maxing out the "BRT" would be a pretty strong boost to the argument.