The Amateur Planner proposes a paved transit way with tracks for not just the B Line but the 57 and BU bus routes.
While we've never done this, other cities have, and it would have a whole host of advantages, including getting buses out of traffic (and bike lanes), stations better able to serve people with disabilities, even some additional parking spaces. Throw in the long-fabled transit priority for traffic signals and you might even see shorter commute times.
It is one of the most heavily-traveled bicycle corridors in the city. Yet we are planning for cars - minority users of the corridor - first, when we should be planning for transit first (by far the largest user of the corridor by the number of passengers carried), then bicyclists and pedestrians. Cars should be an afterthought, put in to the plans after other users have been accommodated, not before.