My personal experience is that the public transit buses are the most predictable and law-abiding vehicles on the road. Of course I'm sure it varies greatly from location to location. I've come to trust that the MBTA buses will always signal when they're done picking up passengers and are merging back in with the traffic. I feel pretty safe passing on the left because they are so consistent with this.

Of course I've probably just jinxed myself now and will get creamed by the next bus I try to pass.