[Russ_Boston]

I could be wrong about MA rules. Perhaps I'm only thinking about the Blackstone River path signs (in RI) since that's the one I use 80% of the time.

I think the big difference, after doing some research, is whether or not there are motorized vechicles on the path. If it's just bikes and walkers then same side is fine. But when you have cars, carts, motor bikes etc then my research indicates that all jurisdictions and safety brochures strongly advise pedestrians on left, bikes and motorized vechicles on right. And there are many accident stats to back up their research.