I find this unnerving. Mainly because you don't EXPECT people in > those places!
>
> How widespread is this, elsewhere?
All lane filtering/lane splitting is Illegal in Massachusetts, not that it matters. The ~25% of motorcyclists who aren't nuts don't do it and the other 75% seem to pretty much do whatever they want, anyway.
I can see why it's legal out West the lanes are huge, the roads are straight and laid out in a sensible way, probably actually helps with biker safety, depending.
But in compact cities with tight winding roads like Boston, Providence, and Worcester it seems like a good way to die. Sort of like how riding any kind of two wheel vehicle in those cities seems to me. I've never seen a motorcyclist pulled over that I can recall, the police here seem to take a let-God-sort-them-out approach to bikers and traffic laws.