At one point, lane splitting was allowed in Massachusetts, but I don't know if it still is anymore. In NH, VT, and Maine, it is strictly illegal. It doesn't mean that it doesn't happen, but if one were to do it and have an accident it would be automatically blamed on the biker. I don't do it myself - see a few near misses and encounters with car doors and vehicles changing lanes. Yes, I sit there on my bike, in the rain in stopped traffic...