What statutes govern the directing of traffic by citizens in Massachusetts? Are there laws or statutes governing the directing of traffic by citizens or "members of the public"?

A scenario I often encounter is one in which someone — typically another rider or pedestrian in a group that has stopped or slowed at road crossing — will signal an approaching driver to proceed across the flow of bicycle traffic. I realize that people who do this are trying to be helpful or courteous to the drivers, but they do so on behalf of others with whom they have not consulted and who they may not be aware of. 

In particular, I'm wondering what the laws are in Massachusetts, though any information about New York City, London, Seattle and Vienna or other areas would be welcome.

Is someone who does this liable in any way for any accidents that might ensue? Are they legally permitted to direct traffic in this way? Do they assume any responsibility for others who may become involved in the consequences of their "traffic direction" (given that those others generally have no idea that such directions have been given)?
Yesterday afternoon during Friday rush hour, I came upon a 2 car head on collision at a blind curve in my area. I am a hospital employee and have training in emergency response so I stopped to help when I saw a young man clutching his eye. 

I put on my poncho and did my best to make clear signals to drivers. All the participants were physically fine, but there was a heavy rain and if I hadn't acted, I suspect there would have been a 2 hour line of traffic at best or a series of further collisions at worst. There were no problems, and I left when the police arrived. 

As I was leaving, a police officer thanked me but cautioned me that it was a legal liability. My thought on the matter is that if an accident had occurred, it would be because drivers ignored my signaling. So who knows who would be in trouble?