I'm all for this kind of citizen action. They're volunteering their own time to improve public safety. Kind of like how neighborhood watch groups can supplement police, these volunteers are supplementing departments of transportation. Of course it'd be even better if they could do it without breaking the law, but I doubt there are rules in place to permit citizens to do their own DIY infrastructure improvements.

Also, unlike what some commenters think, this doesn't really go against democracy. The cities where these groups work have embraced Vision Zero at least on paper, so they're generally for these sorts of improvements. Government just tends to move slowly, and they often have to deal with NIMBY groups that are more concerned with keeping as many parking spaces as possible in their neighborhood than public safety.