BryCowboy -- so how do you propose I tell someone behind me that I'm slowing down?
As Vidaloco indicated, coming up from behind at a canter is dangerous because the front rider can't see what's behind them, and people going into a canter without asking others if it's OK or going out of the group to do so are also out of line. I still don't agree though that as the lead horse I need to somehow tell everyone I'm going to stop now. For the most part it's simply because I don't get the logistics of how to actually do it.
Your point about the signals on cars is well taken, but downhill skiing, skating, x-country skiing, and probably lots of other things I just can't think of right now just don't have those signals and the rule always is: Leave enough room between you and the person in front of you. You are responsible for being able to safely avoid anything in front of you. It's actually one of the formal written safety rules of skiing. Another rule I've heard and used in sports is when overtaking or approaching to warn the other person that you are in their area and which side you are coming up on. These are the guidelines that we use in my area when trail riding.
And, you know, even with cars, we can flash our brake lights and signal all we want, if the person behind hasn't allowed enough room, it doesn't make any difference at all how many signals we use. So the responsibility in the end still comes to the person in the back. Besides, what if something happens that the leader can't control -- like a kid walks across the street or a bear crosses the trail? There is no time to warn -- stop action must be taken immediately and the followers had better get their act together just as quickly.
Yes, horses are herd animals and we had better respect that, both to use to our advantage and to be aware of expected behaviors. But I have also asked people to not be so close to my horse and they have been very able to manipulate their horse away from mine. If someone can't keep their horse a reasonable distance away on a group ride, then they shouldn't be there, IMO.