Just saw this idea on facebook.
Posted by The German Shepherd Dog Community, the gist is in case the link does not work that you tie a yellow ribbon on your dog's lead if they need space
Maybe they have health issues or are training
They may be a rescue dog being rehabilitated
They may have had a bad experience with another dog or just not like the friendly kind of dog who always wants to say "Hi!"
Apparently there is something similar in the Horse community, a red ribbon on the tail for the same message.
This has come over from USA, the GSDC facebook page is from America, but I think it is an excellent idea that would really benefit from a wider audience. They'd like this to become a world wide thing I think.