I agree its an awesome idea, we have a group called Active Adventure dogs, owners and their dogs go on walks, hikes and activities and the guy who leads it is a trainer and he employs this. Dogs that are anxious or aggressive or just not quite social yet wear yellow bandannas and so all other owners and dogs give the yellow wearer a wide berth and no approuching that dog unless permission is granted from the owner. Its the only time I've seen this done though. I wish regular strangers would do this so we know whether to cross the street or step off the path. Though I guess a simple "Is your dog friendly?" would suffice.