Should dogs be allowed on buses?

That is the question being put in this morning's paper. 
Middle Cat took time to take a friend's dog to the vet recently. The friend is not well and is unable to drive. If she had been able to take the dog on the bus she might have been able to go by herself.
Now of course you can take assistance dogs on public transport. Guide dogs and other assistance dogs are supposed to be welcome on public transport. Someone I know can get himself to work because he can take his assistance dog on the train. His dog is trained to detect a medical condition. There have been occasional issues but the dog wears a special coat and the man in question wears a medical alert button. I suspect that most of the train drivers know him.
But what about people who simply want to take their dogs with them for a day at the beach or to help with child minding the grandchildren?
I can see arguments for and against allowing dogs and other pets on public transport. Guide dogs and other assistance dogs go through rigorous training. They are taught to use public transport and to behave in all sorts of situations. It is very, very rare for any sort of assistance dog to be abused by the person it is there to assist. The cost of providing such dogs is so high that nobody wants to risk that.
Other dogs might be trained. They might go to "obedience school" or be given "good manners" training at home. It isn't the same sort of training as assistance dogs are given. Will they always behave? What do you do if someone deliberately goads the dog or the dog attacks someone because their instinct says their human is in danger?
Some time ago I had an interesting experience when a dog I know very well came rushing towards me in a very threatening manner. I froze - and the dog went past me and stopped a man. The dog's owner and I decided that the dog somehow felt I was in danger. The man who was stopped was not happy about it although he did eventually agree that his behaviour had at least been inappropriate. Much as I am grateful that the dog was apparently willing to protect me I did not enjoy the experience. That dog has been to obedience school and is normally extremely well behaved. It is the sort of dog who sits down at each corner on a walk - without being told to do so. What would have happened though if the man  with the inappropriate behaviour and the dog had been on a bus?
Perhaps if you want to take your dog on a bus your dog needs to have reached a level of behaviour which allows it to wear a coat that, like an assistance dog, proclaims, "I am trained - but be wary of behaving badly in front of me"?