I agree that break back traps are more humane than poison, purely because it is generally instant death. The problem is, rats are not easily caught in traps, unlike mice. Rats are a lot more wary and it requires knowledge and experience to eradicate rats by trapping.
Despite modern myths, rats do not suffer agony when poisoned, modern poisons have got to be humane, most modern poisons are anticoagulants, when a rat eats it they bleed internally and will usually die from bleeding to death, whilst it sounds very unpleasant it actually isn't painful. Not pleasant im sure but certainly far more humane than things like glue boards!