well many will be stuck up illegally, and I think that can also reflect your business. MLM uses traffic lights and the cheap and cheerful promo does it not help it's reputation.
It depends what you have in mind. If you want to use gates and railing you need permission form the owner who should know, if they do it regularly enough, if they have council permission.
Did you know that you cannot legally hang flags and banners from your bedroom windows such as England flags for World Cup time? Thought they dont do anything about it (they would lose at the next election if they did!).
If you put vinyl banners up on fences, bridges etc the local council will remove them. Usually they just hold them for you to collect. You can get away with pleading innocence on the first go, but that's all I would think!