Outdoor advertising is classified in two categories – mobile or stationary.  Mobile outdoor advertising refers to any signs mounted on buses, cabs, and other vehicles that are delivering a message about an event, a person, and/or a place.  Conversely, examples of stationary outdoor advertising include banners, billboards, and/or yard signs.  Since we are discussing cost-effectiveness, you can eliminate billboards unless, of course, you have the budget for it.