Adopt Existing Standards and Lexicon
The outdoor advertising industry, the sign manufacturers, urban planning community, and especially the visual pollution watchdogs have already established an extensive number of taxonomies, lexicons, terminology, etc. which describe every aspect of 'Street Graphics' ( of which advertising is only one category, and a fuzzy one at that ). It might seem overkill now, but every other tagging scheme in OSM has evolved in a haphazard manner as people want to add progressively more refinement. A well done hierarchy would allow simple high level task and leave lower levels for those that want the detail. It would also give the data some value to those outside communities because there would be a higher probability there was standard meanings that somewhat matched what the real world uses.