The Collaborative International Dictionary of English v.0.48:

Signboard \Sign"board`\, n.
   A board, placed on or before a shop, office, etc., on which
   ssome notice is given, as the name of a firm, of a business,
   or the like.
   [1913 Webster]

WordNet (r) 3.0 (2006):

signboard
    n 1: structure displaying a board on which advertisements can be
         posted; "the highway was lined with signboards" [syn:
         signboard, sign]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "signboard":
   arrow, blaze, compass needle, direction, direction post,
   finger post, fist, guide, guideboard, guidepost, hand, hour hand,
   index, index finger, lead, lubber line, milepost, minute hand,
   needle, pointer, signpost