I never thought about it but wind probably controls. A sign structure doesn't have much mass; and if it collapsed in an earthquake no one would care - as long as it didn't fall on someone.
If for some reason you had a strange situation with a high mass sign with low wind load you could probably run the numbers through ASCE 7 for a non-building structure. I agree with bridgebuster; even in a high seismic region it seems that wind would control by inspection.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
Is it possible for the arm of a cantilever VMS (Variable Message Sign) support structure under vertical earthquake movement being more critical than wind loading?
If an VMS support structure collapses it will fall onto the road and risk falling onto travelling vehicles.
Sure, that's entirely possible. I'd just do a rough double check using ±(0.2)(Sds)(Dead Load) added to your normal factored dead load and see if that controls. I wouldn't think this would be much more than the a factored dead load only load case.
Just keep in mind that signs can totally have a vertical response from a wind load:
In my opinion, Extreme Event analyses is driven by life safety (by preventing collapse) or immediate serviceability (by limiting damage) considerations. Signs and other ancillary structures are pretty low priority items when it comes to Extreme Event. Invariably the wind loads due to exposed area AND fatigue governs the strength and serviceability design. I have not come across an agency or an owner who mandates seismic analysis of these structures.
