Highway signs are in a somewhat gray area. They are definitely not petroleum, auto service or gas station related, but they do show up at petroliana specialized swap meets. Some petro guys do collect and display them. I get a few complaints about highway signs in the petro For Sale forum, but there are other members who don't care.
Since you ask, we would prefer highway signs to be in the Non-petro For Sale or Showcase forums. That's the reason the Non-Petro For Sale forum was set up to enable trading all those "kind of related" things like soda & beer collectibles, classic car items, general store and gameroom items. Other roadside collectibles like motel & diner, highway signs and Route 66 memorabilia are all welcome there.