Advertising in my opinion has a UX problem, it is however inextricable from a capitalist society. It needs to look great, it needs to be consistent and it should respect user consent and privacy. That's pretty much it.
Billboards don't invade your privacy or require consent, so they just need to look modern/great and be placed in areas where they won't disrupt natural and residential scenery. Unlike web-ads, they're more or less consistent but like i said earlier, I wish they were more "cool" like in scifi movies.