http://leg1.state.va.us/cgi-bin/legp504.exe?000+cod+33.1-373
Posting unlicensed commercial signs is absolutely illegal, and the law applies to every road within the Commonwealth. Of course, enforcement is devolved to the County governments, and some of them are, shall we say, a bit more proactive about it than others.
Personally, I think we'd be better served if the Commonwealth would just work with the phone companies and automatically send people who litter the sides of streets with a bill for the "free" advertising.