Arthur Ravenel Bridge

Are there any laws about candidates posting their signs along the roadway?

Yes, there are several state laws addressing political signs on roadways, as well as county and municipal ordinances.  See SC Code of Laws Sections 57-25-10, 57-25-140, and 7-25-210.  Ultimately, it is the responsibility of the entity that maintains the road (state, county, and municipality) to enforce applicable sign laws.