In Washington state, the law is that signs along the highway have to be things you can actually purchase in the same property.

I think that rule helps strike a decent compromise: Adjacent local businesses can draw attention to themselves, but it blocks the business-plan of erecting a forest of billboards to auction off, flogging cell-phone providers or prescription drugs etc.

https://app.leg.wa.gov/rcw/default.aspx?cite=47.42.040