My question involves real estate located in the State of: Pennsylvania

My property is situated on a corner where a township and State road meet. I am constantly bothered by people posting yard sale and house sale signs in my yard and on my trees. They come in with sledge hammers and post. I either take down the signs and /or return them to their rightful owner or I ask them to please remove their signs and not post them. The posters then argue with me that they can post them because they are in the road right of way and that the township officials have told them they can. My understanding of the road right of way is that it is in place if they need to widen the roads only and they have placed their road identification signs there. What can I do about this? I want to know if any Tom, Dick or Harriet can just walk onto my property and drive a staked sign into my yard or screw them into my trees. Very frustrating that they don't ask permission and then argue with me about it!

Thanks for your help, WJP