If you agree that this was not public property, and assumed it to be a private lot, then you should have been wary about treating it as if it were public property. You would object, I assume, if somebody parked on your front lawn.

If you do not believe that the signage is adequate, you can sue under the provisions of the aforementioned statute. You have the burden of proving inadequate signage so you had best take pictures and measurements - I suggest showing your measuring device in the photographs rather than simply telling the court, "These are the measurements I made."