Even if it's public land, you usually need a permit (though an America the Beautiful Pass is not that expensive and covers almost all federally-owned land).

However, the point was about the signs. You can find quite a lot of neat little things that you otherwise would have no easy way of discovering.

Texas has a bunch of state historical markers along even minor routes. They can be hard to catch at speed (most TX highways have a 70 MPH speed limit, even small ones), but there's typically a space where you can pull over and read it.