Road signs, like power lines, cause uncontrolled growth of trees and bushes. In the case of power lines it has been postulated by many people that it's the electromagnetic field emanating from the lines that causes tall trees always to spring up immediately under the lines.

In the case of road signs, some would say its the vortices formed around the isolated post by the movement of traffic nearby.

Or some would say:

1. manmade object is erected in clear space.

2. bird lands on object to rest and enjoy the view.

3. bird shits.

4. seed in bird shit grows.

5. plant that grows inevitably impairs the intended function of any man made - or man placed - object.

I think I'll go with the last process.

John