Sec. 6. The following signs may not be erected or maintained in an adjacent area:
(1) Signs that are illegal under state statutes or rules.
(2) Signs not securely affixed to a substantial structure.
(3) Signs that attempt or appear to attempt to regulate, warn, or direct the movement of traffic or that interfere with, imitate, or resemble an official traffic sign, signal, or device.
(4) Signs erected or maintained upon trees, or painted or drawn upon rocks or other natural features.
(5) Signs that are not consistent with this chapter.