The Supreme Court of Pennsylvania held that the "incorporated municipality" exception is not available to billboards erected in townships.
Patrick Media Group, Inc. v. Department of Transportation, 620 A.2d 1125 (Pa. 1993).
Robert J. Burnett,
Statutory Construction - The Outdoor Advertising Control Act - Legal Status of Townships,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol32/iss2/11