On December 28, 2016, about 1230 central standard time, a Cessna 172B single engine airplane, N6941X, registered to a private individual, sustained substantial damage after it struck roadway signs while attempting to taxi after a successful emergency landing near Prairie Du Sac, Wisconsin. The private pilot and one passenger were not injured. The flight was being conducted under the provisions of Federal Code of Regulations Part 91. Visual meteorological conditions prevailed throughout the area and a flight plan was not filed. The flight originated from the Sauk-Prairie Airport (91C), Prairie Du Sac, Wisconsin about 1130. The pilot reported that he was flying locally at 4,500 feet after departing from 91C. The engine started to run rough and the pilot applied carburetor heat. He decided to return to the airport for a precautionary landing. About 4 miles from the airport, and lined up for landing on runway 18, the pilot increased the engine throttle but had no response. He decided to land on a roadway about 1 mile from the airport. After landing on the roadway and coming to a stop, the pilot started the engine and attempted to taxi off the roadway. While taxiing, the airplane struck 3 road signs and an iron fence, resulting in structural damage to the left wing. The local Sheriff closed the highway as the airplane was loaded onto a trailer and transported to 91C where it was examined by an FAA inspector. Approximately 4-5 gallons of fuel were present in each wing tank. Small amounts of water were present in the fuel from both wing sumps and the gascolator. The engine was started, ran normally, and both magnetos checked within normal limits. The fuel appeared to be automotive fuel. The pilot confirmed that he used automotive fuel. Review of the logbooks did not disclose that the airplane was approved for the use of automotive fuel.