The pilot flew to the vicinity of his destination by following the path of recent showers. He was a farmer and was interested in the amount of standing water. He had arranged to fly over a friend's house so the friend would pick him up at the airport. After flying over the house and heading for the airport, the engine began running rough. The pilot enriched the mixture, but only momentarily applied carburetor heat. Neither seemed to change the way the engine was running, so he elected to land on a road near the friend's house. Reportedly, he had operated the engine at 16 inches manifold pressure and 2100 RPM for the entire flight and made no changes in his power setting. While landing on a road with a right cross wind, the left gear encountered a soft shoulder. The plane veered into a ditch, then flipped over. An examination of the aircraft and engine revealed no pre-impact malfunctions. The temperature was 59 degrees; at that temperature, carburetor icing conditions would be probable in moist air.