On May 1, 1997, at 1100 central daylight time, a Piper PA-28-180, N5778Z, collided with a road sign during an emergency landing roll, and nosed over into a ditch five miles west of Russellville, Alabama. The personal flight operated under the provisions of Title 14 CFR Part 91 with a flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane sustained substantial damage; the pilot was not injured. The flight departed Quincy, Louisiana, at 0700. According to the pilot, approximately four hours into the flight, while on a long descent to Muscle Shoals, Alabama, he experienced a reduction in engine power. The power reduction continued over a period of eight minutes and resulted in a complete loss of engine power. Following the loss of power, the pilot's attempts to restore full engine power, which included the application of carburetor heat, failed. The pilot selected a nearby paved road for an emergency landing. During the landing roll out, the right wing collided with a road sign, and the airplane swerved into an adjacent ditch and nosed over. During the examination of the airplane approximately twelve gallons of fuel were recovered from the fuel system. During the functional test of the engine, the engine operated normally through all power ranges. A review of current weather information revealed that weather conditions were favorable for the formation of carburetor icing.