On April 5, 2003, at 1327 Pacific standard time, a Cessna 172N, N6272D, lost engine power during cruise and made a forced landing on the 91 Freeway in Anaheim, California. Runway 3-7 operated the airplane under the provisions of 14 CFR Part 91. The airplane was substantially damaged. The private pilot and one passenger were not injured. Visual meteorological conditions prevailed for the local area flight. An instrument flight rules (IFR) flight plan had been filed. The flight originated from Brackett Field Airport (POC), La Verne, California, earlier that day. The return flight to POC departed from Santa Monica Municipal Airport (SMO), Santa Monica, California, at 1249. According to an officer from the California Highway Patrol (CHP), the airplane struck a car and a median fence on the 91 Freeway between the 55 and 57 Freeways. He noted damage to the right wing and right horizontal stabilizer. The CHP officer further noted oil covering the belly of the airplane. In the pilot's written statement, he stated he had rented the airplane for a personal flight. During the preflight inspection he noted that the tire pressure was low. The pilot taxied the airplane over to the fuel pit to put air in the tires. No further discrepancies were noted with the airplane. While at the fuel pit he filed an IFR flight plan to SMO with ground control. He stated that the flight to SMO was "uneventful." At SMO, he and his passenger tied the airplane down and went to lunch. After lunch, he contacted SMO ground to obtain another IFR clearance back to POC. He stated that the flight was again "uneventful" until they were just east of Disneyland. The pilot stated at 1327, he noted a drop in engine rpm. He visually checked the throttle and saw that it was forward, so he applied carburetor heat, with no improvement. He reapplied carburetor heat, but there was still no improvement. The pilot stated that the airplane suddenly "shook violently," and he thought the engine had failed. He declared an emergency with air traffic control (ATC) personnel. The pilot pulled the mixture to idle to stop the shaking, and then noted that the engine was still producing power. He advanced the mixture, and the shaking "got worse." The pilot reported that he thought he had lost part of the propeller blade. At that point he reduced the mixture to idle cut-off. The pilot stated that he queried the ATC controller as to the nearest airport, and was informed that Fullerton Municipal Airport (FUL) was the nearest airport. The pilot stated that at the current descent rate he did not believe he would make FUL. He selected the westbound lanes of the 91 Freeway for the forced landing. The Safety Board investigator interviewed the pilot. The pilot stated that prior to landing; the airplane struck the rear window of a car. He indicated that when he had setup for the landing, there were no cars in front of him, and he did not see the car that the airplane struck. After hitting the car, the pilot lost control of the airplane. The airplane veered to the right, and the right wing hit a fence. The pilot regained control of the airplane and brought it to a stop on its main landing gear. According to engine logbook entries, the engine was overhauled on September 3, 2002, and installed in the accident airplane on March 21, 2003 by the operators maintenance personnel. According to the airframe logbook entry, the airplane received an annual inspection on March 21, 2003. Post accident examination disclosed that oil covered the belly of the airplane and coated the inside of the engine compartment and nose wheel assembly. The oil drain plug was missing. The oil drain plug is normally secured with safety wire. No safety wire remnants were found on the engine. A Federal Aviation Administration (FAA) inspector examined the airplane at Foothill Aircraft Sales, Upland, California. Maintenance personnel removed the cowling. The FAA inspector noted that the engine compartment and nose landing gear area were covered with oil, in addition to oil covering the belly of the airplane. He also found a non-standard plastic plug installed in the engine oil drain, but could not verify when the plug had been installed. The engine had sustained a catastrophic internal failure.