The pilot was in the traffic pattern for the private airport and planning to land in the grass next to the paved runway. As the pilot made a turn to final approach, he heard another airplane report being “short final” for the same runway. The pilot and passenger attempted to make visual contact with the other airplane. Unable to make visual contact with the other airplane, the pilot elected to quickly land his airplane. The pilot conducted a forward slip during his descent, and the airplane’s main landing gear impacted an automobile that was being driven on the adjacent roadway near the approach end of the runway. The airplane landed hard in the grass and came to rest inverted resulting in substantial damage to the vertical stabilizer, rudder, upper right and left wings, and both wing struts. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation. The displaced threshold for the landing runway was located about 242 ft from the approach end of the runway. The roadway that crossed the extended runway centerline was located about 36 ft from the approach end of the runway pavement, about 278 ft from the displaced threshold. The (Obstacle) Departure Procedures for the airport list vehicles on road beginning 20 ft from the runway and left and right of centerline. According to the pilot, after the accident it was reported that the other airplane was about 5 miles from the airport when that pilot radioed he was on “short final” for the runway. The Aeronautical Information Manual states that inbound pilots should self-announce on UNICOM when they are 10 miles out, entering downwind, base, final, and when leaving the runway.