On May 21, 2005, at 1610 eastern daylight time, a Cessna 150M, N66427, collided with a vehicle during an overrun following an attempted aborted landing on runway 11 (2,582 feet by 50 feet, asphalt) at the St. Isabella Airport (D15), Mt. Pleasant, Michigan. The certified flight instructor (CFI) and student pilot on board the airplane were not injured. One occupant of the vehicle received serious injuries and the other occupant was not injured. The airplane was substantially damaged. The 14 CFR Part 91 instructional flight originated from D15 about 1600. The CFI stated the purpose of the flight was to practice takeoffs and landings. The accident occurred on the first landing of the flight. The CFI stated they departed and flew the traffic pattern without incident. He stated the landing approach was normal and the student landed the airplane "a couple hundred feet past the displaced threshold." He stated the airplane bounced and he instructed the student to initiate a "go around." He stated the student added full power, turned off the carburetor heat, and gradually retracted the flaps. The CFI stated the airplane was not climbing at a rate sufficient enough to clear the 41 foot tall trees at the end of the runway. The CFI stated he took control of the airplane, retarded the throttle and brought the airplane back to the runway. He stated that once the airplane touched down, he applied heavy braking and the tires locked up. The airplane slid off the end of the runway and onto a road where it contacted a passing car. The student pilot relayed the same events as the CFI, except he stated they initially touched down 50 feet past the displaced threshold. The CFI stated the local winds were clam at the time of the accident. The Airport/Facility Directory shows that runway 11 has a 1,089 foot displaced threshold and that there are trees at both ends of the runway.