On July 14, 2003, about 1030 Pacific daylight time, a Piper PA-18-150, N8991Y, nosed over during landing rollout on a road about 14 miles south of Ely, Nevada. The airplane was owned by the U.S. Department of Agriculture, and it was operated as a public-use instructional flight by the Department's Animal and Plant Health Inspection Service (APHIS) under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed at the time, and a company flight plan was filed. The airplane was substantially damaged. Neither the airline transport pilot, who held a certificated flight instructor (CFI) certificate, nor the second pilot was injured. The flight originated from Ely about 0830. The CFI reported that the purpose of the flight was to provide a checkout including landing practice to the second pilot, who held a commercial pilot certificate. After performing air work, the CFI directed the second pilot to perform an off field landing. They picked out a road and flew over it to assess its adequacy for landing. The wind was nearly calm in the area and no hazards were noted with the road, which had a gravel surface. The second pilot reported that he lined up with the road and applied full flaps. He planned to perform a three point landing, but upon touchdown the airplane bounced upward about 1 foot. During the rollout, the airplane veered right, and the pilot applied left rudder and full engine power to correct for the drift. The pilot reported that he was not successful at realigning the airplane with the roadway, and he even applied left brake in an effort at maintaining directional control. The pilot opined that the brake was ineffective because of the gravel surface. The right main landing gear wheel dropped off the shoulder of the road. Thereafter, the airplane went into a ditch and nosed over. The CFI reported that when he observed the airplane starting to drift to the side of the road, he began applying pressure to the left rudder to correct for the drift. However, at that time he noticed the second pilot had already applied rudder pressure in his correction effort. The CFI indicated that when he tried to apply full engine power, the second pilot had already opened the throttle.