On June 30, 1996, at 0845 central daylight time, a Cessna A188B, N21744, collided with a ditch while landing on a road during an aerial application operation in Watford City, North Dakota. The 14 CFR Part 137 flight was not operating on a flight plan. Visual meteorological conditions existed. The airplane was substantially damaged and the pilot was not injured. The flight originated from the Watford City Municipal Airport at 0815 cdt. The pilot reported that approximately one-half hour into the aerial application flight the automatic flagger jammed which prevented him from continuing the application. He elected to land on a road approximately 1 mile from the field he was spraying so that he could unjam the flagger. During the landing, the right main landing gear contacted a "deep depression" in the shoulder of the road. This resulted in the right main landing gear separating from the airplane. The propeller and right wing contacted the ground and the airplane slid approximately 100 feet prior to coming to a stop.