On September 5, 1999, about 1630 hours mountain standard time, a Lithuanian Factory of Aviation LAK-12, N612LK, collided with obstacles on the ground while making a forced landing about 12 miles south of Prescott, Arizona. The glider, owned and operated by the pilot, was substantially damaged. The private pilot was not injured. The personal cross-country flight, conducted under the provisions of 14 CFR Part 91, originated from the Pleasant Valley Gliderport, Peoria, Arizona, about 1430, and was en route to the Coyote Run Gliderport in Prescott Valley. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that the glider encountered an extremely heavy sink rate. He could not locate any thermals in the area so he decided to double back to an area where he had previously encountered strong thermal activity. He reported that he still didn't locate any thermals so he decided to look for a suitable landing location. He saw a dead-end service road that paralleled the highway and noted that the road had no traffic and was straight and long. The pilot reported that he made a normal landing, but the right wing clipped a road sign during the landing roll. He estimated that his speed at the time of the collision was approximately 25 to 30 knots. The glider ground looped and rolled onto a grassy knoll between the highway and the service road. The pilot reported that he hadn't experienced any mechanical malfunction with the glider prior to the accident.