The private pilot was on a personal cross country flight under Title 14, CFR Part 91. The pilot said he had fueled the airplane prior to take off, but the indications of the electric fuel gauges made him believe the airplane was losing fuel. He thought the airplane would not make it to the nearest airport, and he elected to make a precautionary landing on a state highway, which was the only suitable landing site in the remote area. He said both fuel gauges were reading empty, and traffic on the road made him land farther down the road than anticipated. During the landing roll, the left wing struck a road sign. He said the airplane sustained substantial damage to the left wing, elevator, and right wing.