On January 18, 2001, at 1545 eastern standard time, a Cessna 152, N704ZF, veered off the left side of runway 13 and collided with an airport sign during an attempted landing at the St John County Airport in St. Augustine, Florida. The instructional flight was operated by the student pilot under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane received substantial structural damage; the student pilot was not injured. The supervised solo flight departed Keystone, Florida, at 1407 hours. According to the student pilot, he was conducting a supervised solo cross-country flight. Upon arriving at St. John County Airport, he established a visual approach to runway 13 for a full-stop landing. During the landing roll-out, the student pilot stated that the airplane entered a fog bank, veered off the left side of the runway, and collided with a airport sign. Witnesses stated that, at the time of the accident, there was no fog in the area. Weather data shows that, at the time of the accident, conditions were clear with 10 miles visibility. Witnesses stated that the pilot locked up the brakes and tried to get off at the first taxiway. Examination of the airplane did not reveal any preexisting discrepancies, that would of contributed to the accident at the time. The pilots did not report a mechanical problem with the airplane.