Actually, a displaced threshold is a little different than that. While displaced threshold does mean that the threshold of the runway, or the beginning of the usable runway for landing, is moved from the edge of the concrete, it is not due to problems with the runway surface. A displaced threshold generally keeps airplanes a little higher that approaching for the very beginning of the runway, so a landing plane can clear obstacles in the normal approach path. At the Purdue Airport in Lsfayette, IN, for example, we have a displaced threshold that keeps aircraft higher than the height of a freight train when we cross the railroad tracks just before runway 23. The area before the displaced threshold can be used for takeoff, but is not usable for landing.