A cantilever bridge is a bridge built using a cantilever. A cantilever is only supported on one end. For small footbridges, the cantilevers may be simple beams. For large cantilever bridges, designed for road or rail traffic the cantilevers are trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when it was first used, because it can span distances of over 1,500 feet (460 m). They can be more easily built at difficult crossings as they do not need to be supported during the construction.