A box girder bridge, an evolution of the plate girder bridge, is fabricated from either poststressed concrete or sheet steel plate, and commonly built for roadway flyovers and light rail transport. Some modern steel trestles, for example, are composed of a number of girder bridge segments. If of concrete, girder bridges may be cast in place, using falsework supports removed after completion, or may be prefabricated (as the steel-plate type usually is) in a fabrication yard, then transported and placed using cranes. The latter method is often used in situations where access for construction is limited to times of light traffic, which may be detoured around the work area, utilizing a limited number of lanes.