A gantry (also known as a sign holder, road sign holder, sign structure or road sign structure) is a traffic sign assembly in which signs are mounted or railway signals are supported on an overhead support. They also often contain the apparatus for traffic monitoring systems and cameras, or open road tolling system. Gantries are usually built on high-traffic roads or routes with several lanes, where signs posted on the side of the highway would be hard for drivers to see. Gantries may be cantilevered or one sided on the left, right and centre (sometimes referred to as a half-gantry or Butterfly gantry), or they may be bridges with poles on each side. Similar gantries are used in railway signalling on multi-track lines.