.jpg)
Anoverpass, called anoverbridge orflyover (for a road only) in the United Kingdom and some otherCommonwealth countries, is abridge,road,railway or similar structure that is over another road or railway. Anoverpass andunderpass together form agrade separation.[1]Stack interchanges are made up of several overpasses.
The world's first railroad flyover was constructed in 1843 by theLondon and Croydon Railway atNorwood Junction railway station to carry itsatmospheric railway vehicles over theBrighton Main Line.[2]
In North American usage, aflyover is a high-level overpass, built above main overpass lanes, or a bridge built over what had been anat-grade intersection. Traffic engineers usually refer to the latter as agrade separation. A flyover may also be an extra ramp added to an existing interchange, either replacing an existingcloverleaf loop (or being built in place of one) with a higher, faster ramp that eventually bears left, but may be built as a right or left exit.
A cloverleaf or partial cloverleaf contains some 270 degree loops, which can slow traffic and can be difficult to construct with multiple lanes. Where all such turns are replaced with flyovers (perhaps with some underpasses) only 90 degree turns are needed, and there may be four or more distinct levels of traffic. Depending upon design, traffic may flow in all directions at or near open road speeds (when not congested). For more examples, seeFreeway interchange.
Apedestrian overpass allows traffic to pass without affectingpedestrian safety.
Railway overpasses are used to replacelevel crossings (at-grade crossings) as a safer alternative. Using overpasses allows for unobstructed rail traffic to flow without conflicting with vehicular and pedestrian traffic.Rapid transit systems use complete grade separation of theirrights of way to avoid traffic interference with frequent and reliable service.
Railroads also useballoon loops andflying junctions instead offlat junctions, as a way to reverse direction and to avoid trains conflicting with those on other tracks.
Media related toOverpasses at Wikimedia Commons