An overpass (called an overbridge in the United Kingdom and Australia, and a flyover in the United Kingdom as well as some other Commonwealth countries) is a bridge, road, railway or similar structure that crosses over another road or railway. An overpass and underpass together form a grade separation. Stack interchanges are made up of many overpasses.
The world's first railroad flyover was constructed in 1843 by the London and Croydon Railway at Norwood Junction railway station to carry its atmospheric railway vehicles over the Brighton Main Line.
The first flyover in India was opened on 14 April 1965 at Kemps Corner in Mumbai. The 48-foot-long (15 m) bridge was constructed in about seven months by Shirish Patel at a cost of ₹17.5 lakh (equivalent to ₹8.3 crore or US$1.2 million in 2017).
Highway and road
In North American usage, a flyover is a high-level overpass, built above main overpass lanes, or a bridge built over what had been an at-grade intersection. Traffic engineers usually refer to the latter as a grade separation. A flyover may also be an extra ramp added to an existing interchange, either replacing an existing cloverleaf 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 see Freeway interchange.
Railway overpasses are used to replace level 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 their rights of way to avoid traffic interference with frequent and reliable service.
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