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A viaduct is a bridge that connects points of equal height in a landscape, usually by bridging a river valley or other eroded opening in an otherwise flat area. Often such valleys have roads descending either side (with a small bridge over the river, where necessary) that become inadequate for the traffic load, necessitating a viaduct for through traffic. Such bridges also lend themselves for use by rail traffic, which requires straighter and flatter roadways. It gets its name from an analogy with aqueduct, which must also be practically flat, while a bridge for people, pack animals, or non-rail vehicles can be hump-backed and gain a structural advantage (i.e., less material or design sophistication needed) from that shape.

Some well-known viaducts include:

• Bloor Street Viaduct
• Millau viaduct

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