A viaduct is a bridge constructed from a series of spans and designed to go across dry land, rather than a body of water. Viaducts can be used in a wide variety of ways, and numerous examples of this type of bridge design can be found around the world, executed in a variety of materials. Viaducts have been constructed since ancient times to carry foot and cart traffic, and today they can be used for cars, pedestrians, cyclists, trains, and trucks.
One common site for viaducts is in valleys. Building a viaduct allows a road or railway to quickly span a valley or chasm, rather than forcing engineers to lay out a path for the roadbed to reach the valley floor, and then establish a route out of the valley. While building a viaduct can be initially very expensive, it is generally cheaper and safer than keeping the road on the flat ground.
A viaduct may also be used in an area where several different streams of traffic are overlaid, allowing traffic to pass on and under the viaduct so that there is less interruption of traffic. Viaducts are common features in regions with lots of freeways, accommodating multiple streams of traffic without forcing traffic to stop. Viaduct construction is also especially common in areas where trains and cars need to travel in roughly the same places. Trains can be run under a viaduct, while cars may drive on it, or vice versa. In either case, car traffic does not have to stop for passing trains, and the risk of train/car collisions is greatly decreased.
In the classic viaduct design, engineers start by constructing a series of towers, and then connecting the towers with arches and installing a roadbed. In the case of a viaduct which is designed so that traffic can flow underneath, the arches must be built especially high to ensure that passing traffic has enough clearance. Viaducts over low valleys and in other situations where traffic underneath is not expected may have much lower arches, since clearance is not an issue.
Many historic viaducts are made from stone and brick, but metal and concrete can be used as well. In communities with older viaducts, preservation organizations sometimes work to keep the viaduct in good working order, as stone and brick viaducts are considered aesthetically valuable. It is also possible to find examples of abandoned viaducts which were left behind when roadways moved or became impassable, and these sites sometimes become popular with walkers.