Bridges make the system more efficient either by saving travel distance or by enabling vehicles or pedestrians to reach places that were previously inaccessible. In addition, pontoon bridges, which actually float on the surface of the water, are used in some situations. While all bridges are built from the basic structural units of bending, tension, and compression members, the design of suspension and pontoon bridges is highly specialized and their construction is usually too costly for small-scale applications. The arch bridge is primarily a compression member, subject to forces that tend to diminish its length. This type of structure built of masonry was widely used by the Greeks and later by the Romans.
Arches continue to be built, but now reinforced concrete or steel is used.
A tension member is subject to forces that tend to increase its length. At the same time, the construction of beam bridges became less costly for spans under 12 m. Eventually they, also, were used for very heavy, longer spans. The site for the bridge should be selected on the basis of minimal cost and maximal convenience for users. Most bridge locations are dictated by such obvious factors as shortest crossing between banks of a river or gulley, the need to join roads of a town, and replacement of an older structure or one that cannot be crossed during floods.