A highway bridge is a major road structure that is designed to go either over or underneath an obstacle. For example, cars can use a highway bridge in order to cross over a body of water or other traffic lanes. Most highway bridges are not used by pedestrians and do not contain pedestrian walking paths. Instead, they are usually intended to allow vehicles, such as passenger cars or semi-trucks, to safely cross over or under the obstacle.
A highway bridge is typically designed by an engineer, who considers the bridge’s purpose in planning its design. The structural design of a highway bridge can vary greatly, depending on its length, location, anticipated load weight, and width requirements. Whether clearance is needed below or above the bridge is another construction consideration. Building budgets and material availability also influence the type of highway bridge design that is used.
Highway bridges can be constructed from a variety of different materials, including concrete, steel, or stone as well as metal alloys or timber. The bridges can be made from beam, truss, or arch systems as well as cable-stayed and suspension systems. In general, the number of traffic lanes a highway bridge contains impacts its overall width.
Two common types of highway bridges are overheads and tunnels. An overhead usually brings the highway over the top of an obstacle. For example, an overhead might run on top of another highway. On the other hand, a tunnel allows a highway to go underneath or through a structure. A tunnel may be used, for instance, to provide a passageway through a mountain or underneath another highway system. Shorter tunnels, usually less than 0.1 miles (about 0.16 kilometers), are sometimes referred to as underpasses or chutes.
Sometimes highway bridge construction involves a complex system of ramps and roads known as an interchange. In general, an interchange provides a way for vehicles to switch between multiple intersecting highways. Structures called connector ramps are usually used by vehicles when changing from one highway to another.
Highway bridges require regular upkeep in order to ensure public safety is maintained. Typically, highway bridge inspection encompasses pinpointing minor issues and ensuring they are properly repaired. Preventative bridge maintenance can help ensure costly replacements are avoided down the road. If conditions with potential risk are noted, bridge inspectors must thoroughly document this data and recommend any remedial measures. In some cases, this may mean reducing bridge traffic and vehicle weight or even temporarily closing the bridge to traffic until it can be fully repaired.