viaduct [Lat.,=road conveyor], type of bridge for carrying a highway or railroad over a valley, over low ground, or over a road. It is commonly constructed in the form of several towers or piers that support arches on which the roadway rests. Viaducts are usually constructed of steel or concrete; in the past they were built of wood or stone. Examples of viaducts include the Pulaski Skyway in NE New Jersey and the Colorado Street Bridge in Pasadena, Calif.