Bridge four-lane highway on the upper level and a pair of railway tracks at the lower level. Tower Bridge in London is different example of a double-decked bridge, with the central section consisting of a low level bascule span and a high level footbridge. A viaduct is made up of multiple bridges connected into one longer structure. The longest and some of the highest bridges are viaducts, such as the Lake Pontchartrain Causeway and Millau Viaduct. A three-way bridge has three separate spans which meet near the center of the bridge. The bridge appears as a "T" or "Y" when viewed
Tower Bridge Corporation of the City of London. In 1974, the original operating mechanism was largely replaced by a new electro-hydraulic drive system, designed by BHA Cromwell House, with the original final pinions driven by modern hydraulic motors and gearing. In 1982, the Tower Bridge Exhibition opened, housed in the bridge's twin towers, the long-closed high-level walkways and the Victorian engine rooms. The latter still house the original steam engines and some of the original hydraulic machinery. A computer system was installed in 2000 to control the raising and lowering of the bascules remotely. It proved unreliable, resulting in the bridge being