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 was suspended from the middle of each walkway (or a red light at night) this indicated that the bridge could not be opened. These signals were repeated about downstream, at Cherry Garden Pier, where boats needing to pass through the bridge had to hoist their signals/lights and sound their horn, as appropriate, to alert the Bridge Master. Some of the control mechanism for the signalling equipment has been preserved and may be seen working in the bridge's museum. Tower Bridge is still a busy crossing of the Thames: it is crossed by over 40,000 people (motorists, cyclists and pedestrians) every