A bridge is a spanning structure that creates a passage over an obstacle such as a river, gorge, valley, road, railway and so on. A movable bridge (or moveable bridge) is a spanning structure that can change position (or even shape) to accommodate different situations, such as the requirements of ships, barges and other types of traffic above or below it. However, they can be inconvenient due to delays caused when one stream of traffic is halted to accommodate the passage of the other.
Movable bridges are typically operated by counterweights (including auxiliary counterweights), or other mechanical systems (such as electric or hydraulic drives). Modern movable bridges may be controlled by stabilising machinery, span brakes and other mechanisms.
There is evidence that movable bridges were first used in ancient times, but it wasn’t until the Middle Ages that they appeared in Europe. During this period, the drawbridge (also referred to as a bascule or lifting bridge) was sometimes incorporated into defensive mechanisms used to protect a castle, village or town. These early drawbridges did not typically include counterweights.
The drawbridge is the most common type of movable bridge. It is also the term that is typically used for all movable bridges in the US. This is not the case in the UK, where the term is more frequently used to describe the defensive version of a drawbridge that is associated with medieval structures.
There are several other types of drawbridges, including:
 Other movable bridges
Engineering advances during the 1800s made it possible to incorporate new technologies and materials into the construction of movable bridges. This resulted in the introduction of several new types of movable bridges, including:
- Retractable (including thrust, floating retractable and Guthrie rolling versions)
- Submersible (also referred to as ducking or low water).
- Swing (or pivot).
- Transporter (also known as a ferry or aerial transfer bridge)
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