Movable bridges are bridges with components that change position to accommodate ship traffic. These bridges can be seen in many regions where transportation officials balance the need for bridges to allow trains, cars, and trucks to pass with the desire to accommodate ships. The alternative would involve building an extremely high and expensive bridge so ships could pass beneath while land traffic passed overhead, and this may not always be an option.
Humans have been using movable bridges on waterways for centuries, and there are a number of basic designs in use. One of the most familiar is the bascule, also known as the drawbridge, where one or both sides of a bridge is fixed on a hinge and can pivot up to create an opening large enough for a boat. Some bascule bridges curl or fold instead of pivoting, depending on the design. This basic moving bridge is easy to install and operate, and it is very popular.
Vertical lift bridges move the platform of the roadway or tracks up on risers so ships can pass underneath, while swing span bridges rotate off and to the side to create an opening. It is also possible to see retractile bridges, with panels that retract for passing traffic. The variation in designs for movable bridges can accommodate different environments and needs. Engineering firms specializing in these types of projects can draw upon a library of bridge designs and styles to determine the best choice for a given application.
An operator on a movable bridge monitors ship traffic and determines when to move the bridge and how long to hold it open. Typically a warning system activates first, with lights, audible alerts, and drop-down gates to encourage cars to get off the bridge and stay off. Once the bridge is clear, the operator can activate the system to move it. After the ship passes, the bridge's position can be restored.
Traffic management with movable bridges can be tricky. Operators may need to consider issues like how opening a bridge will affect rush hour traffic, or the maximum clearance under the bridge. Very large boats may need to go through at low tide, for example, even with the bridge open, because their superstructures could tower above the waterline. Traffic on land could be increased by sports events and other issues, making it necessary to consider holding ships at movable bridges to prevent a traffic snarl on shore.