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A table bridge is a type of movable bridge which operates by raising part of the bridge bed vertically. Like other movable bridges, table bridges are used to accommodate the need to balance ship and vehicle traffic. When the bridge is down, cars can drive over it, and when it is elevated, ships can pass under it. In communities such as ports and areas with numerous waterways, table bridges and other movable bridges can be quite common.
The operation of a table bridge is relatively straight forward. When the bridge is down, it looks like a conventional bridge, and can be designed with a variety of aesthetic schemes. When the bridge needs to be raised, hydraulic pillars underneath the bridge push it up, and then lock in place to hold the bridge up. During this phase, cars cannot pass over the bridge, and barriers may be dropped on the road so that cars do not inadvertently drive into the waterway.
Once a ship has successfully moved under the table bridge, the bridge can be lowered again, the barriers can be raised, and cars can cross it as usual. The apparatus for moving the bridge is hidden, so the bridge presents a low profile, which may be important for aesthetic reasons in some communities. Some people do not like bridges with overhanging equipment used to move the bridge, and may prefer table bridges for this reason.
Movable bridges do present a safety issue for cars, as inattentive drivers could drive onto the strip of the bridge left in place while the bridge is up, and not realize that they are about to drive off the road and into a waterway. For this reason, access to the bridge may be controlled with lights which can be used to signal traffic, and traffic on the table bridge can be further controlled with movable barriers which are used as a safety measure when the bridge cannot be crossed by car.
Before a table bridge is raised, it is typical for alarms to illuminate and sound to alert people in the area to the fact that the bridge is moving. Drivers are generally expected to stop, and pedestrians must do so as well. In some regions, truckers may be required to stop at a movable bridge to confirm that it is safe to cross before moving on, much as trucks stop at railroad tracks to verify that no train is coming.
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