A movable bridge is a bridge that moves to allow passage for (usually) boats or barges. By making the bridge movable it may be made low, which avoids the expense of high piers and long approaches, greatly reducing the cost of the bridge. The principal disadvantage is that the traffic on the bridge must be halted when it is opened for passages. For seldom used railroad bridges over busy channels the bridge may be left open and then closed for train passages. For small bridges bridge movement may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridgeman, sometimes remotely using video-cameras and loudspeakers. Generally the bridges are powered by electric motors, whether operating winches, gearing, or hydraulic pistons.
While movable bridges in their entirety may be quite long, the length of the movable portion is restricted by engineering and cost considerations to a few hundred feet. The George P. Coleman bridge, with a span of 3,750 feet is the longest bridge with a movable section in the U.S and the second longest in the world.
There are often traffic lights for the road and water traffic, and moving barriers for the road traffic.
In the United States, regulations governing the operation of movable bridges, for example, hours of operation and how much advance notice must be given by water traffic, are listed in title 33 of the Code of Federal Regulations[1]; temporary deviations are published in the Coast Guard's Local Notice to Mariners [2].
Jetway - a passenger bridge to an airplane. One end is mobile with height, yaw, and tilt adjustments on the outboard end
Leonardo da Vinci also built an unusual moveable, or portable, bridge. Constructed of merely strong sticks, he entwined them in a way where armies could use this easy bridge to cross rivers too dangerous to cross through.
