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Name: Megan
Status: Student
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993

How does a bridge know when a boat is coming and when to go up?

I think that most of them are pretty low technology, in that there is a human operator standing by to raise the bridge and make sure that traffic is controlled so that no one tries to drive on the bridge while it is discon- nected. It would be possible to do the job automatically, but I do not think it has been done, or at least I have never seen it. For instance one could have a special frequency on the radio waves to access the bridge. The boat operator could signal a computer through this radio frequency that he wanted to go through. The computer could be programmed to change the stop lights to red so that cars would not pass across the bridge. The bridge would then rise allowing the boat to pass. The bridge would then lower when the boat signaled again that it had gone past the bridge (something that could be programmed into the boat's computer). Then traffic would continue across the bridge after the bridge's computer changed the traffic light to green. Of course there would have to be some safe guards on the system, but I think that it is conceptually feasible to do this with available technology (if it has not been done already).

D. Munoz

Up-dated: July 2008
Regarding "Bridge Opening and Closing", the asker wants to know how a bridge knows when a boat is coming. An answerer stated that it is a *human* that operates drawbridges.

Actually, I know of at least one system in which the bridge is normally up, and an oncoming train activates lights and sirens and then drops the bridge, bringing it back up after the train is past. Before and during the dropping of the bridge, sensors verify that there is no boat traffic, and if any is detected, the process is halted.

I learned this by talking to a railroad engineer, but such a system is described in detail here...

I used to do some commercial fishing and some bridges can be raised by calling a certain radio frequency.

P. Bridges

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