One could say by finding the world's largest body of water and placing a structure over it that joins the two sides of land, that then you'd have the longest bridge. Of course it is easy to say, but when assessing crossing logically the best place would be the two points that are the shortest across. To go out of the way to build something gigantic and possibly dangerous is not a good undertaking on any front. So that leaves the question, where does one look to find the world's longest bridge?
It is possibly a very difficult question when trying to define the specifics. A bridge can cross water, but it can partially cross land as well. Knowing this, does it matter if the bridge spans a distance over land or over water? Thinking of it clearly, it would be less costly and difficult to build a long, but not relatively high, bridge over the land. The very fact that it can have many supports to stabilize it undermines the impressive lengths of the great suspension bridges that are more deserving of the title "bridge" simply because they are typically examples that appear in people's minds. People respect majesty and beauty, but when it comes to what is the longest - that doesn't even matter.
The best method of finding the world's longest bridge is to simply leave it to the experts. Everyday people scour the globe searching for things that "beat" other things in number, size, length, looks, and much more, and record it in one place as an organized compilation of data. One such book, the frequently updated Guinness Book of World Records would have the answer in an instant. But it may base its decision on something that differs from another's opinion. With the details on the following site, http://thelongestlistofthelongeststuffatthelongestdomainnameatlonglast.com/long7.html Guinness selected the Lake Pontchartrain Causeway of Louisiana as the longest bridge based on the distance it covers, while the Akashi Kaykio in Japan is considered the longest in span. As these two bridges are both road bridges, it doesn't include the possibility that there may be a rail bridge of longer length elsewhere in the world.
With so many different possibilities for what could be the longest based on road, rail, span, distance, and more, it individualizes the search results. Perhaps with this knowledge it should first be defined what the true measure of length for a bridge is, what mode of travel it aids, and height, weight, support, and other varying difficulties in detail. Until that time when the true bridge length is defined, the search for the longest will have to wait, or settle on the differences accordingly.