WASHINGTON NOW HOLDS the title for the world's longest floating bridge. Again. As with the three other longest and heaviest floating bridges on the planet, the colorfully named SR 520 Bridge makes its home in the Seattle area.

SR 520, which cost $4.5 billion and replaces an aging bridge spanning the same stretch of Lake Washington, recently opened to westbound traffic on State Route 520 (the other side opens soon). It's more than 7,700 feet long, which raises two questions: Why span a lake with glorified pool noodles? And how do you make all that concrete float reliably enough to carry cars and even trains?