The map above is a first crack at part of that task. It shows every single highway bridge in the U.S. -- all 600,000 of them -- and nothing else. Each bridge is represented by a tiny, half-pixel dot. Click on the image for a larger version (trust me, you'll want to do this). The underlying data comes from the National Bridge Inventory maintained by the Federal Highway Administration. It contains every highway bridge in the U.S. greater than 20 feet in length.
It's amazing how just plotting our bridges paints a pretty accurate picture of our cultural geography. Densely populated areas immediately stand out, as do the major highways linking them. You can trace the route of Interstate 95 down the East Coast, as well as Interstate 80's path across the country. You can see the contours of the major street grids in cities like Houston, Phoenix and D.C. Sparsely populated regions with few roads stand out as blank areas on the map.
But you can see the influence of the natural landscape as well. Bridges are much more dense in the eastern half of the country than the west, primarily because of the greater prevalence of rivers and streams there. There's a streak of white where the Mississippi River runs. Looking toward the plains, the bridges follow the branching paths of the rivers that flow eastward. You can see hints of the curve of the Appalachian Mountains in Pennsylvania and Virginia.
The map is inspired by a map of all U.S. streets by information designer Ben Fry, as well as a map of U.S. rivers made by the New York Times' Mike Bostock. It's also something of a distillation of the two -- if the street map and the rivers map got together and had a kid, it would look something like the bridge map.
The map illustrates the ubiquity of America's bridges -- it's basically impossible to go for a drive in any densely populated area and not cross one. But each of those bridges -- all 600,000 of them -- need to be maintained. This maintenance costs money. And some states and localities do a better job of it than others.
The National Bridge Inventory also contains detailed information of the state of those 600,000 bridges -- whether they're well-maintained or falling into disrepair. But illustrating that will require a deeper dive into the data, which I'll do in a subsequent post. Stay tuned! [edit: That post is now up - go check it out!]