Whenever I see a bridge I think about the effort that went into making it. A bridge that supports vehicles takes a long time to build, and it probably takes a long time to engineer and plan for. Even small bridges cost millions of dollars to make, which indicates that they take a lot of human effort and that we value them a lot.
I like wandering under a bridge to get a sense of what I'm using when I'm on a bridge. There's actually a lot of aspects to a bridge--much more than the flat surface we use when crossing over one. I appreciate the effort man has given to bridges. I use them most days. But I also realize bridges are here because we are here.