Is it just a coincidence that Twitter founder Jack Dorsey grew up in a city that embraces Message Bridges?

Give us a pedestrian overpass and a stack of SOLO plastic cups and we’ll find a short message to shout out to thousands.

I know of two bridges in St. Louis that have been adopted by the public in this way; one on Highway 55 near Bayless Avenue, and one of 44 west of Elm Avenue.

But why these two bridges, and not others? There are at least three other pedestrian bridges that span highway 55 north of the Message Bridge. What about the people in the neighborhoods surrounding this bridge determined that it was special? Why is this bridge always covered with messages of love and longing, but other bridges go silent? Why does someone heart Bob so much that they become bridge-worthy, but another Bob up the highway doesn’t make the cut? How did these people determine individually that these events (“Kelly is 16″) were meritorious of SOLO cups, but not permanent tagging?

In the end, Message Bridges are a polite form of graffiti, leaving no damage and providing cups to be recycled into new messages. They disappear as quickly and cleanly as any digital medium.

I want to find others in cities across the country. Is there another name for these? Anyone already talking about them? Send me an image and a map if you know of one. We’ll post them as we get them.