Springfield Bridge

Due to poor civic planning, corruption and an overly yokelized workforce, the Springfield Bridge is the only way in or out of Springfield (except via plane). The bridge was once hit by a missile that was intended to destroy a comet threatening Springfield. When the bridge was rebuilt, city planners went to great lengths to restore the bridges old style architecture and charm, even adding re-creations of its original, innovative graffiti, such as "Class of '78 rules!," "Marge I love Marge You," and "El Barto Hired Someone That Was Here." The bridge is also home to many of Springfield's hobos.

Source
Matt Groening's The Simpsons Guide to Springfield