It's hard to see these days, but in the past Tow Mater was a character that you could meet near Sounds Dangerous and then the Streets of America before he found a home at his current location near the exit to Muppet Vision 3D.
When Mater was visible elsewhere, you could see the entire vehicle including his license plate, which has special signifigance. His license plate reads "A113".
For Pixar fans, A113 should sound very familiar since it's one of the many "easter eggs" found in Pixar films, such as the Pizza Planet truck or John Ratzenberger. The number A113, which refers to John Lasseter, Brad Bird, Pete Docter and Andrew Stanton’s former classroom at CalArts, makes an appearance in every Pixar film.