In the accompanying featurette, the film's co-writer and co-director, John Lasseter ("Toy Story 1 and 2," "A Bug's Life") explains that the virtual abandonment of the America's celebrated Route 66 got him thinking about the story idea (so much so, apparently, that he originally wanted to title his movie "Route 66").
It's got something to do with the lack of a human grounding: the only other time Pixar stumbled was with its similarly bleak A Bug's Life (that picture resorting, like Cars, to racial caricature as its primary tentpole), which is also the only other time the company has neglected to ground its story with homo sapien ballast.
Filmmaker John Lasseter, who directed "A Bug's Life" and both "Toy Story" productions, thinks that the NASCAR in-jokes and quirky peripheral details (such as jet trails in the sky being tire marks) can compensate for the lack of a compelling plot.
Unlike, say, "A Bug's Life," which ingeniously gave pop culture the entomological treatment, "Cars" simply substitutes various makes and models for character types.
This is the first Pixar movie (although you could make a similar case for A Bug's Life) where the presented animated world could completely stand in for the human world because no direct points of reference to actual humans exist.
Specializing themselves in the anthropomorphism of everything from toys ( Toy Story ) to bugs ("A Bug's Life") and fish ( Finding Nemo ), the famed but slightly overrated animation studios give the same treatment to cars in, well, "Cars".