Just about every sci-fi flick that pits the Human Race against our robotic AI overlords show that humans still have a fighting chance, and that as long as there’s a few of us left it will stay that way. Certainly there isn’t a film that depicts what Earth will be like after not only every human has been liquefied, but every living creature? (We'll get back to that...)
Oh yeah, that one. Evidence points to the fact that the events in Cars are in fact, playing out on Planet Earth. They are speaking English and their Judicial System is quite similar to our own.
Perhaps more damning is the repeated mention of Dinoco, a chain of gas stations also appearing in the Toy Story films. The Dinoco Cup is the cup sought after in the racing circuit that Lightning McQueen is a part of. Their 1st race takes place in an ambiguous raceway somewhere in the south. You see sentient cars racing against each other, while being tended to by smaller sentient vehicles in a fully automated pit-crew. Cheering the race on is an audience made up entirely of sentient cars. You get the picture.
Only if you look really closely, somewhere about 5 minutes into the movie, as the cameras pan about the 'Speedway of the South,' you see two rest rooms, clearly marked "Men" and "Women."
Do these cars need to go relieve themselves every once in a while? More likely than, this is a relic of the times before the machines grew self-aware and wiped out every trace of mankind.
Because once the 1st race is over, all ambiguity over what planet this is taking place on is wiped away as the location for the next race is revealed: The Los Angeles International Speedway, which they make quite clear is in that
Los Angeles, California. We see several location shots to further cement this home, of famous LA landmarks like the Hollywood sign, and Mann's Chinese Theatre.
Evidence that they've wiped out all animal life is hinted at by McQueen and Mater's "tractor-tipping" escapade, where they sneak into a field to spook the farm equipment that keeps the grass from growing too tall, now that all the cows have been exterminated as well. The tractors moo like cows, and later in the movie, stampede into town, only to be rounded up with relative ease by the more intelligent machines, proving that their system is more manageable than the sometimes messy dynamic between us humans and our bovine counterparts.
However, the true pinnacle of horror occurs as Sally takes Lightning to the Wheel Well Ruins, and he sees an old pair of gas pumps, replete with a small cloud of buzzing flies. Take a real close look at those insects boys and girls... They aren't flies - those are tiny winged cars
So, either sometime after becoming self-aware, every automobile in good working order decided to wipe out any trace of animal life on earth, or we have a Richard Feynman-style alternate universe thingie going on – which basically presents us with the debate of whether it’s better to have existed only to be extinguished, or to never have existed at all.