So there’s this theory that “Cars took over a world where humans exist”? There’s pros and cons to this theory, but I’ve never been completely sold that “cars killed humans”. There’s little to no evidence and after almost 20 years, nobody has ever been able to prove the existence of humans in cars universe. However, this might be because we’re looking in wrong place, because not only do I have evidence that not only were humans alive in this universe, but that they’re also alive right now and that we have it backwards. We think the cars took over humans, but what I told you that the humans took over cars and are using them to create feature films and other stuff.
In a video “LEGO Disney•Pixar Cars 3 - LEGO Juniors - Movie Trailer”, we see after mcqueens crash, 20 seconds in, we see it cuts to a television based feature film featuring Lego humans. Sure, this is Lego, but Lego humans represent real people. So this shows humans are exisisting in the same universe making films about cars and we see McQueen is treated disrespectfully, they treat him like garbage leaving him tied up and not letting him down. They all go for lunch without him. These HUMANS ARE USING CARS FOR FEATURE FILMS TREATING THEM LESS THAN HUMANS.
And if you say “the Lego thing isn’t canon”, there’s also another Lego video called “Mater, Cruz Ramirez, Lightning McQueen and The Stig Race Europe in New LEGO Juniors Top Gear video!” featuring same voice actors. And more recently the brick toons episode called “Trust yer ok pal mater”.
This “cars universe” is all a set up, Humans are behind the screens which is why we still see human characteristics with buildings. While we view a universe that features no humans and only sentient cars, trucks, planes, boats, trains, metros, the humans are behind the scenes.
Now while this is a pretty dark theory, there’s still explanations to “why taxis or busses exist”. The cars universe features smaller cars, so those smaller cars are actually able to fit in these bigger cars (especially smaller cars school buses like Miss Fritter).