Humans

Always thought the robot apocalypse would look different, ya'know? That it would be like they warned us. You guys finding the loophole and kicking our ass.

Once the ruling species, humans are long gone in the Hummelverse. By causing the nuclear apocalypse, they drove themselves into extinction. From the current state of knowledge, it is unlikely that any are left, but it can't be ruled out that some are still alive, somewhere.
They created the AI machines, living with them for a few decades and thus shaping the machine society and mentality - but after a good hundred years without their human masters, a lot of the doctrine and way of life the humans tried to teach them is lost.
Their last issued commands to the machines were - Fight on. By all means necessary. And the machines did.
Nowadays, reminders and relics of the humans can still be found almost everywhere; the cities and villages they left behind, billboards with ads, stories and legends, documents like movies, music, and literature. Some vehicles like to collect relics that they deem rare. Knowledge about who the humans were and how they lived is mostly common, but there are also some mysteries and misconceptions. Some historians dedicate their research to preserving this knowledge.

The Elusive Race

Among those who believe that there are still humans left, there are many theories as to why no one can seem to find or contact them. Some claim that the humans are simply hiding (sometimes, this hiding place is stated to be the Moon). For various reasons, they could be avoiding contact with the machines where possible - without doubt, there would be vehicles that would love to hunt them down if they learned there are any remaining.
The appearance of human nations would greatly disturb the power structures of the established clans, which is why some clans actively campaign against a race that no one has seen in a century. There is a known case of a clan offering bounties to any vehicle that can show them a human, dead or alive. Many have tried to bring this clan anything that remotely resembles a human body, but so far the bounty has not been claimed.
Machines who claim to have seen or interacted with humans since their extinction are usually labelled as lunatics.

EXTINCT

The Myth of the Return
Many machines firmly believe that one day, their masters will return. The opinions of what this return will entail differ greatly, as do the opinions of the circumstances or causes.

A New Age of War
A widespread belief is that with the eventual return of the humans, a new Great War will happen. This war will be the ultimate end of all the fighting, settling all conflicts once and for all.