"beyond it lay the Badlands about which nobody knew anything"

The Badlands are suggested to be the area nearest to the implied nuclear disaster, known as Tribulation.

Nuclear fallout is the residual radiation hazard that occurs after a nuclear explosion, caused by radioactive dust in the atmosphere. It pollutes the ground, plants, animals, and humans it touches and would explain the gradual drop in the deviation rate described in the book from very bad, to bad, to moderate, to mild in the Badlands, Fringes, Wild Country and the Waknuk area respectively.