What is the symbolism of the "nightmare alley" in the novel, Nightmare Alley?

The nightmare alley serves as a symbol of death and fear. Stan's recurring nightmare is about an alleyway that he runs down toward a light, while a frightening presence pursues him. He never reaches the light in his dream, but the thing he is afraid of keeps closing in on him. When Stan runs away from the police officer, he finds himself in a literal nightmare alley, but instead of dying, which is what he imagines will happen.... he kills the police officer.