What is the main conflict in The Tortilla Curtain by T. Coraghessan Boyle?

The novel's main conflict is centered around borders. The novel contains many physical borders that separate Candidó and América from their rich neighbors. The largest border is the one that they have already crossed, the border that separates the United States from Mexico. No one in the novel can alter the presence of that border, yet the residents of Arroyo Blanco Estates choose to erect the novel's other borders. The wealthy homeowners build a gate to separate themselves from Mexicans. When that fails to keep immigrants out of their neighborhood, they put up a tall fence. These physical structures reflect the borders in their minds that keep them from thinking of Mexicans as their fellow human beings. The road that separates the upper canyon, where Arroyo Blanco Estates is located, from the lower canyon, where Candidó and América make camp, also acts as a border between two types of people. The people with high status live above the road, while the humblest indigents live below it. Many moments of conflict in the novel occur when characters cross that border. For instance, Candidó is hit by Delaney's car when he crosses the road, and Delaney's car gets stolen when he hikes below the road.