On the way to Angola prison there's a sign along the roadside "Do not despair. You will soon be there." Prejean sees it when she first goes to visit Pat (2.8), and it is mentioned a couple of other times. When Millard Farmer sees it, he says, "Somebody knows this road real, real well" (3.50).
The sign seems to acknowledge, ironically, that Angola is a place of despair. It encourages visitors not to despair because they'll soon arrive—but arriving in Angola is not exactly a reason to cheer. The sign offers hope, but it's a bitter hope. In the context of the rest of the narrative, it also seems to refer to execution itself, where arrival is despair and, perhaps, with an eye to heaven, relief from despair.