How do residents know which camion goes where?

I have lived here three years and I still travel mostly on foot, which the cobblestones make an honest choice. The camiones, which is what residents call the local buses, remain a mystery to me. I see them pass near Parque Juarez with hand written signs in the windscreen that I cannot decode. Do residents learn the routes by memory, or is there a simpler way to know which camion goes where?