I would find this slightly useful, I live in Madrid. For most of my trips I take the metro; I have two lines nearby, and they are frequent enough that I just show up. But there are a few parts of town that are just not well connected to my place by metro. If I want to go to Atocha, Cibeles or Piramides, the bus is better. But I have three buses that take me to each of those places, and they show up every 25 minutes. If I'm going there, I want to know which stop will have a bus soonest.