Existing information systems for urban public transportation are empowering travelers to optimize their trips with respect to travel duration. Experience with such systems shows that this is a viable approach. However, we argue that solely relying on trip duration as the primary indicator for satisfaction can be limiting. Especially, in urban settings providing additional information such as the expected number of passengers can be highly beneficial since it enables travelers to further

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... their comfort. As technical basis for determining the number of passengers, we have built an inexpensive hard-and software system to estimate the current number of passengers in a vehicle. Furthermore, we have deployed the system in several buses in the city of Madrid. In this paper, we describe the overall design rationale, the resulting system architecture as well as the underlying algorithms. Furthermore, we provide an initial report on the system's performance. The initial results indicate that the system can indeed provide a reasonable estimate without requiring any manual intervention.