During the period 1998-2000, municipal officials in Bogotá implemented a new transportation system for the city. Transmileniobecame the first major mass transportation system in the world to use only buses. The authors examine here the process that led to the design decision to power all of the buses with diesel fuel. The main finding is that the various public and private partners sought to accommodate themselves to the alternative that was deemed to be less risky. The diesel option was the outcome of contingent negotiation and distribution of responsibilities among the different interests. The dynamics of these processes were heavily influenced by a precautionary posture.