In Switzerland, the health ministry (Office Fédéral de la Santé Publique) has systematically encouraged the evaluation of low threshold services. In this article, we discuss the evaluation of two of these: the buses for syringes exchange in Geneva and Bienne, the implementation of these two services, the success obtained and the contacts established. Even if the design of such an evaluation was relatively complicated, with one monitoring and two specific surveys, the principal aim of this article is not to measure the efficacy as such but to show how an learning process has occurred between the actors: government, administration, police, service's team, drug's users and neighbourhood's inhabitants. The efficacy for a long period of time and the implementation's success are largely linked to such learning processes.