Further to a participant-observation of the training of interviewers in charge of the Household Travel Survey (Enquête Ménages Déplacement, EMD), the author points dysfunctions in certain travel situations. The protocol of investigation appears badly suitable to seize the travels that have an intrinsic utility (undirected travel). When the experience of the mobile individual, the multiplicity of his reasons, the sudden appearance of intentions along the way, or the supervening of a risk on the way, affect the mobility behavior, more open questions are requested. The author defends a crossing of the methods; he suggests in particular the occasional introduction of qualitative and comprehensive brackets in the heart of the quantitative questionnaire when the interviewer notices that the travel situation moves away from the traditional pattern of a travel derived from a demand for activity to destina¬tion (directed travel). Although this adjustment of the protocol would suppose a complex revision of the EMD Certu standard, and a recasting of the recruitment and training process of the interviewers, the author assumes that these obstacles are removed so as to appreciate the possible theoretical prospects which could result. While underlining the richness of the nuances offered by the axial model proposed by MOKHTARIAN and SALOMON (2001), he shows its limits and tries to clarify its ambiguities. This axis comprises eight different keys of determination that can possibly constitute the framework of a mobility behavior model on four plans.