Since the year 2000, the French Ministry of Transport has incorporated a new instrument called the road safety audit (RSA). The audit proposed a singular method, which consists of assessing infrastructures on the basis of drivers’ experiences to get around the limitations of technical knowledge for decreasing the number of accidents. Through an ethnographic study, this article aims to analyze the legitimacy assigned to social and technical knowledge in the auditors’ work. I will study the legitimacy issue through the cognitive change initiated by the audit and the political stakes confronting the Ministry which ascribe, for their part, a central place to technical knowledge. In this way, I will analyze the auditors’ boundary-work around the legitimacy accorded to driving and technical expertise in different moments of RSA assessment.