Abstract-Overspeeding is both a cause and an aggravation factor of traffic accidents. Consequently, lots of efforts are devoted so as to limit overspeeding and consequently to increase the safety of road networks. In this article, a novel approach to compute a safe speed profile to be used in an adaptive Intelligent Speed Adaptation system (ISA) is proposed. The method presents two main novelties. First, the 85 th percentile of observed speeds (V85), estimated along a road section, is used as a reference speed, practiced and practicable in ideal conditions. Second, this reference speed is modulated in adverse weather conditions in order to account for a reduced friction and a reduced visibility distance. The risk is thus mitigated by modulating the potential severity of crashes by means of a generic scenario of accident. Within this scenario, the difference in speed that should be applied in adverse conditions is estimated so that the highway risk is the same as in ideal conditions. The system has been tested on actual data collected on a French secondary road and implemented on a test track and a fleet of vehicles. The performed tests and the experiments of acceptability show a great interest for the deployment of such a system.