The management and control of vehicles transporting hazardous materials (hazmat) on congested highways has attracted growing attention from researchers in recent years. This paper proposes a decision support system (DSS) for monitoring hazmat vehicles, aimed at assessing two problems: evaluating the social risk induced by hazmat vehicles traveling in highways and selecting restoration procedures after an accident involving heavy vehicles. The proposed DSS can estimate in real time and offline the risk of hazmat transportation, by taking into account the type of transported hazmat, the traffic, and the density of populations living close to the highway. Two main modules of the DSS are specified: the risk assessment module and the simulation module (SM) that allows forecasting risk in different contexts and scenarios. In particular, the SM is realized by modeling the highway network in a colored Petri net (CPN) framework. In order to show the effectiveness and the applicability of the DSS, a prototype is described and applied to a highway in the North-east of Italy.