France and Italy are characterized by a huge flow of Dangerous Goods carried by trucks inside their own territory and between them and the European Union. It is well known that the Dangerous Goods Transport (DGT) leads risks for the population, the environment and the human activities, according to the experience feedback of the major accidents occurred during the last 30 years. Due to the particular damages caused by the different types of dangerous goods transported (explosions, toxic release, corrosion, radioactivity etc.) and of the potential subjects exposed to this phenomenon (population, environment, infrastructure etc.), risk prevention and emergency management necessitate the design and development of a Spatial Decision Support System (SDSS) dedicated to the various decision makers involved. This chapter aims to present the methodology that establishes the base of a SDSS dedicated to the risk assessment of a DGT accident and the evaluation of the consequences in order to provide to the decision makers the best information to support their decision. The first part of this chapter presents the definition of DGT accident risk on road, taking into account the specificities of the traffic flow, the weather conditions, the accident feedback in the selected infrastructure and its structural characteristics. The second part is related to an overview of the main phenomena that occur if an accident of DGT occurs, presenting the main model to estimate the effect of these phenomena on the population. The third part is the case study, in the city of Nice (French Riviera), which corresponds to the simulation of two scenarios of DGT accidents in two urbanized places and the damage assessment induced. The last part is a discussion about the interest of this methodology and its results for the public authorities, the Civil Protection and the motorway companies.