The study presented has a twofold objective: an evaluation of the lire risk along the ESCOTA motorway network (430 km in the Provence Alpes-Côte dAzur region) and a modulation of maintenance work according to the level of risk. The main cause of fires on motorways would seems to be lighted cigarettes thrown from vehicles. A preliminary statistical approach aimed at establishing a typology of situations has been undertaken in the case of the motorway network. In the technical approach, aimed at providing a programme of intervention measures two components have been taken into account: the risk of ignition, i.e. the probability of fire outbreak at the edge of the road, and the risk of spread, that is to say the virulence of the fire that may occur after ignition. Calculation of the risk of ignition combines calculation of the trajectory of cigarette and the probability of ignition. Complex notions of fluid mechanics were used to assess the probability of ignition per year and per kilometer of road.