A prototype of a road submersion warning system, providing a rating of road submersion risks every 15 minutes during a storm event, at about 2000 points where roads and rivers intersect, has been developed for the Gard region (French Mediterranean area). The computed risks result from the confrontation between discharges produced by a distributed rainfall–runoff model and the estimated susceptibility to flooding of the intersection points. The warning system is validated against road inundations reported by the local road management service. The comparison of the performances of this framework fed with various quantitative precipitation estimates (1-km2 grid interpolation of point rainfall measurements or radar products) is presented herein. This case study, based on highly distributed rainfall–runoff modelling and on a rich set of indirect observations of the flood magnitudes, provides a priori an ideal framework to evaluate the usefulness of weather radar products for hydrological applications.