A new approach is proposed for quantitative assessment of impact frequency on an element at risk located on a slope under a rock cliff, from the volumetric retreat rate of the cliff, the distribution of the block volumes and the simulation of the block trajectories. The volumetric retreat rate is derived from diachronic terrestrial laser scanning and integration of a power law distribution of the rockfall volume. The frequency and the size of the falling blocks are determined from the power law distribution of the block volume, which is derived from a survey of the blocks fallen on the slope. Finally, the impact frequency is obtained from the simulation of block falls occurring during a given period, using the computer program Rockyfor3D. The method is applied to the Mont Saint-Eynard cliff, which overhangs the Grenoble urban area. The quantitative frequency obtained is compared to the qualitative frequency traditionally used in French hazard maps.