The goal of this study is a cost benefit analysis (CBA) of road accident prevention measures realized in Switzerland in the period from 1975 to 2007. The actual number of road accident casualties is estimated by combining police report and accident insurer data. The societal costs of road accidents (direct costs, productivity losses and intangible costs) and effectiveness of public and private prevention measures in reducing the number of accidents are estimated. The return on investment (ROI) is calculated by comparing the costs and benefits of prevention measures. The main results are a ROI of 1.54 for all public and private prevention interventions, a ROI of 9.43 for public prevention programmes (without investments in the safety of road infrastructure), a ROI of 5.81 for alcohol prevention measures, a ROI of 16.31 for promotion of bicycle helmet wearing, a ROI of 8.06 for the combined prevention measures introduced in the year 2005 and a ROI of 101.03 for the measures aiming at the imposition and promotion of safety-belt. The effect of all interventions between 1975 and 2007 is substantial with 13’484 fatalities and 909’213 casualties prevented and a total of CHF 72’816 million avoided, thanks to prevention.