Safety effects of permanent running lights for bicycles: A controlled experiment

► The evaluation of the safety effects of permanent running lights for bicycles significantly improves traffic safety amongst cyclists. ► The safety effects are most dominant for multiparty accidents involving cyclists under daylight conditions. ► The safety evaluation study has been performed as a controlled experiment applying cluster randomization and self-reporting of accidents. ► The study reflects both the strengths, weaknesses and challenges related to this combination of methods.