That motorists are a lot less likely to hit someone walking or bicycling if more people walk or bicycle surprised researchers. In contrast, the number of car crashes increases proportionally with the number of cars. The evidence of a prevalence effect implies that injury risk is more than just a matter of physics, and that something occurs with human physiology or psychology. Safety in Numbers likely occurs because humans have difficulty detecting rare items. That injury risk decreases with more walking and biking creates opportunity for implementing public policies for reducing damage to the climate and improving health. This non-linear risk also explains why the recent NTSB recommendation for compulsory bicycle helmet laws could increase injury risk.