"Safety in Numbers" re-examined: can we make valid or practical inferences from available evidence?

"Safety in Numbers"(SIN), a recent concept in transportation research, policy and planning, has emerged as a causal inference from the non-linear statistical association between estimates of the numbers of walkers or bicyclists in an area and the rate or number of traffic collisions experienced by pedestrians or cyclists. Proponents of SIN argue that greater numbers of walkers or cyclists modify the hazardous behaviors of motor vehicle drivers thus creating safer conditions. This paper critically examines the research on the non-linear association as an adequate empirical basis for this causal interpretation. Given the paucity of evidence supporting a specific mechanism for the SIN effect, alternative plausible explanations of the non-linear association behind SIN, and a potential for unintended consequences from its policy application, the authors call for caution in the use of SIN in transportation policy and planning dialogue and decision-making.