Introduction

Safety in Numbers is the phenomenon by which the per-walker or per-bicyclist frequency of being struck by motorists declines as the amount of walking or bicycling on a street or in a region increases.1 That is, while the absolute number of walkers or bicyclists struck by motorists may increase with more people walking or bicycling, due to the increase in exposure, the number of such collisions is observed to increase more slowly than the increase in the number of walkers or bicyclists, or even decrease.

The emergence of Safety in Numbers in the 1990s as a widely observed phenomenon reinforces the understanding that the number of injuries suffered by walkers or bicyclists is an imperfect indicator of the danger of walking or bicycling. Rather, the number of walking or bicycling injuries must be adjusted for the numbers of individuals walking or bicycling.

An example from a different context may be helpful here. The fact that no one is attacked in shark-infested waters doesn’t demonstrate that the waters are safe for swimming. The low casualties could simply mean no one swims. Similarly, people tend to refrain from walking or bicycling where they believe they will place themselves in danger by doing so.2 Consequently, safety is best measured by the risk of injury, not by the number of injuries. Safety is indicated by the absence of danger, not by an absence of injuries.

The Safety in Numbers effect for motorist collisions with pedestrians and bicyclists was first described in the context of intersections in Sweden in 1993.3 Thereafter, the effect was also found to apply to entire towns, cities and countries, and even across time periods.1 Numerous additional studies using a variety of data sources confirmed the existence of the Safety in Numbers effect.4

Mathematically, the most …