Based on data from the Netherlands, Denmark, USA and UK, Jacobsen's paper in 20031 identified the non-linearity between the number of cyclists and pedestrians and the risk of injury from being hit by a motor vehicle. In other words, the more people walked and cycled, the fewer the number and rate of traffic collisions and injuries experienced by cyclists and pedestrians—a non-linear relationship. Jacobsen, termed this relationship, ‘Safety in Numbers’ (SIN), which was shown at different levels of scale, whether at an intersection, a city or a country. More recent work has since shown SIN to occur in other countries such as Australia.2

The (SIN) effect quickly grabbed the attention of public health promoters who were seeking ways to promote active travel, including walking and cycling. Efforts to address growing levels of obesity and inactivity in the developed world were, and still are, underway and the concept of SIN was seen to support the clarion calls for measures to promote the number of walkers and cyclists. SIN was also seen as a support by those demonising any measure that might deter the numbers of those walking and cycling, such as the mandatory use of cycle helmets. In fact, people who campaigned for increasing helmet use were lambasted and accused of potentially increasing serious injuries among cyclists by reducing the SIN effect. It is probably the public health fraternity who have done much to increase the impact of this paper, more so than those concerned with injury prevention and/or transportation safety. Arguably, the Jacobsen's paper led to a paradigm shift among planners and engineers who could think about pedestrian and bicycle safety in a different way and not be so fearful that by encouraging increases in walking and cycling they would see an increase in traffic collisions and causalities. …