Bicycle advocates and other stakeholders with an interest in arguing for a shift from motorized to non-motorized travel often rite the concept of ”Safety in numbers” (SiN) as an argument against the concern about a potential increase in numbers of accidents resulting from such a policy. The concept of SiN is used to explain the non-linear statistical relationships between the number of pedestrians or bicyclists and the number of injuries for the same group. The mechanism has been proven in a number of cross sectional and longitudinal studies, summarised in a quite recent meta-analysis. The concept has been subject to debate, regarding its existence, its mathematical characteristics and also related to this, regarding a clear understanding of the mechanism behind the safety in numbers effect. The Scandinavian countries, and in particular Norway are interesting cases to test the SiN effect, as there is a substantial seasonal variation in bicycle use. The seasonal variation is substantial, meaning that every spring there is a dramatic increase in the number of bicycles other road users are exposed to each subsequent week. By studying conflicts and interactions at the same study sites, it is possible to keep a close control with any other potential influencing factors, and only look at the effect of changes in the share of one of the road user groups. In other words, this situation can be used as an experiment of the SiN effect. In the current study, we exploit this variation in cycling levels and infrastructure design in order to give a better explanation of the mechanisms involved in the SiN effect. The same interviews and video recordings that were conducted in Norway were also conducted in Denmark and Sweden. The current report summarizes a three-year research program carried out at the Institute of Transport Economics (Safety in Numbers - uncovering the mechanisms of interplay in urban transport). The project consisted of several work packages, all aiming to either test the existence of the SiN mechanism or to the unravel mechanisms behind it. The report is structured around 15 different hypotheses regarding SiN. In addition potential contributions from infrastructure and traffic culture in explaining the SiN phenomenon are discussed. (Author/publisher)
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