The concept of Safety in numbers 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 (Elvik, 2009). In previous studies the natural seasonal variation in cycling frequency in Oslo, Norway has been exploited (Fyhri et al. 2016). These studies have indicated a short term Safety in Numbers effect through the season. Cyclists experienced fewer occasions of being overlooked by cars and fewer near misses. Video observation data confirmed this. In the current study accident data were collected from a prospective population-based study, during 2014 at the Oslo Emergency Clinic. These data are matched with monthly cycle flow data. Both collisions and single accidents are closely related to the number of cyclists on the road. However, when we look at the relative difference between single accidents and collisions (the ratio), we see that collisions decrease relative to single accidents when cyclist numbers increase. In December, 28 % of all cyclist accidents are collisions, a figure that drops to 10 % in July. The seasonal Safety in Numbers effect that previously has been shown in relation to near misses and conflicts, can also be seen in accident data. The results are discussed in light of potentially confounding factors such as lightning conditions and road conditions.