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Evidence for the ‘safety in density’ effect for cyclists: validation of agent-based modelling results
  1. Jason Hugh Thompson1,2,
  2. Jasper S Wijnands1,
  3. Suzanne Mavoa3,
  4. Katherine Scully1,
  5. Mark R Stevenson1
  1. 1 University of Melbourne, Transport, Health and Urban Design Research Hub, Parkville, Victoria, Australia
  2. 2 Centre for Human Factors and Sociotechnical Systems, University of the Sunshine Coast, Sippy Downs, Queensland, Australia
  3. 3 School of Population and Global Health, University of Melbourne, Parkville, Victoria, Australia
  1. Correspondence to Dr Jason Hugh Thompson, University of Melbourne, Transport, Health and Urban Design Research Hub, Parkville, VIC 3010, Australia; jason.thompson{at}


The safety in numbers (SiN) effect for cyclists is widely observed but remains poorly understood. Although most studies investigating the SiN phenomenon have focused on behavioural adaptation to ‘numbers’ of cyclists in the road network, previous work in simulated environments has suggested SiN may instead be driven by increases in local cyclist spatial density, which prevents drivers from attempting to move through groups of oncoming cyclists. This study therefore set out to validate the results of prior simulation studies in a real-world environment. Time-gap analysis of cyclists passing through an intersection was conducted using 5 hours of video observation of a single intersection in the city of Melbourne, Australia, where motorists were required to ‘yield’ to oncoming cyclists. Results demonstrated that potential collisions between motor vehicles and cyclists reduced with increasing cyclists per minute passing through the intersection. These results successfully validate those observed under simulated conditions, supporting evidence of a proposed causal mechanism related to safety in density rather than SiN, per se. Implications of these results for transportation planners, cyclists and transportation safety researchers are discussed, suggesting that increased cyclist safety could be achieved through directing cyclists towards focused, strategic corridors rather than dispersed across a network.

  • bicycle
  • mechanism, policy
  • interventions, public health
  • interventions, urban
  • populations
  • risk

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  • Contributors JHT conceived the research study collected the data and contributed to the analyses and manuscript writing. JSW contributed to the study design, conducted the analyses and contribute to the manuscript writing. SM contributed to the study design and manuscript writing. KS contributed to the data collection, analyses and manuscript writing. MRS contributed to the manuscript writing.

  • Funding This study was funded by the Australian Research Council (grant no. DE180101411) and the National Health and Medical Research Council (grant nos. 1043091 and 1121035).

  • Competing interests None declared.

  • Patient consent Not required.

  • Ethics approval The research was approved by the University of Melbourne Human Research Ethics Committee (1749212.1).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data sharing statement The video recordings of the intersection interactions can be made available for researchers wishing to use them for additional studies.