Risk factors for unsafe events involving a motor vehicle for group riders (cyclists): A naturalistic case-crossover study

•A naturalistic study examined risk factors for unsafe events while group riding.•Roundabouts increased the risk of an unsafe event involving a motor vehicle.•Traffic islands, speed limits ≥ 60 km/h and rider violations also increased risk.•Riding 2 abreast in a traffic lane or with all riders in a bike lane decreased risk.•Road infrastructure treatments and education could promote safety for group riders. This study aimed to determine the risk factors associated with unsafe events involving a motor vehicle, that occurred while group riding (cycling) in Perth, Western Australia. Naturalistic video footage was collected from 52 group riders and unsafe events identified. A case-crossover study was used to compare the road infrastructure and group behavioural characteristics of 108 case sites where unsafe events occurred to 216 control sites where no unsafe events occurred. After controlling for potential confounding factors, roundabouts increased the risk of an unsafe event compared to midblocks (odds ratio (OR): 3.63, 95% confidence interval (CI): 1.57–8.42, p = 0.003), priority control intersections (OR: 4.36, 95% CI: 1.49–12.76, 0.007) and traffic signal intersections (OR: 5.57, 95% CI: 1.42–21.79, p = 0.014). Raised traffic islands (OR: 2.30, 95% CI: 1.41–3.78, p = 0.001), posted speed limits of ≥60 km per hour (OR: 2.45, 95% CI: 1.55–3.86, p < 0.001) and group rider traffic violations (OR: 2.51, 95% CI: 1.14–5.53, p = 0.022) also significantly increased the risk of an unsafe event. Riding two abreast in the traffic lane (OR: 0.50, 95% CI: 0.32−0.76, p = 0.002) or having all riders in the bicycle lane (OR: 0.14, 95% CI: 0.04−0.51, p = 0.003), significantly reduced the risk of an unsafe event, compared to riding single file in the traffic lane. Simple road infrastructure treatments on popular group riding routes as well as education targeting both group riders and motorists, could reduce unsafe events and promote a safer, more inclusive shared road environment for group riders.