Risk of bicycle collisions and 'safety in numbers': A natural experiment using the local introduction of e-scooters in England

Background: Ecological studies hypothesise a 'safety in numbers' (SiN) effect whereby road safety for bicycles and other micromobility users improves as their numbers increase, due to behavioural changes of motorists. Causal interpretation of these studies is difficult due to confounding and reverse causation. The introduction of electric scooter (e-scooter) rental schemes in selected districts in England meant an increase in micromobility users in these areas, which presented an opportunity to test the SiN hypothesis using a natural experiment. Methods: Time-series analysis of police data on road collisions in local authorities (LAs) in Great Britain, 2015-2023. Random-effects Poisson regression time-series models compared collision rates in LA districts with an e-scooter trial (n=41) versus matched control districts (n=41). Primary outcomes were all road collisions and bicycle collisions. Models adjusted for time; seasonality; baseline collision rate; COVID-19 period; and preintervention/postintervention period (proxied by intervention group/COVID-19 period interaction). Results: The rate of bicycle collisions reduced following the introduction of the schemes, compared with control districts (incidence rate ratio (IRR) 0.78, 95% CI 0.68 to 0.89 during peak COVID-19; IRR 0.87, 95% CI 0.77 to 0.99 in the post-COVID-19 period). This effect was specific to bicycle collisions and strongest in the subgroup of serious/fatal collisions. Conclusions: Findings suggest that the increase of a new and sustainable mode of transport, e-scooters, may have reduced bicycle collisions. This could have far-reaching benefits including reduced injuries, safer environments, and public health and environmental benefits if more people choose bicycles and micromobility over car transport. Findings should be verified in further work.