Safe streets for cyclists? Quantifying the causal impact of cycling infrastructure interventions on safety.

London's Cycle Superhighways (CS) form a network of cycle routes connecting central London to outer boroughs, introduced in 2010 to promote cycling and improve safety. This paper examines their causal impact on cycling volume and safety using detailed road traffic and road safety data from the UK's Department for Transport. To estimate these effects, we employ propensity score-matched difference-in-differences and panel outcome regression models, comparing two distinct infrastructure types: segregated and non-segregated CS. A key contribution of this study is the development of a novel safety indicator - the normalised collision rate - that accounts for changes in cyclist volume (exposure) while incorporating expected non-linearities in the relationship between collisions and exposure. Our findings indicate that non-segregated CS did not increase cycling volume but led to a substantially higher collision rate. This increase appears to be driven by a post-intervention surge in the proportion of new, inexperienced cyclists along these routes. In contrast, segregated CS effectively increased cycling volume without increasing collision rates. Further, an evaluation of a major segregation upgrade along an existing non-segregated CS route revealed a notable reduction in collision rates. These results highlight the crucial role of segregated infrastructure in not only encouraging cycling but also ensuring it remains a safe and viable urban transport option.