Applying a complex systems-informed approach to the evaluation of population health interventions: A case-study example of traffic restriction schemes outside schools

Background: Population health interventions may change physical, social or fiscal environments to shift health behaviours and often operate within a complex system of interacting factors. Systems methods are frequently suggested as a way to provide an overview of the interactions that determine behaviours or actions and may help to understand how or why interventions might (or might not) change health. However, there are few examples of how these methods have been used in empirical evaluations of public health interventions. Methods: We describe our application of a qualitative complex systems approach to evaluate a population health intervention: traffic restriction schemes, often referred to as School Streets, where motor vehicle access outside schools is restricted at pick-up and drop-off times. We intended to use systems methods across our evaluation; here we describe how we use these methods to build theory. Following the five-step approach outlined by Alvarado et al., (2023), we produced research propositions, systems archetypes and a causal loop diagram to understand the underlying system in which traffic restriction schemes are implemented and their potential impact on school-based active travel.Results: In bringing together diverse and multi-disciplinary evidence from different stakeholders we identified unanticipated systems interactions such as increased initial tensions and conflicts between users of different active travel modes (e.g. cyclists vs pedestrians). Using causal loop diagrams and systems archetypes, we developed research propositions focused on funding and implementation, safety, habit formation and the potential for unintended consequences. Conclusions: Taking a complex systems approach has deepened our understanding of how traffic restriction schemes interact with their broader context and helped us develop a guiding theoretical framework for our ongoing evaluation. We intend this work to stimulate discussion, offer insights for future evaluations of population-level health interventions, and encourage public health researchers to adopt systems methods in their own evaluations.Key words: Complex systems approach, population health intervention, traffic restriction schemes, School Streets, Active travel, Causal loop diagrams, Systems archetypes, Qualitative systems methods, public health evaluation