Bridging the Gap: Designing User-Centred Tools to Increase Appropriate Booster Seat and Adult Seatbelt Use Among Children

Despite available safety guidance, knowledge about appropriate booster seat transitions does not consistently translate into accurate parental decision making. This knowledge-behaviour gap represents a critical road safety issue, as it is estimated that almost 75% of Australian children aged 7-12 are prematurely transitioned to adult seat with seatbelt. The misalignment between legal requirements (permitting adult seatbelt use at age 7) and best practice guidelines (requiring safe seatbelt fit assessment) creates confusion among parents. No validated tools exist to support this complex decision making process. This thesis addresses these gaps through a comprehensive three-phase methodology employing the Capability, Opportunity, Motivation – Behaviour (COM-B) model and Behaviour Change Wheel (BCW) framework. Six studies examined parental decision making processes and developed an evidence-based intervention. Phase 1 formative activities established that BCW and COM-B frameworks can be successfully applied in injury prevention contexts. A randomised controlled trial evaluating the existing 5-step test demonstrated significant knowledge improvement but no enhancement in decision making accuracy. Behavioural diagnosis through think-aloud methods identified 17 essential tasks clustered into five domains for safe transitions, while focus groups revealed complex barriers including knowledge gaps, social pressures from peer comparisons, varying parental confidence levels, and children's complaints. Phase 2 utilised systematic behavioural analysis, consumer panel consultation, and health literacy expert input to create a theory-driven digital resource. Phase 3 involved iterative user testing until achieving 80% accuracy benchmarks, followed by final evaluation under realistic conditions with parent-child dyads. The developed website-based decision aid achieved 90% accuracy under controlled conditions but declined to 60% accuracy under realistic testing conditions. While this represents meaningful improvement over existing resources (43% baseline), the 30-percentage-point performance gap reveals persistent challenges in translating behavioural theory into effective real-world safety interventions. The research identified interpersonal negotiation skills as a critical psychological capability component not previously recognised. This research represents the first comprehensive examination of informational requirements and behavioural determinants influencing parental booster seat transition decisions. The work demonstrates that systematically designed, theory-driven support tools can serve as effective interventions while revealing limitations of traditional educational approaches for complex safety behaviours. The established methodology provides a robust framework for intervention development in child safety contexts, though findings indicate that even theoretically grounded interventions may require complementary policy changes to achieve population-level impact.