An Evaluation Framework for Social Return on Investment (SROI) of Low-Cost Road Safety Engineering Interventions

This study develops a framework for evaluating the social and economic impacts of low-cost road safety improvement projects. The proposed framework applies the Social Return on Investment (SROI) approach to compare the socio-economic outcomes of road safety engineering interventions within 4 levels—Tier 1: removing severe conflicts, Tier 2: reducing speed, Tier 3: managing conflicts, and Tier 4: increasing awareness. Traditional evaluation methods focus on crash reduction and economic efficiency; however, they provide limited information on social outcomes and lack a systematic approach for comparing local road safety interventions. To address this gap, data from 15 road safety improvement projects at different study locations are used to assess the social and economic impacts for three stakeholder groups: road users, communities, and government agencies. The social outcome is converted into monetary values over a five-year evaluation period. Statistical and sensitivity analyses are applied to compare differences across the hierarchy of road safety interventions and evaluate the robustness of the SROI results. Results show that higher-level interventions generate the greater social returns. The average SROI ratios for Tiers 1 to 4 are 17.0, 15.6, 11.5, and 8.1, respectively. Statistical analysis shows a significant difference between intervention levels (F(3,11) = 16.51, p < 0.001). Crash reduction and community satisfaction were the main positive impacts, while increased travel time, fuel consumption, and emissions reduced SROI. The proposed framework provides practical guidance for evaluating low-cost road safety measures based on socio-economic performance in decision-making.