County-scale assessment of bridge vulnerability using structural and social indicators

Bridges play a vital role in enabling mobility and economic activity; however, a significant portion of the existing bridge inventory is nearing or has already surpassed its intended service life. This growing problem is intensified by climate change, which is likely to accelerate deterioration and increase the likelihood of damage. At the same time, limited maintenance budgets make it increasingly difficult for bridge managers to decide where to intervene first. Current bridge prioritisation practices typically depend on expert-assigned weighting schemes that emphasise physical condition and network importance, while giving little attention to social equity and community-level consequences. As a result, populations that are most vulnerable to infrastructure failure are often underrepresented in decision-making.

To overcome these shortcomings, this study proposes a new assessment framework that combines structural vulnerability of bridges with social vulnerability of the communities they serve, allowing county-scale prioritisation that supports fairer allocation of maintenance resources. The methodology advances existing practice in several ways. It augments conventional bridge evaluation models by introducing ground subsidence susceptibility as an additional hazard indicator. It also expands the bridge assessment by adding adaptive capacity as a third dimension alongside traditionally used network criticality and damage susceptibility due to structural health and natural hazards. This new adaptive capacity metric captures economic strength, inspection demand and spaceborne monitoring potential of bridges in a county. Indicator weighting is derived objectively through principal component factor analysis, removing reliance on subjective expert judgement. In addition, a bivariate mapping approach is used to jointly visualise and interpret social and structural vulnerability while still preserving their individual contributions.

The framework was implemented for 22298 bridges across 58 counties in California. Results indicate that several counties in Northern California experience the greatest combined vulnerability, where deteriorating bridges coincide with limited institutional capacity and higher social disadvantage. This demonstrates that approaches focused only on engineering conditions and network role can unintentionally reinforce inequalities by failing to identify locations where disruptions would cause the most harm to communities. The analysis also reveals a strong association between socially vulnerable areas and favourable satellite-based monitoring coverage in California, suggesting that remote sensing can be strategically targeted to improve equity in infrastructure management. Overall, the proposed framework offers a practical means of strengthening resilience while more effectively addressing the needs of vulnerable populations.