Infrastructure-Environment Complementarity in African Development: Spatial Thresholds and Economic Returns in Tanzania's BRI Corridors

Conventional infrastructure evaluation in Africa emphasizes economic performance while overlooking environmental conditions that govern long-term sustainability. This study develops a spatial–ecological framework to quantify how environmental quality modifies the developmental returns of infrastructure investments along Tanzania's Belt and Road Initiative (BRI) corridors. A spatial econometric approach integrating remote sensing and socioeconomic indicators captures infrastructure–environment interactions. Using a Spatial Durbin Model (SDM) with the Normalized Difference Vegetation Index (NDVI), the analysis identifies environmental thresholds and estimates complementarity effects on economic activity, measured through Night-Time Lights (NTL). Results reveal a critical threshold at NDVI = –0.8σ, delineating zones where investments generate high returns from areas where environmental degradation limits effectiveness. A significant NTL–NDVI interaction coefficient (6.44, p < 0.001) indicates that higher environmental quality substantially enhances infrastructure-driven growth. Spatial classification shows 63% of the corridor as high-priority zones with optimal returns and positive spillovers, while 15% requires ecological restoration prior to investment. The model demonstrates high explanatory power (pseudo R² = 0.882). The findings provide a replicable decision-support tool for optimizing investment allocation, aligning infrastructure strategies with SDGs 9, 11, and 13.