Modelling the effects of road Geometric design on vehicle emissions.

Abstract

The transport sector is a major contributor to global greenhouse gas emissions, with road transport accounting for nearly 20% of the total CO₂ emissions. In Uganda, rapid traffic growth, reliance on fossil fuels, and declining air quality have intensified environmental and health challenges. Existing emission models, developed mainly for high-income countries, rely on advanced data and modern vehicles, limiting their applicability to Uganda’s older and diverse vehicle fleet. The absence of locally adapted models that integrate road geometry and traffic dynamics constrains the development of sustainable transport infrastructure. This study developed and validated mathematical models to predict CO₂, CO, NOₓ, and hydrocarbon emissions based on geometric and traffic parameters under Ugandan conditions. Vehicle movements were tracked with a high-precision device, and emissions measured directly from tailpipes using a calibrated exhaust gas analyser. Results show that road geometry and dynamic variables significantly affect emissions. In rolling terrain, vehicle speed and higher-order speed terms dominated, producing a Ushaped speed–emissions relationship, particularly for smaller petrol vehicles. In mountainous terrain, gradients, curvature, and interactions between speed, gradient, and curvature were more influential, amplifying emissions and nonlinear effects. Petrol vehicles were more responsive to geometric variations than diesel vehicles. CO₂ and CO models achieved moderate to strong predictive power (adjusted R² up to 0.918), while NOₓ and hydrocarbon models were less predictive. The study concludes that highway geometry, vehicle characteristics, and driver behavior strongly influence emissions, with small-engine vehicles producing disproportionately higher levels. Key gaps in the Uganda Geometric Design Manual were identified, including missing emission-sensitive parameters, environmentally optimal speed ranges, and fleet heterogeneity considerations. Recommendations include integrating environmental criteria into road standards, promoting sustainable transport, and enforcing emission regulations.