Description

This technical working paper presents the second refined proposal for incorporating a vehicle exhaust emissions sub-model into the HDM-4 software package under the ISOHDM initiative. Building on the preliminary conceptual framework, this document advances the mathematical formulation for predicting emission rates based on engine power output, instantaneous fuel consumption, and drive cycle characteristics. Specific attention is given to modeling the influence of traffic congestion, road gradients, and speed fluctuations on engine load and resultant pollutant formation. Emitted compounds analyzed include carbon dioxide, carbon monoxide, unburned hydrocarbons, nitrogen oxides, and exhaust particulate matter. The paper introduces cold-start emissions correction factors, vehicle fleet aging degradation coefficients, and catalytic converter efficiency profiles to reflect modern vehicle fleets across different geographic regions. Guidance is provided on structuring emission lookup tables and default parameter libraries within the HDM-4 database, enabling users to perform rapid environmental screening or detailed project-level environmental cost-benefit assessments. This proposal served as a key technical milestone prior to finalized software coding and model integration. Overall, the report emphasizes rigorous empirical validation, standardized data acquisition protocols, and modular integration into the broader Highway Development and Management analytical engine. By combining physical principles with statistical calibration across international trial sites, this working paper provides a robust theoretical and practical baseline for highway authorities to evaluate long-term infrastructure policy options, allocate maintenance budgets efficiently, and maximize economic returns across regional and national road networks. In summary, this study provides highway engineers, transport planners, and economic analysts with an essential mechanistic-empirical tool for evaluating life-cycle