In light of global environmental degradation, which is largely affected by the transport sector, increasing attention is paid to enhancing the quality of life in urban areas. Policymakers are taking steps to reduce transport-related pollution and accelerate the shift to sustainable city development. They introduce stricter requirements for fuel quality and transport emissions, impose fines for traffic congestion, increase parking fees, establish low-pollution zones, etc. The implementation of measures that may not be perceived positively requires careful planning and analysis. This involves using suitable techniques and software to evaluate and support planned strategies. We propose a comprehensive approach to assess the ecological effect of using a delivery fleet that incorporates cargo bikes, operating in conjunction with light vans within a restricted urban area. The presented methodology was implemented using Python programming language and includes simulation tools for the execution of alternative delivery scenarios and a model for determining emissions generated by the analyzed supply scheme. The developed approach was applied to selected areas of Krakow (Poland) and San Sebastian (Spain), and the obtained models were used to study the influence of transport demand on the decrease of transport-caused environmental pollution. Simulation results indicate a statistically significant influence of the mean consignment weight on the emission reduction obtained due to the use of cargo bikes.