L. Pelkmans, D. De Keukeleere & G. Lenaers Vito- Flemish Institute for Technological Research, Belgium in Urban Transport VII ed. Sucharov & Drebbia 2001, 6 p.

« https://www.researchgate.net/publication/233090883_An_ecological_footprint_for_an_early_learning_centre_identifying_opportunities_for_early_childhood_sustainability_education_through_interdisciplinary_research

« https://www.researchgate.net/publication/267971187_Emissions_and_fuel_consumption_of_natural_gas_powered_city_buses_versus_diesel_buses_in_real-_city_traffic

« https://www.researchgate.net/publication/262944123_Exhaust_Emission_and_Fuel_Consumption_of_CNG_Diesel_Fueled_City_Buses_Calculated_Using_a_Sample_Driving_Cycle

« https://www.mdpi.com/2073-4433/15/2/148

« https://www.sciencedirect.com/science/article/pii/S2352146516304598

« https://www.nrel.gov/docs/fy00osti/28377.pdf

« https://www.mdpi.com/2571-8797/4/4/59

« https://www.degruyter.com/document/doi/10.1515/eng-2021-0038/html

« https://pdfs.semanticscholar.org/db30/f57bc37150133ecaadad678671fe67152a19.pdf

« https://www.iitf.lbtu.lv/conference/proceedings2018/Papers/N483.pdf